Transvascular fluid exchange in the tracheal mucosa

MlCROVASCULARRESEARCH15,287-298(1978)

Transvascular

Fluid Exchange in the Tracheal Mucosa

U. NORDIN,'~~.K~~LLSKOG,

C-E. LINDHOLM,AND

M.WOLGAST

Department of Oto-Rhino-Laryngologv, University Hospital, Uppsala, and Institute Medical Physics, University of Uppsala, Sweden

of Physiology

and

Received February la,1977 The fluid balance of the rabbit tracheal mucosa was investigated with the micropuncture technique and the microsphere method was used for blood flow measurements.Under resting control conditions the blood flow was 0.62 ? 0.41 ml/mm. g of tissue; it increased to 3.22 + 1.55 ml/min.g when the trachea was divided by a midline incision and lixed with two clamps for the micropuncture experiments. The hydrostatic pressure in the early part of the capillary was 28 mm Hg, in the middle part 17 mm Hg, and in the late part 14 mm Hg. The pressure in the dense network of sinusoidal submucosal veins was 12 mm Hg. The interstitial pressure was 3-4 mm Hg. The plasma colloid osmotic pressure, as estimated from protein data, was 21 mm Hg, and that in the interstitium or terminal lymph was 19 mm Hg; thus they were almost identical. This was due to a heavy leakage of protein resulting from the irritation caused by the incision and clamp-fixation and resembled any case of irritation. With a horizontal body posture there is an outwardly directed filtration in all vascular segments.Part of the fluid will form the fluid layer on the tracheal epithelium and the mucus. The rest is drained, together with the proteins, by the richly developed lymphatic system. In an upright body posture, significant resorption will take place via the submucosalvenous plexus, with less risk of edema.

INTRODUCTION As part of an evaluation of pathophysiological events taking place in the tracheal mucosa upon trauma from an inflated tracheal tube cuff (Nordin et al., 1977a, b, c; Nordin and Lindholm, 1977) it was considered of interest to ascertain the perfusion pressure on the mucosal capillaries, as this would give an idea about when the pressure of the cuff on the mucosa could be expected to stop the capillary blood flow, i.e., to causelocal ischemia, with a consequentrisk of tracheal damageand stenosis, Unfortunately no reports of direct measurementsof perfusion pressure in the vessels of the tracheal mucosa appear to have been published, and we therefore decided to carry out such investigations, using methods applied earlier in the kidney (Kiillskog et al., 1976). MATERIALS

AND METHODS

Sixteen white rabbits (New Zealand strain) weighing 2.4-3.2 kg were used for the experiments, which were conducted under urethane (20% in saline) anesthesia administered intravenously in a dose of 1 g/kg body weight. The animals were allowed to breathe spontaneously and the body temperature was kept constant at 37O by a heating lamp. The frontal part of the trachea was dissected free and divided by a longitudinal incision of about 3-4 cm. In order to eliminate movements of the tissue, r Address reprint requests to Ulf Nordin, M.D., E.N.T. Clinic, University Hospital, S-750 14 Uppsala, Sweden. 0026.2862/78/0153-0287902.00/O 287 Copyright @ 1978 by Academic Press, Inc. AU right of reproduction in any form reserved. Printed in Great Britain

