Microcirculation of the alimentary tract

GASTROENTEROLOGY

PROGRESS

1983;84:846-68

ARTICLE

Microcirculation I. Physiology Exchange

of Transcapillary

D. NEIL GRANGER Department and Faculty Canada

of Physiology, of Medicine,

of the Alimentary

and JAMES

College Memorial

of Medicine, University

Review Contents Introduction Anatomic Considerations Capillaries Salivary glands Pancreas Stomach Small intestine Colon Pathways for transcapillary exchange Interstitium Lymphatics Stomach Small intestine Colon Salivary glands Pancreas Physiology of Capillary Fluid and Solute Exchange Capillary Fluid Exchange Lymph flow Capillary filtration coefficient Capillary pressure Interstitial fluid pressure Osmotic reflection coefficient Oncotic pressure gradient Interactions of Capillary and Interstitial Forces Enhanced capillary filtration-edema safety factors Reduced capillary filtration-safety factors against tial dehydration Net transepithelial fluid transport Secretion Absorption Filtration secretion

Fluid and Solute

A. BARROWMAN University of South Alabama, Mobile, Alabama, of Newfoundland, St. John’s, Newfoundland,

Transcapillary Solute Small solutes Macromolecules

intersti-

Received August 3, 1982. Accepted November 29, 1982. Address requests for reprints to: Dr. D. Neil Granger, Department of Physiology, College of Medicine, University of South Alabama, Mobile, Alabama 36688. Dr. Granger is the recipient of Research Career Development Award HL00816 from the National Heart, Lung, and Blood Institute. Part II will be published in the following issue of GASTROENTEROLOGY. 0 1983 by the American Gastroenterological Association 0016-5085/83/04846-23$03.00

Tract

Exchange

It is well recognized that large amounts of water and solutes escape the circulation to enter the interstitial spaces of the alimentary tract. In health, this extravasation is considered to result from the relatively high density [and consequent large surface area) and permeable nature of the capillaries supplying these tissues. The magnitude and direction of the movement of water and solutes between capillaries and interstitium varies in relation to the physiologic state of gastrointestinal function. Modulation of capillary fluid movement during these conditions results from the balance of hydrostatic and oncotic pressures exerted across the capillary wall. Adjustments in these forces allow the microcirculation to provide the fluid for epithelial transport during secretion and to remove fluid from the interstitium during absorption without overexpansion of the interstitial spaces. Perturbations in hydrostatic and oncotic pressures, alterations in vascular permeability, and lymphatic obstruction, however, can lead to engorgement of the interstitium with capillary filtrate. Edema, a manifestation of excess accumulation of fluid in the interstitium, is a common feature of many diseases of the alimentary tract. The overall aim of this review is to summarize the available information regarding transcapillary fluid and solute exchange in the alimentary tract in health and disease. The functional anatomy of blood and lymph microcirculations and the interstitium is briefly described. This is followed by a detailed description of the physiologic factors involved in the regulation of capillary fluid and solute exchange in resting and functional states (absorption and secretion). The final section (Part II of this review) examines the pathogenesis of interstitial edema in the

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alimentary tract in terms nisms, i.e., hydrostatic, lymph edemas.

Anatomic

of the four classical mechacolloid, permeability, and

Considerations

Capillaries Salivary glands. Dense capillary networks supply both the striated ducts and the secretory endpieces of salivary glands. The two capillary circulations are separate and this may reflect the later embryologic development of the secretory endpieces. It is unclear to what extent and how these two microcirculations are linked. Venous sinusoids draining the capillaries associated with the secretory endpieces are noted to come into close proximity with both the secretory endpiece epithelium and the striated ducts (1). Pancreas. The arrangement of the microcirculations of the pancreas has recently received considerable attention. The peculiar arrangement of these circulations may well have a functional significance. Capillaries resembling a glomerular tuft supplying the islets of Langerhans drain to neighboring acini via short portal vessels (2). It is estimated that 75% 90% of acinar capillaries receive their blood supply from the pancreatic arterial blood while the remainder receive portal blood from the endocrine tissue (3). There is also some evidence that a proportion of the capillary circulation in the acini is drained by another portal venous system to supply a periductal capillary plexus, thus some pancreatic blood appears to pass through three capillary circuits arranged in series (4). Stomach. The mucosa contains dense leashes of arborizing capillaries, which are found in all areas of the stomach, particularly surrounding the gastric pits. There has been some controversy as to whether this mucosal circulation can be short-circuited by submucosal arteriovenous anastomotic channels (57), but presently the general agreement is that such anastomoses do not exist. Small intestine. There are three microcirculations in the small intestinal wall coupled in parallel, i.e., the mucosa. the submucosa, and the muscularis propria. The mucosal circulation can be subdivided into a deep mucosal circulation and the villus vessels. The villus vessels have a greater autoregulatory capacity and react differently to local vasoconstrictor nerve stimulation than the deeper mucosal vessels, and both arterial and venous vessels of the villi act independently of the deeper vessels. In the villi, the capillaries that arise from a centrally placed arterial vessel form a lacy network ramifying below the

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enterocyte layer. The fenestrae are most frequently in the upper part of the mucosa, and the juxtacapillary space between the base of the epithelial cells and the luminal aspect of the capillary wall is estimated at -2 lrn (8). Colon. The blood circulation in the colonic wall has many similarities to that of the small intestine. From a submucosal plexus, arterial vessels arise and extend to the base of the mucosa, where they break up into capillary vessels that pass vertically to the superficial mucosa to form a plexus whose capillary vessels lie just below the surface epithelium. It has been estimated that the capillary wall lies as close as 1 pm to the base of epithelial cells, that is, about half the corresponding distance in the small intestinal villi (8). The fenestrae of colonic mucosal capillaries as in the small intestine are noted to be most frequent in the superficial onethird of the mucosa, and always face the enterocytes. Pathways for transcapillary exchange. Morphologic analyses and ultrastructural tracer studies have produced a detailed description of the nature, organization, and functional role of the structures which account for the transport characteristics of capillaries in the alimentary tract (g--14). Figure 1 is a diagram of the various morphologically defined pathways for exchange of water and solutes across a capillary. The capillary depicted falls under the classification of fenestrated capillaries, the most abundant type of capillary found in the alimentary tract. However, there are several salient features of this hypothetical capillary that are found in continuous (nonfenestrated) capillaries, the other type of capillary found in the alimentary tract. The enumerated transport pathways are as follows: CELL MEMBRANE. Small perforations (J-10 A radius] in the lipid bilayer of cell membranes allow water, small nonpolar solutes, and lipid-soluble solutes to cross the cells. Due to a dominance of transport pathways that do not traverse cell membranes, the endothelial cell pathway accounts for less than one-tenth of the total capillary hydraulic conductivity (i.e., the rate of fluid filtered across the membrane per unit pressure gradient] particularly in fenestrated capillaries (13-15). OPEN FENESTRAE. Fenestrae are circular openings of 200-300 A radius in the capillary endotheliurn. The fenestrations are present only in the thin part of the endothelial cell, which comprises 2%,30% of its total surface. Fenestrae not having a diaphragm offer minimal restriction to transcapillary movement of macromolecules. Tracer molecules ranging in size from 25 to 150 A radius readily permeate open fenestrae. The frequency of fenestrae (both open and diaphragmed) increases from arterial to venous ends of the capillary (lo--12,16).

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Figure

1. Diagram of transport pathways in capillaries 3, diaphragmed fenestrae; 4, intercellular membrane.

84, No. 4

of the alimentary tract. Transport pathways: 1, cell membrane; 2, open fenestrae; junction; 5, pinocytotic vesicles; 6, transendothelial channels; 7, basement

DIAPHRAGMED FENESTRAE. In the mucosa of the small bowel over 60% of the fenestrae are provided with an aperature or diaphragm. Although the porosity of the diaphragm is unknown, these structures are considered to account for the observation that tracer molecules of >50 /i radius exit through only a fraction of the fenestral population (10,11,16-18). INTERCELLULAR JUNCTIONS. Open intercellular junctions have been described in continuous and fenestrated capillaries. Intercellular junctions, however, are relatively infrequent in fenestrated capillaries. Ultrastructural tracer studies indicate that open intercellular junctions measure 20-60 A in width. Intercellular junctions of arteriolar and capillary endothelium appear morphologically closed and functionally impermeable to solutes of 20 A diameter. However, 25%30% of the junctions appear open in the endothelium of postcapillary venules (10,17,18).

PINCYTOTIC VESICLES. A relatively large volume of endothelial cells in continuous and fenestrated capillaries is occupied by vesicles with an internal radius of -250 A. The vesicles are considered to move freely (by thermal kinetic energy) from one side to another and fuse with the plasma membrane, carrying either plasma or interstitial fluid. Tracers ranging in size between 25 and 150 A radius have access to the vesicles. The population density of vesicles within the endothelium increases from arterial to venous ends of the capillary. Vesicles are considered by some investigators as a major pathway for transport of macromolecules across continuous capillaries. Ultrastructural tracer studies in the intestine indicate that transport of macromolecules by vesicles is three to eight times slower than exit through fenestrae (10,13,16-19). TRANSENDOTHELIAL CHANNELS. Patent transendothelial channels are formed by one or more vesicles open simultaneously on both sides of the endothelium. The maximal internal radius of these transient channels approaches that of a single vesicle, i.e., 250 A, yet they have strictures at their necks and points of fusion between vesicles that reduce the

internal radius to 50-200 A. Occasionally, the channel opening is provided with a diaphragm that has an exclusion limit between 25 and 55 A molecular radius. The relative frequency of transendothelial channels increases from arterial to venous ends of the capillary (lO,li’,lg). BASEMENT MEMBRANE. The basement membrane surrounding fenestrated and continuous capillaries is formed by a layer of fine fibrillar material, presumably collagen and mucopolysaccharides. Although there are no structurally recognizable pathways across the basement membrane, there is evidence that this structure reduces the rate of transport of larger tracer molecules. After penetration through the fenestrae, tracer particles (62-150 A radius) transiently accumulate in the subendothelial space against the basement membrane to form small clusters opposite permeable fenestrae. Tracers of 25-55 A radius are not temporarily retained by the basement membrane (10,18). From the previous description, it is clear that the various pathways for transendothelial transport are more densely distributed on the venous side of the microvasculature. In continuous capillary beds, there is evidence that transport of macromolecules primarily takes place across the postcapillary venule rather than the capillary (20). The reader should bear this notion in mind when the term “capillary permeability” is used in this review. Interstitium As a gel interposed between the capillary wall and the terminal lymphatic, the interstitium exerts profound influences on the exchange of water between the capillaries and transporting epithelia of the alimentary tract. With modern techniques, the interstitium, i.e., the noncellular domain in which cells of a tissue are dispersed, has become accessible to quantitative studies of its dimensions and physicochemical properties. The interstitium is composed mainly of collagen fibers and mucopolysaccharides mechanically en-

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1983

tangled and cross-linked to produce a gel-like structure. The primary mucopolysaccharide in the interis hyaluronic acid. The physiologic stitium significance of hyaluronic acid lies in its ability to immobilize interstitial fluid; therefore, the hydraulic conductivity of the normally hydrated interstitium is very low. Since the hydraulic conductivity of the interstitium is inversely proportional to the hyaluranic acid concentration, overhydration of the interstitial spaces increases the hydraulic conductivity of the interstitial matrix many-fold [a doubling of interstitial volume could produce >lOOO-fold increase in hydraulic conductance). In addition to its effect on water movement, the interstitial matrix plays an important role in determining (a) the pressure-volume characteristics of the interstitial space, (b) the oncotic pressure generated by interstitial proteins, and (c) the rate of movement of macromolecules within the interstitial spaces (21,22). An important property of the interstitial matrix is its ability to exclude solutes from a portion of the space available to water. Plasma proteins are distributed in only a fraction of the matrix water volume because these macromolecules cannot fit into certain parts of the meshwork with a high-matrix density. Albumin is excluded from 35%-40% of the space available to water in normally hydrated intestinal interstitium (24), results comparable to that reported for liver (25,26) and other tissues (22). This degree of exclusion of albumin is consistent with a matrix perforated by pores of 200 A radius and indicates that the rate of diffusion of albumin in the intestinal interstitium is reduced by more than one-third of its velocity in water because of the frictional interaction between albumin and the matrix (2x,22). Stimulation of net water absorption increases interstitial volume and reduces the degree of albumin exclusion by the intestinal interstitium (24)-results consistent with the concept that the degree of exclusion of a molecule is inversely proportional to matrix hydration (21). The reduction in albumin exclusion during absorption reflects an expanded matrix with pores exceeding 1000 A radius. This expansion of the matrix, which would also occur as a result of excess capillary filtration, should greatly enhance the diffusive and convective movement (bulk flow) of macromolecules within the intestinal interstitium. In spite of the profound influences the interstitium exerts on transcapillary water and solute exchange, relatively little attention has been given to this “structure” in the alimentary tract. The ultrastructural appearance of the interstitium in tissues such as the pancreas, salivary glands, and the various layers of the intestinal wall suggest that there may be dramatic differences in interstitial volume and other properties of the interstitium between tissues in the

MICROCIRCULATION

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849

alimentary tract. Future experimentation should be directed toward establishing whether or not such differences exist, as well as toward further characterization of the responses of the interstitial matrix to various physiologic (absorption and secretion) and pathologic (edema) conditions.

Lymphatics Initial lymphatics are structurally similar in many ways to blood capillaries, although these vessels, when filled, can have a larger diameter than their blood-vessel counterparts. The lymphatic endothelium is a flattened, rather featureless cell, with sparse intracellular organelles but with prominent pinocytotic vesicles (27). The basement membrane is fragmentary, contrasting with the continuous basement membrane surrounding most blood capillaries. A basement membrane, however, is strikingly absent from the sinusoidal endothelium of the liver. Intercellular adhesion devices between the lymphatic endothelial cells are generally found to be less plentiful than those associated with endothelial cells of capillaries. Thus, initial lymphatics present a highly porous appearance and, in conditions of edema, wide intercellular gaps have been described that would appear to offer little resistance to entry of macromolecules and particles into the lymphatic lumen. There is some debate, however, as to whether fixation creates artifactual gaps of this kind (28). Nevertheless, there is no doubt that although the initial lymphatic vessels have only a single layer of endothelium, attached collagenous fibrils anchored to other cells offer the means of distracting the endothelial junctions and opening overlapping flaps formed by adjacent endothelial cell processes (29). Fenestrae are not a feature of lymphatic endothelium (27). These various structural features have led to the assumption that interstitial fluid has free access to the lymphatic lumen and that the lymph from any region is identical in composition to interstitial fluid in that area. Much needed evidence to support this has come from studies of subcutaneous interstitial (30). fluid and lymph obtained by micropuncture Certain pathologic processes in the liver, such as cirrhosis and chronic Budd-Chiari syndrome, result in “capillarization” of the sinusoids with ultrastructural appearances such as development of a basement membrane, suggesting a reduction in sinusoidal permeability (31). It is possible that similar pathologic alterations can occur in initial lymphatics, altering the permeability and resulting in a composition of lymph no longer representative of interstitial fluid. In this connection, it is interesting that such a change has recently been described in the

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ultrastructural appearances of lymphatic vessels in Crohn’s disease (3 2). Another important consideration regarding initial lymph vessels is their anatomic relationship within the interstitium to blood capillaries and transporting epithelia of the gastrointestinal tract and digestive glands. This idea embodies the concept of compartments within the interstitium. Clearly, the closer the lymph vessels are to capillaries, the more nearly will the lymph resemble a capillary filtrate in composition. On the other hand, if an appreciable interstitial space separates the lymph from the blood vessels, then modification of tissue fluid by the interstitial structure and by metabolically active cells in this interstitium would be anticipated. Thus, in the lamina propria of the small intestinal mucosa, for example, the central lymph vessl is -50 pm from the capillaries, which lie only -2 pm from the enterocyte layer (33). Stomach. There is a plexus of lymph vessels in all three layers of the stomach wall with short perpendicularly arranged vessels linking the lymph vessels of each layer. The initial lymph vessels of the mucosa lie very close to the gastric glands (34), and these mucosal vessels drain to a network of lymphatics that lie just superficial to the muscularis mucosae. Small intestine. The central lacteal of the intestinal villus is a well-described structure running axially down to anastomose with a plexus of submucosal lymphatic vessels. Although this initial lymphatic has a wall consisting of a single endothelial cell layer, some smooth muscle cells arranged coaxially to this vessel have a loose association with it and may have some functional importance in promoting fluid movement into and along the lymphatic channel (35,36). A problem that applies generally to studies of the stomach, small intestine, and colon is the fact that there are layers in the bowel wall, each of which presumably makes a contribution to the total lymph flow from the organ. Most attention is naturally focused on the mucosa in view of its high capillary density and the proximity of the enterocyte layer concerned with transport of large amounts of water and solute. Nevertheless, contributions are presumably arising from submucosal and muscle layers. Presently, there are no techniques available to study the extent of these contributions, but it is probable that submucosal contributions are considerable in pathologic processes such as Crohn’s disease since submucosal edema is a prominent feature in this condition. The compliance of the submucosal interstitium, which would be important in this respect, is unknown but may be relatively high. Colon. While small intestinal initial lymph

GASTROENTEROLOGY

Vol. 84, No. 4

vessels in the mucosa are readily identified, colonic mucosal vessels are rather inconspicuous. It has been claimed that there are no lymphatics in the colonic mucosa (33, but recent studies demonstrate that a network of horizontally arranged lymphatics is located in the deeper one-third of the mucosa (8,381. These vessels are of considerably greater diameter than the blood capillaries. Salivary glands. Relatively little information is available on the disposition of lymphatic vessels in the salivary glands. In several species, however, valved vessels have been identified in the capsule and parenchyma. The latter follow the blood vessels and duct system (39). In the dog, initial lymph vessels are described as lying very closely apposed to the base of the acinar epithelium (40). Similarly, in the human parotid, initial lymphatics appear to arise close to the secretory endpieces (al), but in the rabbit, these vessels lie close to the finest ramifications of the duct system rather than the secretory endpiece (42). Pancreas. As in the salivary glands, fine lymphatic vessels have been described lying in close proximity to the basement membrane of acinar cells (431, although some injection studies have failed to define these vessels and suggest that lymph vessels only reach the interlobular areas (44,45). It is generally agreed that no lymphatic vessels penetrate the islets of Langerhans. Collecting lymph vessels accompany the blood vessels (46).

Physiology of Capillary Solute Exchange

Fluid and

It is well recognized that large amounts of fluid and solutes are filtered across the microvasculature of the alimentary tract under normal physiologic conditions. In this section, we summarize the available information regarding transcapillary fluid and solute exchanges in the alimentary tract and describe the factors and characteristics that allow these capillaries to exchange large amounts of fluid and solute under resting conditions and during periods of net transepithelial secretion or absorption. Capillary

Fluid Exchange

Since the hydrostatic and oncotic pressure gradients across the capillary wall, as well as the permeability and hydraulic conductivity of capillaries, determine the rate and direction of fluid movement across the capillary wall, capillary fluid exchange will be discussed relative to the Starling hypothesis (47), i.e.,

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Table

I, Factors

Governing

Transcapillary

Colon” Lymph

Fluid

Exchange Pancreas”

in the Alimentary Salivary glands

OF THE ALIMENTARY

TRACT

851

Tract SIlklll intrstlnr”

Stomach

tlox\

(ml/min

100 g)

0.015

0.009

0.14

0.045

0.06”

0.204

0.293

0.300

0.168

0.058

Capillar!; filtration coefficient (mhmln

mm&.

Capillary hydrostatic: pressure (mmHg) [nterstitial fluid pressure (mmHg) Osmotic reflection coefficient Transcapillary onrotic pressure gradient (mm&l

100 g)

3.0

10.0

-6.6” 0.85

12.8

References: Colon (48.49); pancreas (50,51); salivary pressure of o mmHg. all other values were obtained

0.85

11.3

15.9

111.2 2.1”

0.53

0.92

0.78

IX0

glands (52-54): small intestine (55.56); stomach (57-60). ” \‘alues obtained ” Calculated from the measured parameters. at normal venous pressures.

where Jv,c., is the net rate of capillary filtration (or absorption), Kf,c is the capillary filtration coefficient, P,, is the capillary hydrostatic pressure, Pt is the interstitial fluid pressure, ad is the osmotic reflection coefficient, rrj is the plasma oncotic pressure, and r, is the interstitial oncotic pressure. Steadystate values for each of the parameters in the Starling equation are presented in Table 1 for organs in the alimentary tract under resting conditions. Net capillary filtration rate (lymph j7ow). It is generally assumed that the rate of lymph flow from a tissue provides an estimate of net capillary filtration rate under isovolumetric or isogravimetric conditions (when the tissue is neither gaining nor losing volume or weight). Steady-state values for lymph flow have been obtained for most organs of the alimentary tract (Table 1). The lymph-flow data suggest that there are significant differences in net capillary filtration rate between organs of the alimentary tract, with salivary glands and pancreas exhibiting the highest and lowest rates, respectively. Some of the differences in lymph flow may not reflect differences in capillary filtration rate if significant transepithelial fluid secretion or absorption occurred during the period of lymph-flow measurement. In secreting organs (salivary glands, stomach, pancreas), the capillary filtration rate could be significantly underestimated by lymph flow since a proportion of the capillary filtrate would be removed from the interstitial spaces via the transporting epithelia rather than the lymphatics. During net transepithelial fluid absorption (small and large intestines), lymph flow will not reflect the capillary filtration rate since the absorbed fluid is also removed from the interstitium via the lymphatics. Because most of the lymph flow data in Table 1 were obtained under conditions favoring minimal trans-