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two clamps were attached to the edgesand fixed with steel rods to the operating table. Two seriesof experiments were carried out, comprising (1) studiesof the mucosal blood flow and (2) micropuncture investigations. The trachea was prepared and fixed in the same way for both series.The microvascular anatomy of the tracheal mucosa has been studied in detail previously (Nordin and Lindhohn, 1977). As a complement,in order to demonstrate the capillary pattern in parallel with the micropuncture experiments, a preparation was obtained from one rabbit weighing 0.9 kg. The animal was anesthetized as described above, and 100 mg of horseradish peroxidase (HRP), type I, dissolved in 2 ml of saline, was injected intravenously. Fifteen minutes later a lethal injection of urethane was given. Immediately after death, 2.5% glutaraldehyde was injected through the larynx into the trachea, which was filled. After 15 min the larynx and the trachea were dissected out en bloc and stored in fresh 2.5% glutaraldehyde over night. The specimen was then transferred to 0.1 M phosphate buffer and kept there for 3 hr at room temperature. Thereafter it was incubated for 20 min in a medium of 5 mg of 3,3’diaminobenzidine tetrahydrochloride (Sigma Chemical Co., Saint Louis, Missouri) in 10 ml of Tris buffer, to which was added 0.1 ml of l-3% H,O, freshly prepared from 30% H,O,. The incubation medium was changed at lo-min intervals. The staining reaction was observed through a dissecting microscope. Following incubation the specimen was rinsed in 0.1 M phosphate buffer for 30 min and stored in 2.5% glutaraldehyde. Series 1. Blood flow investigations. This series was performed on nine animals, in which, in addition to the preparation of the trachea, the two femoral arteries were catheterized and a third catheter was introduced into the left ventricle of the heart. The right femoral arterial catheter was used for blood pressure measurements (Statham strain-gauge transducer P 23 fed to a photokymograph, Ultralet Model 6260, AB EM, Sweden).The left femoral arterial catheter had an outer diameter of 5 mm, but the tip was pulled out to about 1 mm in outer diameter. The wider part of the catheter was long enough (20 cm) to contain the two blood samples. Previous to sampling the catheter was filled with a heparinized saline solution. The catheter was connected to a suction pump which sampled at a constant rate of 0.82 ml/min. Microspheres (seebelow) were administered through the catheter placed in the left ventricle. This latter catheter, which was 1 mm in outer diameter, was introduced into the ventricle through the right external carotid artery, without ligation of the internal carotid artery, via the common carotid artery and aortic arch. The microspheres (i4iCe-labeled and 85Sr-labeled,3 M, 15 + 5 @ microspheres, St. Paul, Minnesota) were first submitted to ultrasonic agitation and then separatedby a sedimentation procedure in large glass vials. With this technique the mean size of the sphereswas 15 pm and the variation in size could be reduced to about +2 ,um, and no aggregatesof spheres were found in the suspensions.The spheres, numbering about 1 million, were then suspendedin 0.6 ml of rabbit plasma, since plasma suspensions had been found to be more stable than saline suspensionsof spheres,in which rapid sedimentation occurred. For details, seeK8llskog et al. (1972). For each determination of mucosal blood flow, 0.6 ml of the plasma-microsphere suspension was injected through the catheter into the left ventricle. The catheter was then rinsed with l-2 ml of saline. The time of injection was about 20 set, and the blood