10.6”

11.5 at portal

epithelial fluid transport, the values probably represent reasonable estimates of capillary filtration rate. Lymph flow from the alimentary tract is altered by a wide variety of conditions that influence transcapillary fluid exchange. Acute elevation of venous pressure has been shown to increase lymph flow from the stomach (59), small intestine (61,62), and colon (49). Increases in lymph flow approaching 30 times control have been reported for the small intestine with venous pressure elevation (30 mmHg) (63). Significant increases in lymph flow have also been observed in the small intestine during plasma dilution. Plasma dilution generally produces larger elevations of lymph flow than venous hypertension in the small intestine (63,64). Several vasodilator drugs and hormones have been shown to increase lymph flow in the small intestine. These include histamine (65), bradykinin (661, isoproterenol [SS), glucagon (67), cholecystokinin (68,69), secretin (68,70), prostaglandin El (71), and diuretics (72). Lymph flow from the small intestine increases during net fluid absorption (33,36) and decreases during net fluid secretion (63); however, net transmucosal fluid movement does not alter colonic lymph flow (8). Intraenteric distention enhances intestinal lymph flow in a manner similar to venous pressure elevation (73). Conditions that decrease intestinal lymph flow include arterial hypotension (74), local intraarterial infusion of hypertonic glucose (75), vasopressin (76), theophylline, and vasoactive intestinal peptide (VIP) (77). According to the classical view of lymph formation, the interstitial-to-initial lymphatic hydrostatic pressure gradient serves as the major driving force for lymphatic filling and thus is the primary determinant of lymph flow (78). While there is direct evidence to support this concept in studies on collect-

852

GRANGER

AND

-2

BARROWMAN

0 lnterstltial

Figure

2

intestine and 108.

2 fluid

pressure

4 (mm

6 Hg)

lymph flow, interstitial fluid fluid pressure in the small of the cat. Based on data from references 62

Relationships

volume

GASTROENTEROLOGY

between

and interstitial

ing lymph vessels of rat and guinea pig mesentery (79,80), no direct evidence exists for whole organs in the alimentary tract. However, there is rather convincing evidence that small intestine lymph flow is related to the steady-state interstitial fluid pressure (Figure 2). The relationship between lymph flow and tissue fluid pressure in the small intestine is nonlinear, presumably due to the interstitial compliance characteristics of the intestine. Capillary filtration coefficient. Capillary filtration coefficients (Kf,,) are a measure of the hydraulic conductance of a capillary bed and are influenced by the size and number of pores in each capillary as well as the number of perfused capillaries (81,82). Capillary filtration coefficients have been estimated for most organs in the alimentary tract using volumetric or gravimetric techniques. These techniques require a sudden elevation of venous pressure in an isovolumetric or isogravimetric organ. The volume change after venous pressure elevation is characterized by two distinct components: an initial rapid increase that is generally attributable to venous distention, and a slower, more prolonged phase of volume change that is considered to represent capillary filtration. The slope of the slow [filtration] component of the volume change is divided by the

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84, No. 4

increment in capillary pressure to yield an estimate of Kf,c. This technique suffers from two fundamental problems: determining the end-point of the passive increase in vascular volume and assessing capillary filtration rate (slow component) before readjustment of tissue forces after venous pressure elevation (81). Recent modifications of the volumetric technique help minimize these problems in the intestine (83). A new method for estimating Kf,, in the intestine was recently described that eliminates the need for weighing the organ or placing it in a plethysmograph (84). The technique should prove to be useful since it minimizes the problems of exteriorization, denervation, and tissue handling usually associated with the conventional approaches. The data in the literature suggest that control Kf,c values vary greatly between the splanchnic organs [Table 1). The stomach, an organ with a relatively large muscle mass, has the lowest filtration coefficient, while the salivary gland exhibits the highest filtration coefficient. The difference in Kf,(: values between the small intestine, stomach, and colon may represent different resting capillary porosities or densities, or may be due to the different techniques used to estimate Kf,, (82). Caution should be exercised when attempting to interpret differences in mean Kf,c between organs since this parameter has been reported to vary by approximately an order of ml/min * mmHg . 100 g) in a magnitude (0.03-0.56 single organ, e.g., the intestine (81,82). This variability, however, is more readily explained by physio-

Table

2. Effects of Physiologic, Pathologic, and Pharmacologic Conditions on the Capillary Filtration Coefficient (K,,L) in the Small Intestine”

Conditions or agents that Glucose absorption Arterial hypotension Hyperthermia Denervation Hemorrhagic shock Nitroglycerin Isoproterenol Phentolamine Neostigmine Bradykinin Sodium nitroprusside Sodium nitrite

increase

Conditions or agents that decrease Sympathetic nerve stimulation Lumenal distention Portal hypertension Acute arterial hypertension Hypothermia Serotonin ” References

81, 85-89.

Kt,, Histamine Secretin Aminophylline Glucagon Acetylcholine Serotonin Cholecystokinin Prostaglandin Propranolol Epinephrine Cholera toxin

E,

Kf,, Pentagastrin Adenosine Norepinephrine Phenylephrine Angiotension Ergotamine

II

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MICROCIRCULATION

logic factors rather than technical difficulties with the techniques used to estimate Kf,c. Table 2 summarizes the effects of various physiologic, pathologic, and pharmacologic interventions on Kf,c in the intestines. [Although control Kf,c values are available for other splanchnic organs, there is very little information regarding the response of Kf,, in these organs to various perturbations. Of the perturbations studied, there appears to be no qualitative difference in the Kf,, response between these organs and the small intestine.) In general, the data suggest that conditions or agents that produce vasodilation in the intestines increase Kf,c, while a reduction in Kf,c is generally associated with vasoconstriction. Capillary filtration coefficients can also change without any alteration in blood flow if small doses of drugs are administered (81). It is well recognized that Kf,, is inversely related to capillary hydrostatic pressure in the small intestine (Figure 3). This inverse correlation is thought to result from myogenic control of “precapillary sphincters,” which regulate perfused capillary density. The myogenic theory proposes the existence of tension receptors that modulate precapillary sphincter muscle tone in response to changes in transmural pressure (90,91). As expected from such a system, acute elevation of either arterial or venous pressure in the denervated small intestine leads to capillary derecruitment (reduction in Kf,J, while a reduction in either pressure leads to capillary recruitment (62,74,92). Changes in venous pressure exert greater

o Arterial

hypotenslon

Venous

hypertension

??

“00 0

5

10

Capillary Figure

3

15 pressure

20 (mm

25

30

Hg)

Relationship between capillary filtration coefficient and capillary pressure in the small intestine of the cat. Capillary pressure was altered by venous pressure elevation (62) or arterial pressure reduction (74). The inverse correlation is believed to result from myogenic control of perfused capillary density.

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effects on Kf,, than comparable alterations in arterial pressure because a smaller proportion of the pres.sure change is transmitted to the precapillary sphincters in the latter instance. Although redur . tions in either arterial or venous pressure cause capillary pressure to fall, the blood flow responses trr the two perturbations are different-arterial pressure reduction decreases flow while reducing venollr pressure increases flow. Because of the reduction in blood flow, a metabolic control mechanism could also account for the rise in Kf,c that accompanies a decrement in intestinal arterial pressure (88). According to the metabolic theory, a reduction in blood flow leads to an accumulation of metabolites and a reduction of oxygen tension in the tissue (93). The metabolites or reduced tissue PO,, or both, then cause a relaxation of precapillary sphincter smooth muscle and a rise in Kf,<:.The Kf,, increases when arterial pressure is reduced or venous pressure is increased in the pancreas (49) and colon (481, suggesting that metabolic factors are more important (than myogenic) in regulating precapillary sphincter tone in these tissues. Neither metabolic nor myogenic control mechanisms can explain the direct correlation between Kf,c and blood flow produced by most vasodilators and vasoconstrictors. Isoproterenol, a potent intestinal vasodilator, produces a significant linear relation, ship between Kf,, and blood flow (94). This direct correlation is considered to result from progressive relaxation of both resistance and precapillary sphincter smooth muscles with increasing doses of isoproterenol. These observations, coupled to the responses produced by other vasodilators and vasoconstrictors, indicate that vascular elements controlling perfused capillary density (precapillary sphincters) possess specific receptors for a wide variety of humoral substances and drugs. Although most of the Kf,,: changes presented in Table 2 have been attributed to alterations in the tone of precapillary sphincters (and the number of perfused capillaries), there is probably a component of the increase in Kf,c that is due to increased microvessel permeability. There is evidence for the involvement of an increased capillary permeability in the Kf,c changes produced by glucose absorption (64), hemorrhagic shock (95), bradykinin (661, histamine (65), and glucagon (67). Even though small changes in capillary pore size should markedly influence Kf,, (since flow through cylindrical channels is proportional to the fourth power of the radius), intestinal Kf,, changes >50% are rarely observed with agents (e.g., histamine, bradykinin) or conditions (e.g., fat absorption) that dramatically alter the permeability of intestinal capillaries to macromolecules. The latter observation may be explained by

854

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AND BARROWMAN

GASTROENTEROLOGY

the fact that most of the hydraulic conductance across intestinal capillaries can be attributed to the “small pores” (47 A radius), and yet agents that increase capillary permeability to macromolecules do so by affecting only the relatively few “large pores” (-250 k!radius) (96). The hydraulic conductivity of single capillaries in rat intestinal muscle has been measured using microocclusion techniques (97,98). These data suggest a remarkably high axial gradient in capillary filtration coefficient, i.e., Kf,, increases by a factor of 7 between the arterial and venous ends of the capillary. This technique has not yet been applied to the mucosal capillaries of the small intestine or to those of other splanchnic organs. Capillary pre&ure. Capillary pressure has been measured in the small intestine (62,99,100), pancreas (50), and salivary glands (54) using the gravimetric or volumetric technique, and has been estimated in stomach (51)from the balance of forces in the Starling equation (from the sum of the other forces and lymph flow). The available data suggest that the resting values for capillary pressure in the small intestine and stomach (-16 mmHg) are substantially greater than that reported for the pancreas (12 mmHg), yet lower than that of skeletal muscle (20 mmHg) (101). The different values for capillary pressure in these organs presumably reflect variations in the resting precapillary (R,)-to-postcapillary (R,) resistance ratio. The R, to R, ratios in the small intestine and stomach (15:1) and pancreas (29: 1) greatly exceed that reported for skeletal muscle (5: 1). The lower capillary pressure in splanchnic

Figure

4

Vol. 84, No. 4

organs (compared to skeletal muscle) may be of homeostatic significance since it prevents excessive capillary filtration in tissues with a high capillary exchange capacity. The very high R, to R, ratios reported for the pancreas are comparable to that predicted for the liver and may be explained by the existence of portal circulations between islets and acini, and the acini and ducts (4). Capillary pressure in splanchnic organs is also influenced to a large extent by arterial and venous pressures. When arterial pressure is increased in the of the incremental small intestine, only 5%10% change is transmitted to the capillaries. In the same organ, venous pressure elevation has a more profound effect with 60%70% of the incremental change transmitted to the capillaries (62,99). By contrast, 90% and 100% of an increment in venous pressure is transmitted back to the hepatic sinusoids (102) and pancreatic (50) capillaries, respectively. The rise in capillary pressure produced by venous pressure elevation in the small intestine is dampened by a rise in precapillary resistance and a fall in postcapillary resistance (99). The increased R, to R, ratio caused by venous hypertension reduces the change in capillary pressure by 6.5 mmHg for an increase in venous pressure from 0 to 30 mmHg (62). The distribution of pressures in the microcirculation of the small intestine has been measured in the rat (Figure 4). The results of micropuncture studies clearly indicate that capillary pressures in the mucosal villi are significantly lower than capillary pressures in the intestinal muscle layers (103). The very low resistance in mucosal venules is considered the

Distribution of pressures in the microcirculation of the rat small intestine. Modified from reference 103.

Venules

Artertoles 0 60

I

I

I

I

I

50

40

30

20

10

3

I

I

I

I

I

I

10

20

30

40

50

60

Internal diameter (microns)

April

1983

primary determinant of the low mucosal capillary pressure. Capillary pressures and relative flows in the various layers of the intestine have been used to calculate a weighted average capillary pressure of (104), 16.8 mmHg for the whole small intestine which compares favorably with results from isogravimetric studies. Because of the tendency for R, to decrease and R, to increase when arterial pressure is reduced, it has been suggested that capillary pressure is “autoregulated” in the small intestine (99). Such a mechanism is homeostatically pleasing since it would protect the small intestine, with its high capillary exchange capacity, against drastic loss or accumulation of interstitial fluid in the event of an accidental reduction or elevation in arterial pressure. Whole organ and microcirculatory estimates of capillary pressure over a wide range of arterial pressures, however, suggest that this parameter is poorly autoregulated (74,103). Poor autoregulation of capillary pressure is supported by the observation that intestinal lymph flow (capillary filtration rate) is not autoregulated in the small intestine (74). Humoral and pharmacologic agents have been shown to alter capillary pressure in the small intestine. Vasodilators, such as glucagon (671,adenosine (105),bradykinin (55), and nitroglycerin (87), increase capillary pressure while vasopressin (761,a vasoconstrictor, decreases capillary pressure. Indirect estimates Interstitial fluid pressure. of interstitial fluid pressure (P,) have been obtained for the small intestine (106)and stomach (57)using the capsule technique. Values for Pt in the small intestine have also been determined using micropuncture techniques (107)and calculated using the balance of Starling forces (62) assuming Pt = P, - o;l (5 - ~~1 - JL/Kfsc, where JL equals the steady-state lymph flow. The calculated values for Pt compare favorably with those obtained by the capsule technique (108).At a normal arterial pressure and portal pressures of O-5 mmHg, interstitial fluid pressure in the small intestine (calculated from the balance of filtration forces or measured with capsules) ranges from -3.0 to 0 mmHg (55,62,105).When portal pressure exceeds 5 mmHg, intestinal interstitial fluid pressure is consistently positive. One of the limitations of the capsule technique is that the size of the fluid pressure-measuring device prevents the measurement of P, in a single layer of small intestine. Micropuncture techniques yield a value of 0.50-1.0 mmHg for Pt in the mucosa of the rat small intestine (107). This value increases during intestinal absorption (107) and decreases during cholera toxin-induced intestinal secretion (109). Interstitial fluid pressure, calculated from the balance of Starling

MICROCIRCULATION

OF THE ALIMENTARY

TRACT

855

forces, increases in the small intestine in response to intraarterial infusion of glucagon (67)or bradykinin (55), and decreases during local arterial hypotension (74). A relationship between interstitial fluid pressure and interstitial volume has been reported for the small intestine (Figure 2). As observed in other tissues, there are two distinct components to the interstitial compliance curve in the small bowel (62,106): a low-compliance portion (0.4 ml/mmHg * 100 g) at interstitial fluid pressures between -2 and ~15 mmHg) and a + 3 mmHg (venous pressures high-compliance portion (4.0 ml/mmHg * 100 g) at higher interstitial fluid pressures (venous pressure >l5 mmHg). Thus, at a normal tissue hydration, small changes in interstitial volume cause large increases in Pt, yet when the tissue becomes edematous, considerable interstitial fluid can accumulate without altering P,. The abrupt change in interstitial compliance at large interstitial volumes implies a structural alteration in the interstitium. There is indirect evidence that suggests that the intestinal interstitium is more compliant when fluid is absorbed via the mucosal membrane than when capillary filtration is enhanced by venous pressure elevation (108). Such a difference in interstitial compliance could be explained by a compressive effect of vascular distention on the interstitium subsequent to venous pressure elevation. Interstitial fluid pressure of the submucosa of the gastric fundus has been measured using the capsule technique (57,110). The resting Pt in this organ is similar to that reported for the small intestine at normal portal pressure. Gastric Pt is not modified by placing hypertonic solutions in the stomach in spite of a large increase in net fluid flow into the lumen. Intraarterial infusion of hypertonic mannitol, however, significantly reduces P,. Gastric secretion produced by increased arterial pressure and intraarterial infusion of histamine is associated with a rise in P,. Acetylcholine infusion also increases gastric P,; however, this effect appears to be related to acetylcholine’s ability to contract the muscularis mucosae. Increasing venous pressure produces a rapid rise in Pt, which parallels the rise in venous pressure, suggesting that vascular distention rather than enhanced capillary filtration accounts for most of the rise in Pt. Intraarterial infusion of norepinephrine or atropine decreases gastric Pt. Osmotic reflection coeficient. Since capillaries of the alimentary tract are permeable to plasma proteins, only part of the oncotic pressure generated by plasma proteins is exerted across the capillary wall. The osmotic reflection coefficient (~~1) describes the fraction of the total oncotic pressure

856

GRANGER AND BARROWMAN

GASTROENTEROLOGY Vol. 84, No. 4

generated across a capillary membrane (impermeant proteins generate 100% of their maximum oncotic pressure and cd = 1, while freely permeable proteins do not generate an effective oncotic pressure and Ud = 0). Most organs of the alimentary tract are supplied with two types of capillaries, i.e., fenestrated and continuous, with the possible exception of salivary gland and pancreas, which appear to be purely fenestrated. Based on ultrastructural estimates of pore dimensions in fenestrated capillaries and the normally high lymph-to-plasma protein concentration ratio (L/P), one would predict a low V,j for intestinal, gastric, pancreatic, and colonic capillaries (a ad of -0.1 for albumin is predicted from fenestral pore sizes and hydrodynamic theory). Using the same criteria, one would predict that ad = 0 for all plasma proteins across the liver sinusoids. Estimates of the osmotic reflection coefficient of splanchnic capillaries to plasma proteins have been recently obtained using lymph protein data (111). The technique is based on the assumption that at high capillary filtration rates the diffusive exchange of macromolecules across a capillary wall becomes infinitesimally small, at which point the lymph-toplasma solute concentration ratio describes the separative capacity of the capillary wall and provides an accurate approximation of Ud (i.e., Vd = 1 - L/P). Using this approach, a Ud value of 0.92 is derived for capillaries in the small intestine of cat (111) and rat (112). When the same analysis is applied to lymph protein flux data derived from cirrhotic patients

Table

3. Effects of Physiologic and Pharmacologic Interventions on the Osmotic Reflection Coefficient of Intestinal Capillaries to Total Plasma Proteins Experimental

condition

Controls Isoproterenol Bradykinin Secretin Cholecystokinin Fat absorption Glucagon EDTA E. coli endotoxin Arterial hyperglycemia Ischemia Ischemia + superoxide

(20 mM)

dismutase

Histamine Cimetidine + histamine Benadryl + histamine Compound 48/80 Goldblatt hypertension Angiotension II u Value

derived

from dog, all other

Reflection coefficient

Reference(s)

0.92 0.92 0.65 0.91 0.89 0.70 0.81 0.73 0.78 0.64 0.59 0.86 0.56 0.90 0.56 0.76 0.55” 0.91”

111 66 66 68 68 68 67 96 115 116 95 95 65 65 65 117 118 119

values

from cat.

(113), a value for Ud > 0.95 is predicted for the human small bowel. The same technique yields values that differ considerably between splanchnic organs, suggesting differences in capillary permeability (Table 1). The cd data for total plasma proteins indicate that 85% 78%, 92%, and 85% of the total oncotic pressure gradient is transmitted across capillaries in the pancreas (51), stomach (591, small intestine (ill), and colon (49), respectively. Although the reflection coefficient of salivary gland capillaries has not been measured, ultrastructural studies suggest that these capillaries are less permeable to macromolecules than capillaries in the intestine (114). Many physiologic and pharmacologic interventions have been shown to alter the fld of intestinal capillaries to total plasma proteins. Table 3 summarizes the available information regarding the effects of various agents and experimental conditions on the reflection coefficient of intestinal capillaries. While the permeability response of intestinal capillaries to various stimuli (e.g., bradykinin, endotoxin) are in general accord with that observed in other tissues, some of the responses may be unique to the alimentary tract and deserve additional comment. Of particular significance is the observation that Ud is reduced by a normal physiologic process, i.e., fat absorption (68). This response appears to be readily reversible and cannot be reproduced by intraarterial infusion of postprandial levels of secretin or cholecystokinin. The effect of absorption of other nutrients (e.g., glucose) on Ud has not been studied and warrents attention. It is conceivable that glucose absorption may reduce Ud since increases in blood glucose by as little as 20 mM (caused by local intraarterial infusion of hyperosmotic glucose) significantly reduced U,j in the nonabsorbing small bowel (116). The responses of intestinal capillaries to histamine also differ from those observed in peripheral organs (65). The reduction in Ud caused by local intraarterial infusion of histamine is unaffected by benadryl pretreatment, yet it is completely prevented by cimetidine pretreatment, suggesting that the effects of histamine on intestinal capillary permeability are mediated through Hz receptors. In the stomach, both HI and Hz antagonists are effective in preventing the reduction in Ud produced by intraarterial histamine infusion (239). Transcapillary oncotic pressure gradient. If one assumes that lymph provides a valid reflection of interstitial fluid, the transcapillary oncotic pressure can be estimated from lymph and plasma using either an oncometer or equations that relate protein concentration to oncotic pressure (see Colloid Edema section). The data in the literature suggest that