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flow values reported in the following thus refer to the mean flow during this time interval. A few secondsbefore the injection, blood was continuously sampled from the right femoral arterial catheter. The systemic blood pressure was monitored continuously. All experiments in which the blood pressure changed by more than 10 mm Hg were excluded from the results. A further prerequisite for inclusion was that the pressure recording from the left ventricular catheter before and after the injection had the typical appearanceof a true intraventricular tracing. The first injection, as a rule with the i4iCe-labeled microspheres, was given under resting conditions, i.e., when the trachea was still intact. The secondmicrosphere injection was given after the trachea had been exposed and fixed with the clamp arrangement described above. Piecesof the tracheal mucosa in the region accessiblefor micropuncture, as well as from the region cranial and caudal to this site, were then collected by blunt dissection from the underlying perichondrium and connective tissue stroma, with the guidance of an operation microscope with a magnification of 10 times. The radioactivity of the tissue pieces and of the blood samplewas analyzed in a twochannel y spectrophotometer (Nucab, Gtiteborg, Sweden).The channels were put on the inflection points of the photopeak of the two isotopes. As a rule, about 20% of the 85Sr activity was recorded in the 141Cechannel, whereasno crossover from 14Ceto 85Srwas found. For calculating blood flow, the formula F/M = F,/M, was used, where F is the blood flow to the region in question, M the activity of the isotope, F, the blood sampling rate of 0.82 ml/min, and M, the radioactivity in the blood sample. Cardiac output was determined from the sameformula, replacing the factor M by the total amount of radioactivity injected; the factor F is now the cardiac output. With the microsphere size used, about 15 m, the sphereswould only become trapped in capillaries with diameters less than this figure. This was, in fact, verified by inspection of the tracheal mucosa with the operation microscope (magnification, 120 times). No extremely large spheres or aggregations of sphereswere found, nor were any spheresfound in the venous plexuses located in the submucosal region. The obtained blood flow will thus refer to capillary or nutritive flow. The number of spheresescaping capillary trapping was l-3% as determined from the radioactivity in the lungs. Series 2. Micropuncture investigations. This serieswas performed on sevenanimals prepared as described above. As a rule, the mucosa was covered with a fairly thick layer of fluid and therefore no superperfusion with saline or heated mineral oil was needed. The different structures in the mucosa were punctured with glass capillaries with the tip extended to a diameter of 2-12 m. The smaller tips of 2-4 pm were used for injection of dye (Vollgriin) and for pressure measurements,and the larger ones for injection of colored mineral oil and collection of fluid samples. For pressure measurementsthe glass pipet was attached to a servo-nulling pressure measurement device, as described by Wiederhielm (1964), in a modification by Intaglietta et al. (1970). With this system the mean pressure as well as the variation in pressure synchronous to the pulse rate could be recorded. The location of the tip was verified by injection of colored saline solution. The capillary tree was easily identified as a network oriented longitudinally in the

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trachea. The capillaries were characterized in terms of (1) the early part of the capillary, (2) the midcapillary, and (3) the late part of the capillary. This subdivision was made mainly on anatomical grounds, but also from the direction of the bolus of the injected dye. The late part of the capillaries was easily identified from the venules originating in them and ending in the venous plexuses.The pressurein the early part of the capillaries or in the arterioles could also be measured indirectly. This was done by measuring the capillary pressure after blockade of the blood stream in the downflow direction, either with a spherical glass rod or by the injection of colored mineral oil with a secondpipet. In the caseof a complete blockade there will be a stagnant pool of blood between the site of the blockade and the branching point just proximal to the site of puncture. Hydrostatic pressureswithin the venous system were measuredin the sameway by direct punctures of venules, small veins, and larger veins located in the submucosal layer. The interstitial pressure was obtained by direct puncture into the space betweenthe capillaries or puncture into the terminal lymph vessels. No significant difference between these two pressureswas found with the technique used, even though the fluid transport from the interstitium to the lymph channels will require at least a slight pressure drop. The lymphatic system was identified by injecting colored saline solution into the interstitium. The injection in itself causeda slight pressureincreasewhich, however, was transient. After a few minutes the dye appearedin the very richly developed lymphatic plexus. In some cases, however, the lymphatic system could be identified directly. The colloid osmotic pressure was investigated from the protein concentration in systemic blood and in the tracheal interstitium or terminal lymph. The concentration in the interstitium or lymph was determined from samplesobtained by direct punctures of interstitium or small terminal lymphatic vessels(seeAnatomical Remarks section). The sample volume was 10 nl or more and each sample was drawn in about 5 min. The volume of the samples was measured as the length in constant-bore glass tubes (Microcaps, Canton Laboratories, Massachusetts). The protein was measured by a microadaption of the Folin method (Lowry et al., 1951). In some experimentsthe turnalbumin was injected over of protein was also determined. In these cases 1251-labeled into the systemic circulation. A seriesof samplesof systemic plasma and interstitial or lymph fluid was then taken from time zero up to 2 hr after the albumin injection. The total activity of the interstitial sampleswas 500 cpm or more, and was counted for 1 hr. RESULTS Anatomical Remarks On inspection of the tracheal mucosa in the operation microscope (magnification 120 times), the cilia were clearly visible, continuously moving the fluid covering the mucosa in the rostra1 direction. The microvascular anatomy is visualized in Fig. 1. The capillaries formed a network oriented in the longitudinal direction of the trachea. They originated in comparatively large vesselslocated in the region between the cartilages. These vessels,presumably arterioles, were usually only seenas red dots branching into the capillary tree. The capillaries, originating from the different arterioles, did not seem

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to be separated from each other but to be anastomosed freely, forming a network supplied by several arterioles. The capillary system drained into the venous system via characteristic connecting vessels or venules, forming a loop from the superficially located capillaries to the more deeply located veins. The diameter increasedfrom 10 p at the proximal end to about 50 ,umat the site of drainage into the venous system. A most prominent feature of the vascular anatomy of the trachea was the venous system,which consisted of a very densenetwork of almost sinusoidal veins (Nordin and Lindholm, 1977). The diameter of these vesselsranged from about 100 to 300 pm. The

FIG. 1. The microvascular anatomy of the rabbit tracheal mucosa as visualized by iv injection of horseradish peroxidase (HRP), type I. The capillaries are mainly oriented in the longitudinal direction of the trachea, especially at the paries membranaceus (upper part of the picture).

network was drained by small veins about 5OOpm in diameter, which in most cases were oriented longitudinally. The general vascular arrangement is illustrated in Fig. 4. The lymphatic system was very richly developed, almost covering the circumference of the trachea. The lymphatic tree ended in sac-like structures, which may be called terminal lymphatic sacs. When a dye was injected into the interstitium, it first entered theseterminal lymphatics and then the lymphatic collecting tree. These collecting lymph vessels,however, proved to be impermeableto the dye in question. When, on the other hand, dye was injected into the lymphatic tree, it remained in the collecting vessels.F’or larger injections the dye could be forced in the backflow direction and it then penetrated into the interstitium through the terminal lymphatic sacs. Valves could be identified, but were not present in the terminal branches. Both the capillaries and the small veins were permeable, and injection of dye into these structures was followed by rapid passageout into the interstitium and then into the vesselagain. This sequenceof eventswas completed in a few second.In contrast, the dye injected into the interstitium took 0.5-l hr to disappear. All the vascular structures identified were open, i.e., there were no signs of any

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appreciable vasomotion. In the venous plexuses, however, the blood flow at different branching points in some cases changed its direction. This finding would seem to indicate at least a minor degreeof vasomotion. Blood Flow Conditions

The data on the mucosal blood flow in series 1, under control resting conditions and after the preparation and fixation of the trachea, are summarized in Table 1. In the region used for the micropuncture experiments the resting blood flow was estimated at TABLE 1 MUCOSAL BLOOD Fmw IN THE TRACHEA (SERIES 1) UNDER RESTING CONDITIONS AND AFTER PREPARATION OF THE TRACHEA FOR MICROPUNCTUREZ EXPERIMENTS WITH CORRESPONDING CARDIAC OUTPUT

Puncture site @/min. g of tissue)

Cranial to puncture site @/min. g of tissue)

Caudal to puncture site (ml/min. g of tissue)

Cardiac output (ml/mine kg body wt)

After Resting Resting After Resting After Resting After conditions preparation conditions preparation conditions preparation conditions preparation 0.76 0.73 1.00 2.30” 0.21 0.19 0.28 1.34 0.47 0.62 +0.41b N=9

2.13 3.78 2.51 2.70 4.86 2.50 2.15 6.09 1.76 3.22 +1.55 N=9

1.32 1.21 1.14 1.52 0.28 0.23 0.38 1.83 0.06

0.91 4.57 2.14 4.22 4.59 4.46 3.49 6.05 2.12

0.81 kO.65 N=9

3.54 k1.69 N=9

1.31 0.68 0.62 2.16 0.10 0.23 0.16 1.31 0.36 0.59 +0.48 N=9

0.96 1.53 1.82 3.31 4.56 2.35 2.45 4.68 2.42 2.59 +1.34 N=9

121.4 134.6 174.6 128.9 201.7 148.8 238.8 251.6 141.1 176.6 k49.2 N=9

91.0 102.9 151.8 103.0 162.1 138.5 200.7 173.2 136.3 144.6 L35.9 N=9

a From the statistics. bMean + SD.