April

MICROCIRCULATION

1983

the resting transcapillary oncotic pressure gradient is similar for stomach, small intestine, pancreas, and colon [Table l), while the salivary gland exhibits a slightly larger gradient. In tissues where cd >O, an increase in capillary filtration rate should increase the transcapillary oncotic pressure gradient, the magnitude of the increase being dependent on capillary surface area, cd, lymph flow, interstitial compliance, and the degree of solute exclusion by the interstitial matrix. In all splanchnic organs, except the liver (120), the transcapillary oncotic pressure gradient increases in response to an increase in capillary pressure if capillary permeability is unaltered. Although there is some controversy regarding the magnitude of the increase in 7~~- ni in the small intestine for a given increment in capillary pressure (61,62,100), this organ appears to display a greater propensity for interstitial protein dilution than the other splanchnic organs. Several substances and experimental conditions have been shown to alter the transcapillary oncotic pressure gradient in the small bowel. Those which increase the gradient include acute venous hypertension (62), plasma volume expansion (112), and net transmucosal fluid absorption (12 1). The transcapillary oncotic pressure gradient is reduced by acute arterial hypotension (741, vasopressin (76), and net transmucosal fluid secretion (122), all of which produce a concomitant reduction in lymph flow, and bradykinin (55), histamine (65), and glucagon (67), which increase lymph flow. Some substances, e.g., isoproterenol(66), cholecystokinin (68), and secretin (68), increase lymph flow, yet do not influence the

Interstitial fluid pressure

TRACT

oncotic pressure gradient. Such a response ally attributed to recruitment of previously fused (or nonfiltering) capillaries. Interactions Forces

of Capillary

857

is genernonper-

and Interstitial

Enhanced capillary filtration-edema safety factors. If capillary pressure is increased or the plasma oncotic pressure is decreased, fluid moves from the vascular system into the interstitium. The fluid accumulation within the interstitium causes interstitial fluid pressure and lymph flow to increase and tissue oncotic pressure to decrease; these changes oppose further filtration out of the capillaries and a new steady-state condition is achieved with a slightly more hydrated interstitium. Therefore, for small imbalances of the Starling forces, the capillary and tissue forces are able to resist edema formation. The resistivity to edema formation resulting from readjustment of interstitial forces and lymph flow has been referred to as the “edema safety factor” and the magnitude of the compensation by each can be estimated (101). Figure 5 compares the safety factors against edema in cat small intestine (62) and dog colon (49) for an increment in capillary pressure of 12-13.2 mmHg. The data indicate that the relative contribution of each “safety factor” to the prevention of edema differs between these organs. In both tissues, the increased oncotic pressure gradient and interstitial fluid pressure are the major safety factors while lymph flow plays a more minor role, particularly in the colon. There is some contro-

Figure

Transcapillary oncotic pressure gradient

OF THE ALIMENTARY

5. The safety factors against interstitial edema in the cat small intestine (62) and dog colon (49) for an increment in capillary pressure of 12.0-13.2 mmHg. The magnitude of the changes in interstitial forces and lymph flow during interstitial dehydration resulting from a 6.5 mmHg decrement in capillary pressure (74) is also depicted.

858

GRANGER

AND BARROWMAN

versy regarding the role of interstitial fluid pressure (P,) in the prevention of edema in the small bowel. In contrast to the data presented in Figure 5, some investigators contend that P, is not altered by enhanced capillary filtration and the increased transcapillary oncotic pressure gradient accounts for virtually all of the safety factor against edema in the small bowel (100).Such results would not be expected if the bowel were normally hydrated and it displayed an interstitial compliance curve comparable to that reported for other tissues (101).Thus, differences in tissue hydration between preparations could account for the discrepancies regarding the relative roles of interstitial fluid pressure in preventing interstitial edema in the small bowel. The relative roles of lymph flow, interstitial fluid pressure, and the transcapillary oncotic pressure gradient in preventing interstitial edema have not been systematically analyzed in salivary gland, stomach, and pancreas. Lymph flow and lymph protein data obtained at several venous pressures in stomach (59) and pancreas (Kvietys PR, personal communication) indicate, however, that an increased transcapillary oncotic pressure gradient is likely to be a major safety factor against edema in these tissues. The low resting lymph flows in stomach and pancreas (Table 1) indicate that the lymphatic safety factor is likely to be of minor importance in these tissues. In most tissues, the individual safety factors can prevent interstitial edema until a given increment in capillary pressure or decrement in plasma oncotic pressure is imposed, at which time large quantities of fluid enter the interstitium and edema ensues. The increment in capillary pressure or decrement in plasma oncotic pressure required to produce edema is referred to as the “total safety factor” against edema (101).In the small intestine, the total safety factor ranges between 12 and 15 mmHg (62,123). Increments in intestinal capillary pressure in excess of 15 mmHg lead to unrestrained interstitial edema and ultimately to an exudation of interstitial fluid into the lumen. The total safety factor against edema has not been determined for other splanchnic organs. If one associates interstitial edema with ascitic fluid formation by the liver, then the total safety factor would be only l-2 mmHg. It has been suggested, however, that ascitic fluid formation by the liver serves as an additional safety factor against interstitial edema (102),thereby limiting liver swelling to a level that does not compromise the metabolic functions of this organ. If the same rationale is applied to the gastrointestinal tract, then one might consider exudation of interstitial fluid into the bowel lumen as an additional safety factor against edema.

GASTROENTEROLOGY

Vol. 84, No. 4

Reduced capillary filtration-safety factors against interstitial dehydration. If capillary pressure is reduced or plasma oncotic pressure increased, interstitial fluid volume decreases. The reduction in interstitial volume causes interstitial fluid pressure to fall and interstitial oncotic pressure to rise. Since interstitial fluid pressure is the primary driving force for lymphatic filling, lymph flow also falls. These changes in interstitial forces and lymph flow tend to promote capillary filtration and, therefore, serve to prevent excess dehydration of the interstitium. The resistance to interstitial dehydration resulting from readjustment of interstitial forces and lymph flow has been studied in the small bowel (74). Figure 5 illustrates the magnitude of the changes in interstitial forces and lymph flow in the small intestine for a 6.5 mmHg decrement in capillary pressure caused by reducing superior mesenteric arterial pressure from 125 to 50 mmHg. The results of this study indicate that reductions in interstitial fluid pressure account for 55%60% of the total buffering capacity against dehydration, while an increased interstitial oncotic pressure (30%-35%) and decreased lymph flow (9%13%) play a more minor role. The dramatic changes in interstitial fluid pressure during interstitial dehydration presumably reflect the normally low compliance of the intestinal interstitium, i.e., a 10% reduction in interstitial volume should produce a 4.5 mmHg decline in interstitial pressure (see Figure 2). Net transepithelial fluid transport. The maintenance of a normal interstitial fluid volume in most tissues results from the near equality of net transcapillary and lymphatic volume flows. In transporting tissues, interstitial fluid volume is also influenced by the rate and direction of solute-coupled fluid movement across the epithelia, i.e., absorption and secretion. Large amounts of digestive juices are secreted each day by the salivary glands, stomach, pancreas, liver, and small intestine. Over the same period of time, the small and large bowel must reabsorb the secreted juices plus an additional volume of ingested water. Although much emphasis has been placed on the importance of the epithelium in the transport of fluid in digestive organs, the roles that the microcirculation and Starling forces play in transporting fluid to and from the epithelium have received relatively little attention. This is true in spite of the fact that the water for secretion of digestive juices is derived from the vasculature and that capillaries are the principal conduits for removal of absorbed water from the mucosal interstitium of the small and large bowel. In the following sections, the available information regarding the role of the blood and lymph microcirculations and Starling

April

MICROCIRCIJLATION

1983

NET

A decreased interstltlal

A

L

release

volume

increased

interstitial

oncotic

859

of

humoral

agents

J

L

precapillary sphincter

reststance

pressure

TRACT

SECRETION

/\ fluid

ALIMENTARY

TRANSMUCOSAL

FLUID

reduced arterlolar

decreased

OF THE

dilatation

h

tissue

increased

pressure

vascular

permeability

increased capillary

capillary

pressure

?i\\

recruitment

increased

formation

capillary capacity

VASCULAR

FLUID

FILTRATION

NET

Increased Interstitial

increased fluid

>“‘lD

ABsoRPT’oN~

oncotic

of agents

tissue pressure

vascular

capillary increased

release humoral

volume

decreased

interstitial pressure

TRANSMUCOSAL

permeablltty

pressure

lymph exchange

.

A

VASCULAR

capacity

FLUID

ABSORPTION Figure

6. Changes in the Starling forces and capillary supply fluid for epithelial transport during

membrane secretion

parameters which allow the microcirculation (A] and to remove fluid from the interstitium

forces in the secretory and absorptive functions of the digestive organs are summarized. SECRETION. Figure 6A describes the changes in Starling forces and capillary membrane parameters that allow the microcirculation of digestive organs to supply fluid for epithelial transport during secretion. Although most of the events described in the figure are likely to occur in all digestive organs,

of the alimentary during absorption

tract to (B).

the relative contributions of the various forces and membrane parameters to fluid movement appear to differ from one digestive organ to another. In the salivary gland there is evidence that an increased capillary hydrostatic pressure is the primary factor enhancing capillary filtration to supply the fluid necessary for saliva formation (52). During maximal salivary secretion, blood flow increases by as much

860 GRANGER AND BARROWMAN

as 15 times the resting value (125).Assuming the reduction in vascular resistance occurs exclusively at the arteriolar level, capillary pressure should increase to -45 mmHg during maximal salivary secretion. The arteriolar dilatation associated with salivary secretion is believed to be accompanied by dilatation of precapillary sphincters since the capillary filtration coefficient (I&) increases threefold to sevenfold during maximal vasodilation (52). Inasmuch as bradykinin is considered to mediate the functional hyperemia of the salivary gland (125), part of the rise in Kr,, may result from an increase in vascular permeability rather than capillary recruitment. Nonetheless, the available information on salivary glands strongly suggests that a rise in both capillary pressure and capillary exchange capacity (Kf,,) enhance capillary filtration to the level required to supply the fluid necessary for salivary secretion. An increased capillary hydrostatic pressure may also play a role in providing the fluid for secretion in the stomach and pancreas since blood flow to these organs increases during digestion (85). Some secretagogues (e.g., secretin, pentagastrin), however, can stimulate active secretions in the pancreas and stomach to near maximum without changing blood flow (125). Thus, an increase in capillary pressure is not necessary to drive the fluid for secretion in the pancreas and stomach. During pentagastrin-stimulated acid secretion there is a 5O%-100% increase in Kr,,. Indirect evidence suggests that Kr,c may increase during secretin-stimulated pancreatic secretion (51). Although an increase in capillary exchange capacity would facilitate the transfer of fluid between the capillaries and epithelia, a rise in Kf,= alone could not account for the increased capillary filtration rate required for postprandial rates of gastric and pancreatic secretion (126). In the absence of changes in capillary pressure, the increment in net filtration pressure, [(PC - P,) - ad (rP - rt)], must result from an alteration in the interstitial forces, i.e., an increase in interstitial oncotic pressure or a reduction in interstitial fluid pressure, or both. Theoretically, as protein-free fluid is transported out of the interstitium via active secretion, interstitial fluid volume will decrease, thereby causing tissue oncotic pressure to increase and interstitial hydrostatic pressure to decrease. Assuming the interstitial volumes and compliances of the pancreas and stomach are the same as that reported for the small bowel, a 5% reduction in interstitial volume due to secretion would produce a 2.25 mmHg reduction in interstitial fluid pressure and a 0.75 mmHg increase in interstitial oncotic pressure. These changes in interstitial forces should enhance capillary filtration rate and

GASTROENTEROLOGY Vol.84, No. 4

depress lymph flow thereby providing the fluid for secretion of pancreatic and gastric juices. Support for this view is provided by the observation that secretagogues such as acetylcholine, histamine, and secretin depress gastric lymph flow (127). Although the small intestine is generally considered to be an absorbing organ, excess stimulation of active secretory processes can lead to net fluid movement into the bowel lumen. Virtually all the Starling forces are altered in a manner consistent with enhanced capillary filtration during cholera toxin-induced secretion in the small intestine. Villus lymph (lacteal) pressure and total intestinal lymph flow decrease after exposure of the mucosa to cholera toxin (63,109)and other active secretagogues (777, suggesting that interstitial fluid pressure is reduced due to interstitial dehydration. There is also evidence that the capillary exchange capacity Kf,, increases during cholera toxin-induced intestinal secretion (128).Some investigators have suggested that capillary pressure is increased during cholera toxin secretion because of the intestinal hyperemia associated with this condition (128,129). ABSORPTION. In the nonabsorbing small bowel, the balance of forces across the capillary wall favors net fluid filtration and the lymphatics prevent an accumulation of the filtrate in the mucosal interstitium. In the absorptive state, however, both capillaries and lymphatics are involved in preventing an accumulation of absorbed fluid in the mucosal interstitium. Capillary and interstitial forces are believed to be primarily responsible for driving absorbed fluid from the interstitium into lymphatics and capillaries. Fluid absorption leads to an expansion of the interstitial spaces of the mucosa (241, which in turn, increases interstitial hydrostatic pressure (130) and reduces interstitial oncotic pressure (121). The change in tissue forces significantly alters the balance of pressures across the capillary wall in favor of net fluid movement from interstitium to blood. An increased capillary exchange capacity, resulting from capillary recruitment (86) and an increased vascular permeability (68), enhances the removal of absorbed fluid by the capillaries. Lymph flow also increases because of the rise in interstitial fluid pressure (36). Since blood flow to the mucosa of the small bowel can increase substantially during absorption (1311,capillary pressure may also increase. An increased capillary pressure would tend to oppose vascular removal of absorbed fluid. Figure 6B summarizes the series of events that allow the blood and lymph microcirculations to remove absorbed fluid from the mucosal interstitium of the small bowel. As a consequence of the reaction initiated during absorption, two significant changes occur: (a)

April

1983

filtering capillaries are converted to absorbing capillaries and (b) the rate of lymph formation is increased. The magnitude of the reduction in tissue oncotic pressure and the increase in mucosal fluid pressure during absorption appear to be related to the rate of fluid absorption. Changes in interstitial fluid pressure, however, appear to play a more important role in driving absorbed fluid into intestinal capillaries at low absorption rates while the reduction in tissue oncotic pressure becomes more important at the higher absorption rates. The differential responsiveness of P, and n, at various absorption rates presumably reflects an increase in interstitial compliance at high absorption rates (33,121). The relative fractions of absorbed fluid removed by the capillaries and lymphatics of the small intestine have been estimated by numerous investigators (132-l3 7). Values for the lymphatic contribution range between 1% and 85% (33). This extremely large variability has been attributed to differences in the tonicity of fluid placed into the lumen, portal vein pressures, lumenal distention pressures, presence or absence of motility, and the use of lymph contaminated by contributions from other tissues, e.g., liver (36,135). If these factors are held constant or eliminated, the rate of fluid absorption becomes a major determinant of the absolute and relative amounts of absorbed fluid removed by the intestinal lymphatics. At absorption rates >0.15 mlimin * 100 g, the relative contributions of each system remain constant at 80%-85% capillary removal and 15%20% lymph removal (121). Although one might expect the same alterations in capillary, interstitial, and lymphatic forces and flows during absorption in the colon, a recent study indicates that the response of the colonic blood and lymphatic microcirculations to absorption differs considerably from that of the small bowel (8). Colonic lymph flow and lymph oncotic pressure are not affected by net transmucosal fluid movement (absorptive or secretory). These findings were explained by the paucity of lymphatic drainage from the colonic mucosa and the close proximity of fenestrated capillaries to the absorptive epithelium. Blood capillaries were considered, therefore, to be the sole conduits for removal of absorbed fluid from the colonic interstitium. FILTKATIONSECKETION. Net movement of fluid and electrolytes into the lumen of the small bowel can result from an alteration in the forces governing mucosal transcapillary fluid exchange (138).The terms “filtration secretion” and “secretory filtration” are used to describe this process. An imbalance in forces across the capillary wall in excess of 12

MICROCIRCULATION OF THE ALIMENTARY TRACT

861

mmHg is required to cause filtration secretion in the small bowel (62,123). Such an imbalance can be induced by acute portal hypertension, increased intraenteric pressure, plasma dilution, lymphatic obstruction, and substances that increase capillary permeability or pressure, or both. Filtration secretion does not occur with imbalances in the capillary forces <12 mmHg (threshold value) for two reasons: a low mucosal hydraulic conductance and a low mucosal interstitial fluid pressure (138). When the net capillary filtration pressure exceeds the threshold value, sustained net capillary filtration occurs. The increased capillary filtration causes mucosal interstitial volume to increase, which in turn causes an increased mucosal fluid pressure (61,623. When mucosal fluid pressure increases by 4-5 mmHg, large channels are opened in the rnucosal membrane at the villus tips (139-141). The width of the intercellular channels between mucosal epithelium, a under normal conditions (142), which is -8-30 increases to an extent sufficient to allow solutes >37 A radius (albumin) to enter the lumen and the hydraulic conductance of the mucosal membrane increases (61). Ultrastructurally, the changes in the mucosal membrane vary from a widening of the mucosal intercellular space during plasma volume expansion (139) to villus tip erosion with prostaglandin El (71) and glucagon (67) infusions and bileoleic acid instillation in the lumen (143). The increased mucosal conductance allows for filtration secretion rates of -1.0 mlimin . 100 g at a portal pressure of 30 mmHg (63). If the mucosal fluid pressure remains at 5 mmHg, the mucosal hydraulic conductance would be -0.20 mUmin. mmHg * 100 g. a value 2000 times greater than that reported for normal mucosa (142). Because of the structural changes in the mucosal membrane, the composition of the secreted fluid closely resembles lymph (61,771, suggesting that the process represents an exudation of interstitial fluid into the lumen from the mucosa. Although considerable attention has been given to the filtration secretion process in the small bowel, less information is available on this phenomenon in other organs in the alimentary tract. There is some evidence that suggests that the threshold for filtration secretion in the colon exceeds that of small bowel (144), presumably reflecting the lower permeability of the colonic mucosal membrane. In the stomach, there is evidence that changes in interstitial fluid pressure are associated with fluid secretion across the gastric mucosa (110).Flow of interstitial fluid across the gastric mucosa has been observed under a variety of experimental conditions, e.g., elevation of arterial and venous pressure, applica-

662

GRANGER AND BARROWMAN

tion of sulfhydryl-reducing agents and salicylic acid to the gastric mucosa, and intraarterial infusion of acetylcholine. Transcapillary

Solute Exchange

Small solutes. Two experimental approaches have been used to assess the permeability of capillaries in the alimentary tract to small molecules: the osmotic transient method (116,145) and the indicator diffusion technique (146-150). Results from osmotic transient studies indicate that the osmotic reflection coefficient (gd) of intestinal capillaries for NaCl, urea, glucose, and maltose range between 0.0006and 0.0017,with no correlation between u and solute size (116). These values are one-tenth to one-hundredth the values obtained for organs perfused primarily by continuous capillaries, i.e., skeletal muscle (151), heart (1521, and mesentery (145). The equivalent radius of capillary pores predicted with this technique in the small bowel ranges between 200 and 350 A (116), which contrasts with the 35-45 A radius pores predicted for skeletal muscle and heart capillaries. While the results obtained with the osmotic transient method tend to agree with ultrastructural estimates of pore dimensions in the fenestrated capillaries of the intestine, several difficulties with the technique in general and as applied to small intestine (not the least of which is that the osmotic bolus increases capillary permeability) may tend to limit the reliability of the results (116). The principle of indicator dilution has been used to measure the permeability of capillaries to small solutes in many organs. Information obtained from this technique can be used to produce an estimate of the permeability-surface area product (PS) for small tracer molecules. The PS values for raffinose, inulin, and p-lactoglobulin A have been obtained for small intestine (149) and stomach (59) capillaries over a wide range of blood flows. The PS value for all solutes increases with plasma flow in both organs. Evidence for blood flow-limited exchange was obtained only for raffinose in the stomach at resting and moderately increased blood flows. The direct linear correlation between PS and plasma flow for inulin and p-lactoglobulin A, therefore, has been attributed to a progressive rise in capillary surface area (S) during isoproterenol-induced vasodilation in the intestine and stomach. The PS values for raffinose and inulin in the small intestine are 20 times larger than values reported for hyperemic skeletal muscle. This difference may be due to a larger surface area or a greater permeability, or both, of the capillaries in the intestine compared to skeletal muscle. Since differences in capillary surface area may account for a threefold to fourfold greater PS in intestine than muscle, intestinal capillaries are like-

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ly to be five to seven times more permeable to raffinose and inulin than skeletal muscle capillaries. The PS values for small solutes in the stomach are three times larger than those obtained in the intestine. There is no evidence of restricted diffusion of raffinose and inulin in either tissue; however, plactoglobulin (28 A radius) is restricted and the degree of restriction is consistent with an equivalent capillary pore radius of 53-59 A (59,149). The small intestine and stomach are composed of two ultrastructurally different capillary beds in parallel. The capillaries of the mucosa-submucosa are of the fenestrated type whereas those in the muscularis are of the continuous type. In the resting and isoproterenol vasodilated small bowel and stomach, the mucosa-submucosa receives 80% or more of the total blood flow, therefore, it is likely that the aforementioned values for PS primarily represent the permeability characteristics of the fenestrated capillaries. The permeability characteristics of the continuous capillaries of the muscularis have been studied using the indicator diffusion technique in intestinal preparations where blood flow was redistributed in favor of the muscularis with adenosine (105,149). This approach yielded lower PS values for inulin and raffinose at any given plasma flow. There was evidence of restricted diffusion of inulin compared with raffinose and the results were consistent with an equivalent pore radius of 40 A for muscularis capillaries. These results suggest that the permeability of capillaries in the muscularis layer of the intestine to small lipid-insoluble solutes is very similar to that reported for skeletal muscle. Another vascular bed that has been studied with the indicator dilution technique is the salivary gland (146,147). The PS values for raffinose and inulin in this tissue are -10 times larger than the values reported for the small intestine. Although these higher values may be largely explained by differences in capillary surface area between intestine and salivary gland, the 120 A equivalent pores predicted for salivary gland capillaries indicates that differences in vascular permeability is a likely explanation. In glands perfused at constant flow, parasympathetic stimulation leads to a decrease in PS for ethylenediametetraacetate (EDTA), cyanocobalamin, and insulin (147). It is suggested that this may be the result of a redistribution of flow from the acinar microcirculation to the less permeable ductal vasculature. Ligation of the submandibular duct for 3-12 days also significantly reduces the PS for small lipidinsoluble molecules (147). Relatively little is known about the permeability of capillaries in the pancreas to small lipid-insoluble molecules. The PS for EDTA in the maximally vasoto one-sixth the dilated pancreas is -one-eighth