0.62 + 0.41 ml/min . g of tissue. Essentially the samevalues were obtained for the areas cranial and caudal to this region. After the manipulation of the trachea the blood flow increased markedly up to 3.22 2 1.55 ml/mm. g, and a similar increase was noted for the area cranial and caudal to the region used for micropuncture. The cardiac output under resting conditions,was 176.6 + 49.2 ml/mine kg, and had decreasedslightly at the secondmicrosphere injection. Micropuncture

Data

The results of 74 pressure measurementsin different vascular structures in seven rabbits are given in Table 2. Arteriolar pressure in the table refers to only four measurements, and these figures are therefore somewhat uncertain. The capillary pressure is presented as the pressure in the early, middle, and late parts of the capillaries. The subdivision of the capillary in this way is obviously somewhat subjective. The pressuresin the venous plexuses are given here as a mean value, although it

MUCOSAL

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IN THE TRACHEA

shouldbe emphasized that a series of punctures in thedownflowdirectionrevealed a continuouspressuredrop, which was obviously to be expected.The absolutepressure drop within the venous system was, however, very slight--of the order of a few millimetersof mercury. The results thus indicate a hydrostatic pressure of about 28 mm Hg in the early part of the capillary, about 17 mm Hg in the midcapillary and about 14 mm Hg in the late part of the capillary. The pressurein the venous plexus was estimatedat 12 mm Hg, i.e., not very different from that in the small veins draining the mucosa. The pressurein the interstitium was estimated at 3-4 mm Hg, as investigated from either direct puncture into the interstitium or puncture into the lymphatic channels. The colloid osmotic conditions were analyzed in six additional experiments. The TABLE 2 HYDROSTATTCPRESSURESIN THE ~~UCOSAI.VASCULARBED AND IN THE INTERSTITIUM(SERIES2) Capillaries (mm W Arterioles (mm Hg)

Early

31 32 44

27.0 -

-

28.0 32.0 27.0 24.0

31.1 t6.0“

27.6 k2.9

Mid

Late

-

14.0 -

17.5 21.0 15.0 14.0 15.0 16.5 +2.8

20.0

12.0 13.5 10.0 12.0 13.6 k3.4

Sinusoidal plexus (mm Hg) 12.0 15.0 15.5 8.0 13.0 9.5 11.5 12.1 22.1

Small veins (mm I-W

Interstitium Mean arterial or lymph pressure (mm Hd (mm W

-

-

10.5 15.0

5.2

1.5

3.0

15.0 11.0

3.5 3.5

11.8 +3.2

-

3.8

$1.0

80 85

100 110 110 120 115 102.9 k15.2

plasma protein concentration was found to be 6.1 + 0.2% (N= 6). Adopting the formula of Landis and Pappenheimer (1963), the corresponding colloid osmotic pressurewas calculated to be 21 mm Hg. The protein concentrations in the interstitium and lymph were analyzed by injecting lZSI-labeledrabbit albumin into the systemic circulation. A series of sampleswas then taken from the terminal lymphatics a few minutes to 2 hr after the injection of the labeled albumin (seeFig. 2a). As seenin Fig. 2a, the plasma activity slowly decreased. The radioactivity in the terminal lymph rose steeply to values close tc the plasma activity itself, whereafter the two curves ran in parallel. Another experiment is shown in Fig. 2b; here the two curves showed the samecourse as in the other experiment, but the curve for lymph activity stabilized at a level clearly below that for plasma activity. The experiments do not allow a detailed analysis of the transit times (due to technical difficulties), but neverthelessindicate a rapid turnover of proteins. The half-time will be of the order of 30 min. Thirteen terminal lymph samplesfrom four animals were also analyzed with respect to the total protein concentration, and a value of 5.7 + 0.8% was obtained. The colIoid