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value reported for salivary gland and 20 times greater than skeletal muscle (148). It is uncertain whether these differences result from organ to organ variation in capillary permeability or surface area, or both. Studies employing several solutes of different sizes are necessary to describe more fully the permeability of pancreatic capillaries to small solutes. Macromolecules. It is well recognized that the alimentary tract accounts for a large proportion of the total transcapillary escape rate of proteins in humans and lower animals (153,154). The rate of protein leakage across capillaries varies considerably between organs of the alimentary tract, ranging from 0.50 mg/min * 100 g in the colon (49) to 2.0 mg/min * 100 g in salivary glands (53). Protein leakage in the nonabsorbing small intestine is -1.5 mg/min * 100 g, a value -10 times greater than that reported for skeletal muscle (101). The relatively high rate of capillary protein leak in the alimentary tract is generally attributed to a large capillary surface area and a high permeability to macromolecules. Since the reflection coefficient of capillaries in the alimentary tract to plasma proteins generally exceeds those reported for other organs, capillary surface area may be the more important factor accounting for the high rates of protein leak. The results of several studies (49,53,56,59,155158) clearly establish that the capillaries of the alimentary tract selectively restrict blood-to-interstitium movement of macromolecules in accordance with solute size. At normal capillary filtration rates, the data show a steep fall in the permeation of solutes with a radius <60 A. Above 60 A there is an extension of residual permeability with no decrement in permeability for molecules as large as 135 A radius. The steep fall in permeability with solute radius below 60 A suggests that there are restrictive porosities approximating this dimension. The constant residual permeability up to 135 A radius has been attributed to either large pores (>135 A radius) or vesicular transport (13,158). Estimates of the osmotic reflection coefficient for plasma proteins of varying size in stomach, colon, and small intestine also demonstrate selective restriction of macromolecules in accordance with solute size. For intestinal capillaries, the osmotic reflection coefficient (0;1) is 0.90 for albumin and rises progressively with molecular size up to P-lipoprotein, where ad = 0.99 (111). The reflection coefficient data from stomach (59). colon (96), and small intestine (159) have been applied to irreversible thermodynamic and hydrodynamic principles to estimate the dimensions and number of transport pathways for macromolecules in these capillaries. The reflection coefficient data in all organs are consistent with two equivalent pore populations, i.e., a popula-

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tion of small pores of 46-53 A radius and large pores with radii ranging between 180 and 250 A. The relative number of small pores to large pores varies from 6400: 1 in capillaries of the small intestine to 500 : 1 in colonic capillaries. The morphologic equivalents to the small and large pore populations predicted for capillaries of the alimentary tract have not been firmly established. It is likely, however, that the structural equivalent of the large pores is the open fenestrae (2OO400 A radius). The internal radius of the cytoplasmic vesicles (2250 A) is also in agreement with the physiologic large pore estimates. It is also possible that differential porosities within the fibrillar structure of the basement membrane account for a component of the large pore equivalency. While the permeability of the fenestral diaphragm is unknown, it is generally considered that the porosity of this structure is the morphologic equivalent to the small pore system (10,17). The concept that the presence or absence of size-limiting structures within the fenestral diaphragms differentiates subpopulations that correspond to small and large pore systems seems tenable, yet, the relative frequency of open and diaphragmed fenestrae (11) appears to be much higher than the relative frequency of small and large pores predicted by the physiologic data. In most capillary permeability studies, the molecular probes and capillary pores are treated as rigid structures exhibiting no net electrical charge. Data from the kidney clearly indicate that the electrical charge and configuration of macromolecules greatly influence transglomerular exchange (160). Recent ultrastructural (161,162) and physiologic (163) studies suggest that solute charge is also an important determinant of transcapillary exchange in the alimentary tract. The distribution of anionic: sites on the blood front of the fenestrated endothelium of pancreatic and jejunal capillaries has been assessed using cationized ferritin (161,162). From the pattern of binding of cationized ferritin to the capillary surface, a high density of anionic sites was demonstrated on the fenestral diaphragms. Binding could not be demonstrated on the membrane of plasmalemma1 vesicles and transendothelial channels. These studies suggest that the fenestral diaphragms of visceral capillaries will discriminate against anionic molecules while vesicles and transendothelial channels may favor the penetration of anionic molecules and discriminate against cationic molecules. Lymph protein studies in the small bowel indicate that, for a given molecular size, the capillary reflection coefficient increases as a function of the isoelectric point of the molecule (163). This indicates that the intestinal capillary wall as a whole behaves as a positively charged barrier that impedes the move-

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ment of cationic molecules from blood to lymph. The effects of solute configuration on transcapillary exchange have not been systematically analyzed in capillaries of the alimentary tract. Ultrastructural studies have demonstrated, however, that large dextrans (linear polymers) appear to unravel and move end-on through the fenestral diaphragm of intestinal capillaries (18). The mechanisms that account for the transfer of macromolecules across capillaries in the alimentary tract are poorly understood. Convection, diffusion, and vesicular exchange are considered to be the principal mechanisms by which macromolecules cross the capillary wall. The relative contributions of diffusion and convection to macromolecule transport across intestinal capillaries has been estimated from lymphatic protein flux data in humans (113) and cat (164). From the relationship between lymphatic protein clearance and lymph flow in the intestine of cirrhotic patients, Witte et al. (113) have deduced that diffusion is the dominant process responsible for transcapillary protein exchange in the intestine. Lymphatic protein flux data from the cat intestine, which was analyzed using phenomenological transport equations, indicate however, that convection accounts for -800!-90% of total transcapillary protein movement at normal and increased capillary filtration rates. When intestinal capillaries were converted from filtering to absorbing vessels by enhancing transmucosal water movement, the convective and diffusive protein fluxes occur in opposite directions, i.e., significant quantities of plasma proteins move from interstitium to blood by convection (164). The existence of a blood-tissue circulation of plasma proteins during absorption may be advantageous for the removal of protein-bound nutrients (e.g., fatty acids) from the mucosal interstitium. Many physiologic, pharmacologic, and pathologic interventions have been shown to enhance capillary protein leakage in the alimentary tract. While all of the agents or conditions listed in Table 3 have been shown to increase transcapillary protein flux, the relative contributions of capillary recruitment and increased vascular permeability to the enhanced protein leakage is unknown in most cases. Many agents (e.g., histamine, bradykinin, glucagon) increase vascular permeability, although it is uncertain whether or not they also increase capillary surface area. Other substances (e.g., isoproterenol, secretin) increase capillary protein leakage without increasing vascular permeability, suggesting that capillary recruitment occurs. Much of the difficulty in discerning the mechanism by which capillary protein leakage increases results from the inability of available methods to measure a change in capillary surface area when capillary permeability is increased.

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Some conditions that increase vascular permeability and capillary protein leakage in the intestine do so by preferentially affecting the dimensions of the large pores. Osmotic reflection coefficient values for several endogenous proteins obtained during fat absorption (68) or after 1 h of ischemia (115) indicate that the size of the large pores increases from 200 A to 300-330 A, while the dimensions of the small pores remain relatively constant at 47-50 A. The mechanism by which these conditions selectively influence the large pore system is not readily apparent. There is substantial evidence from other tissues (e.g., mesentery), however, that indicates that various pharmacologic agents reversibly increase vascular leakage of tracer molecules by forming large interendothelial gaps (large pores) in the venous capillaries (165). The gaps are believed to be formed as a result of receptor-mediated contraction and subsequent separation of endothelial cells. The physiologic implications of these observations is that endothelial cells of the microvasculature may behave as a functional unit that can selectively respond to changes in the composition of its external environment, There are several electron microscopic studies in the literature that describe the effect of some pharmacologic or pathologic insult on intestinal vascular permeability to macromolecules. Perfusion of the rat intestine with histamine has been shown to cause partial removal of the fenestral diaphragms, occasional detachment of the endothelium from the basement membrane, and focal separation of the intercellular junctions of capillaries in the mucosa (166). Histamine treatment allowed carbon particles to transverse the fenestrae; however, the basement membrane prevented most of the particles from entering the interstitium. Perfusion of the capillaries with EDTA produced similar effects to histamine, yet the intensity of the structural alterations was greater (166). The ultrastructural alterations produced by histamine presumably account for the reduction in ad for total proteins from 0.92 to 0.56 predicted from lymph protein studies in the intestine (65). There is also morphologic evidence indicating that intestinal capillary permeability is increased (as judged by the restriction to carbon or ferritin particles) in early experimental hypertension (167), in acute angiotensin-induced hypertension (1681, and after enteric instillation of either cholera enterotoxin (169) or mustard oil (170). References Blair-West JR, Coghlan JP, Denton DA, et al. Ionic, histological and vascular factors in the reaction of the sheep’s parotid to high and low mineralocorticoid status. J Physiol 1969;205:563-79.