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osmotic pressurewas calculated at 19 mm Hg. It was observed,however, that when the manipulation of the trachea was very gentle, the protein concentration tended to be lower than when the mucosa was more irritated. The results thus indicate a heavy leakage of protein out of the vascular bed into the interstitium, the leakage being proportional to the degreeof irritation.

a

0 y 0

I 50

I 100

I 150

I 200

I 250

Time cpmlnl 100

b 1

01

I

50

I

100

I

150

I

200 min

FIG. 2. iz51 Activity in the systemic plasma (open circles) and interstitial fluid (filled circles) after injection of 1251-labeled albumin. In (a) the lymph activity has increased rapidly, approaching the activity in the blood; and in another animal (b) it has stabilized at a lower level after a rapid increase. The larger scatter of the interstitial fluid activity is probably due to technical errors in the sampling of these small volumes. In addition, the samples were drawn from different sites of the interstitium.

DISCUSSION From the micropuncture data obtained, the fluid dynamics of the tracheal mucosa can be evaluated, as shown in Figs. 3a and b. In Fig. 3a, a superficially located capillary is seenjust beneath the epithelial cell layer. The capillary system will drain into the more deeply located venous plexusesvia typical interconnecting venules,forming a cone, with the diameter of the vesselincreasing from about 10 p in the capillary end of the vessel to about 50 ,um in the venous end. In the region betweenthe capillaries there is a richly developedlymphatic system.A terminal lymphatic is visible in the figure.

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The hydrostatic pressure in the early part of the capillary was estimated at 28 mm Hg, in the midpart it was 17 mm Hg, and in the late part of the capillary it had decreasedto 14 mm Hg. In the sinusoidal vein the pressure was about 12 mm Hg, and approximately the same pressure was also obtained in the small veins draining the mucosa. The hydrostatic pressurein the interstitium, which was identical to the pressure in terminal lymph vessels,was estimated at 4 mm Hg. The pressuredrop neededfor the fluid transfer was beyond the scope of detection with the present technique. The colloid osmotic pressure of the systemic plasma was 21 mm Hg, a figure which obviously was the same for the blood in all the structures investigated. The corresponding colloid a

b

FIG. 3. Fluid dynamics of the tracheal mucosa.Driving forces for the fluid exchange across capillaries and submucosal veins. P = hydrostatic pressure (mm Hg), n=colloid osmotic pressure (mm Hg). (a) Horizontal body posture, (b) upright body posture. A lymph vessel with a terminal sac (dotted line indicating permeability to proteins) is intermingled with the capillary and venous systems.

osmotic pressure in the interstitium was astonishingly high, about 19 mm Hg. The determination of colloid osmotic pressure assumesa reflexion coefficient of 1.0. This figure can obviously be criticized, as a heavy leakage of proteins was found to occur. It should be emphasized, however, that this figure is only the mean value. In an irritated mucosa the figure could exceed the colloid osmotic pressure in plasma. In a mucosa which was manipulated very gently it could be about 8 mm Hg. The protein concentration in the fluid coating the epithelium is close to zero, which was checked in pilot experiments. This is in accordance with reports by Lorin et al. (1972) and by Deuschl and Johansson (1977). From these figures it is evident that there will be an outward filtration of fluid from all the vascular structures identified. The driving force for this fluid turnover is 22 mm Hg in the early part of the capillaries, decreasingprogressively to 6 mm Hg in the submucosal venous plexus. The high permeability of the structures (as indicated by the rapid turnover of dye injected) will also allow for such a transport of fluid. The fluid filtered will also contain protein, which has been transported across the capillary endothelium through large pores (Pappenheimer et al., 1951; Grotte,

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1956; Mayerson, 1961; Mayerson et al., 1962) or by pinocytosis (Simionescu et al., 1975). The present data do not, however, allow an analysis of the sites of this protein leakage. The amount of fluid flowing into the interstitium will, in principle, be drained by two separate routes (Fig. 4). Part of the fluid will be reabsorbed by secretory glands and goblet cells for subsequentproduction of mucus. This is probably an active processvia the secretion of Cl- and Na+ (Olver et al., 1975; Marin et al., 1976). This part of the fluid contains negligible amounts of proteins and will finally be removed by the cilia. By this process the protein within the interstitial fluid will become increasingly concentrated, reaching values close to the systemic plasma concentration. This part of the fluid, which in fact is equivalent to pure plasma as regards the protein concentra-

FIG. 4. Fluid dynamics of the tracheal mucosa.Fluid transported from the early part of the capillaries into the interstitium will be drained by two separate routes. An essentially protein-free fraction of the fluid will be reabsorbed by the secretory glands and goblet cells for subsequent production of mucus. Part of the fluid, with a high concentration of protein, will enter the lymphatic system through the walls of the terminal lymphatics and will then be drained by the richly developed lymphatic system. Granules in the arrows indicate protein concentration.

tion, will enter the lymphatic system through the walls of the terminal lymphatics and then be drained by the richly developedlymphatic system. As pointed out previously, the figures obtained refer to a mucosa prepared for micropuncture and not to the physiology of the undisturbed resting mucosa. Such preparation will mean some irritation to the mucosa, giving reactions equivalent to those caused by the introduction of a foreign body into the trachea, as found in another investigation (Nordin et al., 1977c). As shown in both studies, the blood flow will be increasedto five to six times that under resting conditions. In conclusion, irritation of the tracheal mucosa will lead to the releaseof histaminelike substances, causing dilatation of the arterioles. This will result in an increasing blood flow in the mucosa, but also in a raised hydrostatic pressure within the small vesselsof the trachea. The permeability to large molecules such as proteins will also be increased. These reactions will lead to an enhanced filtration of fluid out of both the capillaries and the venous system. The fluid will thus contain proteins. A protein-free

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fraction of this fluid will be secretedinto the tracheal lumen and thus form the mucus fluid lining the tracheal epithelium, a fluid which obviously will contribute to removing the agent primarily irritating the mucosa. Extraction of a protein-free fraction will clearly lead to an increasein the protein concentration of the interstitial fluid and thereby also to a rise in the colloid osmotic pressure of the interstitium, causing a greater filtration of fluid out of the blood stream. All these reactions lead to an edema, with increased interstitial pressure, even though the well-developedlymphatic system constitutes an effective route of drainage. The above discussion refers to a horizontal body posture, which explains the fairly high venous pressureof about 10 mm Hg. In the case,of an upright position, the hydrostatic pressure within the veins of the upper part of the body will decreasetoward zero (Jonson and Rundcrantz, 1969). Such a condition is visualized in Fig. .3b. It is evident here that the pressures within the mucosal microvasculature are somewhat decreased (this assuming an unchanged blood flow). In this case there will still be an outward filtration of fluid in the early part of the capillaries. In the late part, and especially in the richly developed submucosal venous system, there will be a resorption of filtrate. The lymphatic drainage will obviously also be more efficient. The upright body position will thus lead to a decreasednet outward filtration of fluid, with a consequentreduction of the interstitial pressure. Thus, under this condition, the risk of edema will be considerably diminished. ACKNOWLEDGMENTS We wish to express our sincere gratitude to Mrs. Birgitta Jansson for skilful technical assistance,Mrs. Pia Houe for typing the manuscript, and Mrs. Karin Sahlman for drawing the figures. This work was supported by the Swedish Medical Research Council (Project No. B76-17X4259-03).