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2. Henderson JR, Daniel PM, Fraser PA. The pancreas as a single organ: the influence of the endocrine upon the exocrine part of the gland. Gut 1981;22:158-67. KG, Mayrand RR, et al. Blood flow to 3. Lifson N, Kramlinger the rabbit pancreas with special reference to the islets of Langerhans. Gastroenterology 1980;79:466-73. 4. Lifson N, Lassa CV. Note on the blood supply of the ducts of the rabbit pancreas. Microvasc Res 1981;22:171-6. 5. Barlow TE, Bentley FH, Walder ND. Arteries, veins and arterio-venous anastomoses in the human stomach. Surg Gynecol Obstet 1951;93:657-71. 6. Delaney JP. The paucity of arteriovenous anastomoses in the stomach. Surgery 1975;78:411-3. 7. Guth PH. In vivo microscopy of the gastric microcirculation. In: Granger DN, Bulkley GB, eds. Measurement of blood flow. Baltimore: Williams and Wilkins, 1981:105-20. 8. Kvietys PR, Wilborn WH, Granger DN. Effects of net transmucosal volume flux on lymph flow in the canine colon. Structural-functional relationship. Gastroenterology 1981;81:1080-90. 9. Karnovsky MJ. The ultrastructural basis of transcapillary exchanges. J Gen Physiol 1968;52:641-96. 10. Palade GE, Simionescu M, Simionescu N. Structural aspects of the permeability of the microvascular endothelium. Acta Physiol Stand 1979;463(Suppl):ll-32. 11. Casley-Smith JR, O’Donoghue PJ, Cracker KWJ. The quantitative relationship between fenestrae in jejunal capillaries and connective tissue channels: proof of “tunnel capillaries.” Microvasc Res 1975;9:78-100. 12. Casley-Smith JR. Endothelial fenestrae in intestinal villi: differences between the arterial and venous ends of the capillary. Microvasc Res 1971;3:49-68. 13. Renkin EM. Multiple pathways of capillary permeability. Circ Res 1977;41:735-43. 14. Renkin EM. Relation of capillary morphology to transport of fluid and large molecules: a review. Acta Physiol Stand 1979:463(Suppl):81-91. 15. Solomon AK. Characterization of biological membranes by equivalent pores. J Gen Physiol 1968;51:335-64. 16. Clementi F, Palade (;. Intestinal capillaries. Permeability to peroxidase and ferritin. J Cell Biol 1969;41:33-58. 17. Simionescu N, Simionescu M, Palade GE. Structural-functional correlates in the transendothelial exchange of water soluble macromolecules. Thromb Res 1976;8:257-69. 18. Simionescu N, Simionescu M, Palade G. Permeability of intestinal capillaries. Pathway followed by dextrans and glpcogens. J Cell Biol 1972;53:365-92. 19. Palade GE, Bruns RR. Structural modulations of plasmalemma1 vesicles. J Cell Biol 1968;37:633-49. 20. Fox J, Galey F, Wayland H. Action of histamine on the mesenteric microvasculature. Microvasc Res 1980;19:10826. 21. Comper WD, Laurent TC. Physiologic function of connective tissue polysaccharides. Physiol Rev 1978;58:255-315. 22. Granger HJ. Physicochemical properties of the extracellular matrix. In: Hargens AR, ed. Tissue fluid pressure and composition Baltimore: Williams and Wilkins, 1980:43-61. 23. Mailman D. Blood Bow and intestinal absorption. Fed Proc 1982;41:2096-100, 24. Granger DN, Mortillaro NA, Kvietys PR, et al. Role of the interstitial matrix during intestinal volume absorption. Am J Physiol 1980;238:G183-9. 25. Barrowman JA, Perry MA, Kvietys PR, Granger DN. The exclusion phenomenon in the liver interstitium. Am J Physiol 1982;243:G410-4. 26. Goresky CA. The nature of transcapihary liver. Can Med Assoc J 1965;92:517-22. 27. Papp M, Kohlich

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50. Kvietys PR, McClendon JM, Bulkley GH, et rll. ‘The pancreatic circulation: intrinsic regulation. Am J Pbysiol 1982:242:G596-602. 51. Granger culation 1982;41

DN. Perry MA, Kvietys PK. The intestinal and fluid transport in digestive organs. [in press).

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52. Elieasen E, Folkow B, Hilton S. Blood flow and capillary filtration capacities in salivary and pancreatic glands. Acta Physiol Stand 1973;87:11A. 53. Koo A, Smaje LH, Spencer PD. Low permeability to macromolecules of the fenestrated capillaries in the cat submandibular gland. Bib Anat 1981;20:301-4. 54. Lundwall J, Holmberg J. Mechanisms involved in transcapillary fluid movement in the secreting cat submandibular gland. Acta Physiol Stand 1978;102:16A. 55. Barrowman JA, Perry MA, Kvietys PR, et al. Effects of bradykinin on intestinal transcapillary fluid exchange. Can J Physiol Pharmacol 1981;59:786-9. 56. Granger DN, Taylor AE. Permselectivity of intestinal capillaries. Physiologist 1980;23:47-52. 57. Altaminaro M, Requena M, Perez T. Interstitial fluid pressure in canine gastric mucosa. Am J Physiol 1975;229:141420. 58. Bill A. Regional lymph flow in unanesthetized rabbits. Ups J Med Sci 1979;84:129-36. 59. Perry MA, Crook WJ, Granger DN. Permeability of gastric capillaries to small and large molecules. Am J Physiol 1981;241:G478-86. 60. Jansson G, Lundgren 0, Martinson J. Neurohormonal control of gastric blood flow. Gastroenterology 1970:58:424-9. 61. Yablonski ME; Lifson N. Mechanism of production of intestinal secretion by elevated venous pressure. J Clin Invest 1976;57:904-15. 62. Mortillaro NA, Taylor AE. Interaction of capillary and tissue forces in the cat intestine. Circ Res 1976;39:348-58. 63. Granger DN, Mortillaro NA, Taylor AE. Interactions of intestinal lymph flow and secretion. Am J Physiol 1978;232:E13-8. 64. Granger DN, Parker RE, Quillen EW, et al. Lymph flow transients. In: Malek P, ed. Lymphology. Stuttgart: G. Thieme Publishers, 1977:61-3. 65. Mortillaro NA, Granger DN, Kvietys PR, et al. Effects of histamine and histamine antagonists on intestinal capillary permeability. Am J Physiol 1981;24O:G381-6. 66. Granger DN, Richardson PDI, Taylor AE. Effects of isoprenaline and bradykinin on capillary filtration in the cat ileum Br J Pharmacol 1979;67:361-6. 67. Granger DN, Kvietys PR, Wilborn WH, et al. Mechanisms of glucagon-induced intestinal secretion. Am J Physiol 1980;239:G30-8. 68. Granger DN, Perry MA, Kvietys PR, et al. Permeability of intestinal capillaries: effects of fat absorption and gastrointestinal hormones. Am J Physiol 1982;242:G194-201. 69. Turner SG, Barrowman JA. The effects of cholecystokinin on intestinal lymph flow in the rat. Can J Physiol Pharmacol 1977;59:1339-96. 70. Lawrence JA, Bryant D, Roberts KB, et al. Effect of secretin on intestinal lymph flow and composition in rat. Q J Exp Physiol 1981;66:297-305. 71. Granger DN, Shackleford JS, Taylor AE. Prostaglandin Elinduced filtration secretion in the feline ileum. Am J Physiol 1979;236:E788-98. 72. Szwed JJ, Maxwell DR, Elliott R, et al. Diuretics and small intestinal lymph flow in the dog. J Pharmacol Exp Ther 1977;200:88-94. 73. Granger DN, Kvietys PR, Mortillaro NA, et al. Effect of luminal distension on intestinal transcapillary fluid exchange. Am J Physiol 1980;239:G516-23. 74. Granger DN, Mortillaro NA, Perry MA, et al. Autoregulation of intestinal capillary filtration. Am J Physiol 1982; 243:G475-83. 75. Levine SE, Granger DN, Brace RA, et al. Effect of hyperosmolality on vascular resistance and lymph flow in the cat ileum. Am J Physiol 1978;234:H14-20.

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76. Quillen EW, Granger DN, Taylor AE. The effects of arginine vasopressin on capillary filtration in the cat ileum. Gastroenterology 1977;72:474-8. 77. Granger DN, Cross R, Barrowman JA. Effects of various secretagogues and human carcinoid serum on lymph flow in the cat ileum. Gastroenterology 1982;83:896-901. 78. Nicoll PA, Taylor AE. Lymph formation and flow. Ann Rev Physiol 1977;39:73-95. 79. Granger HJ. Role of the interstitial matrix and lymphatic pump in regulation of transcapillary fluid balance. Microvast Res 1979;18:209-16. 80. Hargens AR, Zweifach BW. Contractile stimuli in collecting lymph vessels. Am J Physiol 1977;233:H57-65, 81. Richardson PDI, Granger DN, Taylor AE. Capillary filtration coefficient: the technique and its application to the small intestine. Cardiovasc Res 1979;13:547-61. 82. Richardson PDI, Granger DN. Capillary filtration coefficient as a measure of perfused capillary density. In: Granger DN, Bulkley GB, eds. Measurement of blood flow. Applications to the splanchnic circulation. Baltimore: Williams and Wilkins, 1981:319-36. 83. Granger DN, Richardson PDI, Taylor AE. Volumetric assessment of the capillary filtration coefficient in the cat small intestine. Pflugers Arch Eur J Physiol 1979;381:25-33. 84. Chen HI. An extracorporeal reservoir device for continuous monitoring of tissue volume change. Am J Physiol 1982;242:H698-704. 85. Chou CC, Kvietys PR. Physiological and pharmacological alterations in gastrointestinal blood flow. In: Granger DN, Bulkley GB, eds. Measurement of blood flow. Applications to the splanchnic circulation. Baltimore: Williams and Wilkins, 1981:477-507. 86. Shepherd AP. Intestinal capillary blood flow during metabolic hyperemia. Am J Physiol 1979:237:E548-54. 87. Chen HI, Yeh FC, Ho W. Direct effects of nitroglycerine on the resistance, exchange and capacitance functions of canine intestinal vasculature. J Pharmacol Exp Ther 1981;218:497503. 88. Granger DN, Perry MA, Kvietys PR. Role of exchange vessels in the regulation of intestinal oxygenation. Am J Physiol 1982;242:G570-4. 89. Ohman U. Studies on small intestinal obstruction. Blood circulation in the obstructed small intestine. Acta Chir Stand 1975:452(Suppl). 90. Johnson PC. Myogenic nature of increase in intestinal vascular resistance with venous pressure elevation. Circ Res 1959;6:992-9. 91. Granger DN, Kvietys PR. The splanchnic circulation: intrinsic regulation. Ann Rev Physiol 1981;43:409-18. 92. Johnson PC, Hanson KM. Capillary filtration in the small intestine of the dog. Circ Res 1966;19:766-73. 93. Granger DN, Richardson PDI, Kvietys PR, et al. Intestinal 1980;78:837-63. blood flow. Gastroenterology 94. Folkow B, Lundgren 0, Wallentin E. Studies on the relationship between flow resistance, capillary filtration coefficient blood volume in the intestine of the cat. Acta and regional Physiol Stand 1963:57:270-83. 95. Granger DN, Rutili G, McCord JM. Superoxide radicals in feline intestinal ischemia. Gastroenterology 1981;81:22-9. 96. Taylor AE, Granger DN. Exchange of macromolecules across the circulation. In: Renkin EM, Michel CC, eds. Handbook of physiology, microcirculation. Chap 11. Washington, D.C.: American Physiological Society, 1983 (in press). 97. Gore RW, Schoknecht W, Bohlen HG. Filtration coefficients of single capillaries in rat intestinal muscle. In: Grayson J, Zingg W, eds. Microcirculation. New York: Plenum Press, 1976:331-2. 98. Gore RW. Fluid exchange across single capillaries in rat

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