REFERENCES DEUSCHL,H., AND JOHANSSON, S. G. 0. (1977). Specific IgE antibodies in nasal secretion from patients with allergic rhinitis and with negative or weakly positive RAST in the serum. Clin. AZlergy 7, 195-202. GANONG,W. F. (1975). “Medical Physiology.” Lange Medical, Los Altos, California. GROW, G. (1956). Passage of dextran molecules across the blood-lymph barrier. Acta Chir. Stand. (SuppZ.)211, 1. INTAGLIETTA, M., PAU~LTLA,R. F., AND TOMPKINS, W. R. (1970). Pressure measurements in the mammalian microvasculature. Microuasc. Res. 2212-220. JONSON, B., AND RUNDC~ANTZ,H. (1969). Posture and pressure within the internal jugular vein. Actu Otolaryngol. 68,271-275. KXLLSKGG, b., LMDBOM, L. O., ULFENDAHL,H. R., AND WOLGAST,M. (1976). Hydrostatic pressures within the vascular structures of the rat kidney. PjZgers Arch. 363,205-210. K~~LLSKOG, G., ULFENDAHL,H. R., AND WOLGAST,M. (1972). Single glomerular blood flow as measured with carbonized %e labelled microspheres.Acta Physiol. Scand. 85,408-413. LANDIS, E. M., AND PAPPENHEIMER, J. R. (1963). Exchange of substancesthrough the capillary wall. In “Handbook of Physi9logy. Section II: Circulation,” pp 96 l-1034. Wiiams and Wilkins, Baltimore. L~IUN, M. I., GAERLAN, P. F., AND MANDEL, I. D. (1972). Quantitative composition of nasal secretions in normal subjects. J. Lab. Clin. Med. 80, No 2. LOWRY, 0. H., ROSEBROUGH, N. J., FARR, A. L., AND RANDALL, R. J. (1951). Protein measurementwith the Folin phenol reagent. J. Biol. Chem. 193,265-275. MARIN, M. G., DAVIS, B., AND NADEL, J. A. (1976). Effect of acetylcholine on Cl- and Na+ fluxes across dog tracheal epithelium in vitro. Amer. J. Physiol. 231, 1546-1549.

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NORDIN

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MAYERSON, H. S. (1961). Observations and reflections on the lymphatic system, Trans. Stud. Coil. Physicians Philadeghia 28, 109-127. MAYERSON, H. S., PA~RSON, R. M., MCKEE, A., LEBRIE, S. J., AND MAYERSON, P. (1962). Permeability of lymphatic vessels. Amer. J. Physiol. 203,98-106. NORDIN, U., ENGSTR~M, B., JANSSON, B., AND LINDHOLM, C. E. (1977a). Surface structure and vascular anatomy of the tracheal wag under normal conditions and after intubation. Acta Oto-Laryngol. (Suppl.) 345. NORDM, U., ENGSTRSM, B., AND LINDHOLM, C. E. (1977b). Surface structure of the tracheal wag after different durations of intubation. Actu Oto-Lmyngol. (Suppl.) 345. NORDIN, U., AND LINDHOLM, C. E. (1977). The vessels of the rabbit trachea and ischemia caused by cuff pressure. Arch. Oto-Rhino-Laryngol. 215, 1 l-24. NORDIN, U., LINDHOJ..M,C. E., AND WOLGAST, M. (1977~). Blood flow in the rabbit tracheal mucosa under normal conditions and under the influence of tracheal intubation. Acta Anaesthesiol. Stand. 21,8 l-94. OLVER, R. E., DAVIS, B., MARIN, M. G., AND NADEL, J. A. (1975). Active transport of Nat and Cl- across the canine tracheal epithelium in vitro. Amer. Rev. Resp. Dis. 112,81 I-815. PAPPENHEIMER, J. R., RENKIN, E. M., AND BORRERO, L. M. (1951). Filtration, diffusion and molecular sieving through peripheral capillary membranes: A contribution to the pore theory of capillary permeability. Amer. J. Physiol. 167, 13. SMIONESCLJ, N., SIMIONESCU, M., AND PALADE, G. E. (1975). Permeability of muscle capillaries to small heme-peptides. J. Cell Biol. 64, 586-607. WIEDERHIELM, C. A., WOODBLJRY,J. W., KIRK, S., AND RUSHMER, R. F. (1964). Pulsatile pressures in the microcirculation of frog’s mesentery. Amer. J. Physiol. 207, 173-176.