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Blood flow velocity in capillaries of brain and muscles and its physiological significance
MICROVASC!ULAR
Blood
RESEARCH
22, 143-155 (1981)
Flow Velocity
in Capillaries of Brain and Muscles Physiological Significance K. P.
and Its
M. K. KALININA, Yu. I. LEVKOVICH
IVANOV,
AND
Pavlov Institute of Physiology of the Academy of Sciences Leningrad 199164, USSR
of
the USSR,
Received April I, 1980
Intravital microfilming by means of a dark-field contact epiobjective was used for measuring capillary blood flow velocity in the brain and skeletal muscles of the rat. The linear flow rate in capillaries was determined by measuring the rate of motion of plasma-filled “gaps” in the continuous erythrocyte flow. The mean linear red cell velocity for 100 cerebral capillaries 2-5 pm in diameter was found to be 0.79 t 0.03 mm/set. In the temporalis muscle the velocity was equal to 1.14 f 0.04 mmisec in 123 capillaries and 2.43 2 0.08 mm/set in 34 arterioles and precapillaries not more than 5 pm in luminal diameter. The experimentally obtained average values of blood flow velocities in cerebral capillaries indicate that these velocities vary mainly from 0.5 to 1.5 mm/set. This agrees with previously performed calculations based on the mathematical model which suggests that this range of velocities is optimal to adequately supply neurons with oxygen.
INTRODUCTION Oxygen pressure (PO,) in living tissues depends on many factors such as the oxygen capacity and actual saturation of the blood with oxygen, density of the capillary network, shape and location of the oxyhemoglobin dissociation curve, the blood flow velocity through capillaries, and so on. Barcroft (1934) was the first to develop a theoretical analysis of the role of blood flow velocity in the regulation of tissue ~0,. Studies of blood flow velocity through capillaries and its physiological variations are of interest for the brain where there are few or no reserve capillaries (Fulton, 195.5; Opitz and Schneider, 1950; Schneider, 1950). An increase in blood hemoglobin concentration or a shift of the dissociation curve to the right takes time. That is why for the brain a change in the velocity of blood flow through capillaries is supposed to be the most important physiological mechanism to improve oxygen transport as the oxygen content in the blood falls or as the demand of nerve cells for oxygen increases. It is well known that reserve capillaries do exist in muscle and their mobilization takes place during transition of muscles from a resting to a contractive state when oxygen demand increases considerably. This powerful physiological reaction 1.0 increase oxygen delivery was discovered by Krogh (1924). A detailed 143 0026-2862/81/050143-1)$02.00/0 Copyright 0 1981 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in U.S.A.
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by Grtinewald and Sowa (1977). Thus, the oxygen supply to muscle tissue is regulated in a different manner than that to cerebral tissue. The question naturally arises as to whether the oxygen supply to muscles can be regulated not only by opening up of reserve capillaries but also by changes in the velocity of capillary blood flow. Up to now direct measurements of the blood flow velocity through capillaries of various organs have been extremely difficult. There are few articles on this subject, which was reviewed several years ago by Fronek and Zweifach (1977). Rosenblum (1969, 1971, 1975) and Koo and Cheng (1976) measured the blood flow velocity in pial arterioles and venules, and Ma et al. (1974) performed similar measurements in capillaries of subarachnoid space of anesthetized animals. A few attempts to measure directly the microvessel flow velocity were carried out on dissected skeletal muscles (Fronek and Zweifach, 1977; Branemark and Eriksson, 1972; Eriksson and Lisander, 1972; Eriksson and Myrhage, 1972; Burton and Johnson, 1972; Johnson et al., 1976; Henrich and Hecke, 1978; Intaglietta et al., 1975; , Tuma et al., 1975). The purpose of the present study was to measure directly the red cell velocity in capillaries of cerebral cortex and skeletal muscles of unanesthetized animals by a specially developed technique. MATERIALS
AND METHODS
The ,studies were carried out using a cinema-TV complex for observing and microfilming minute blood vessels of brain and muscles in situ with the help of a contact dark-field epiobjective (20 x 0.60). The objective was brought into contact with the object under investigation (surface of brain or muscle). The depth of vision was controlled by varying the effective length of the microscope body tube, i.e., the distance between the objective and the eyepiece. With such focusing the distance between the objective and the object studied did not change and the objective merely touched the object without damaging it. With the effective magnification of 300 x we could observe and film blood vessels 2-5 pm in diameter and more at the tissue depth to 70 pm. The best picture contrast at maximum actinic brightness was obtained while illuminating the object by an ultra-high-pressure mercury-quartz lamp ( 11P Ill250-2 or HBO-500 type) without contrast-light filters. Two zones of line emission of these lamps (405-436 and 546-577 nm) coincide with those parts of the spectrum where the absorption of light by hemoglobin is the largest. The light flux emitted by this type of lamp is absorbed completely by erythrocytes whereas cerebral and muscle tissues absorbing light nonselectively reflect the incident light. A movie camera was used for filming blood vessels at the rate of 40 frames/ sec. Image focusing throughout the experiment was performed on a television screen with weak blue light, since the TV camera iconoscope is highly sensitive in the short-wave region of the spectrum (about 400 nm). The blue filter was removed for the period of filming, approximately lo-20 sec. Figure 1 is a schematic diagram of a cinema-TV complex with a contact epiobjective. From a lamp (1) the light flux is directed by a condenser (2) to a ring diaphragm (5) through a heat-protective filter (3) and a blue light filter (4). An elliptical mirror (8) turns a ring-shaped light flux to the periphery of the
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epiobjective (7), so that only the preparation surface which contacts with a frontal lens of the objective is illuminated. Nonilluminated underlying layers of the preparation produce the effect of dark field, thus greatly increasing the picture contrast. Focusing is achieved by moving an eyepiece (9), which focuses the image on the vidicon target (16) with the help of an objective (15); visual control is performed on a TV kinescope (17). For filming, the camera (13) is set against the eyepiece by moving along the AA-A’AI axis (9), and the control of field of vision is accomplished through a view-finder (14) of the camera. An animal was fixed on a special stage of the installation equipped with a mechanical device for vertical and horizontal displacement of the stage. Using this device the surface of brain or muscle of the animal was brought into contact with a frontal lens. The site for filming was selected by a horizontal motion of the stage with the animal. The depth of vision made possible by the relative transparency of cerebral and muscle tissue structures was regulated optically, as noted above, by changing the effective length of the microscope body tube (1) (Fig. 1). A special scale on the body tube was provided for measuring the depth at which filming was carried out by a known expression: 1 = V& * Au/n, where 1 is the magnitude of variation of the effective length of the body tube, mm; A (T is the change in the depth of vision, mm; V,, is the objective ratio; n is the refractive index of a tissue. Surface structures of a tissue served as a reference point for counting 1. As mentioned above, the limiting depth of visual observation and filming of vessels was equal to 0.070 mm (1 being 21 mm) for brain and muscles. Structures lying closer to the surface disappeared from the field of vision, since the depth of focus was not more than lo-15 pm. The experiments were performed on white Wistar rats (female) weighing 260-280 g. Animals were mounted on the stage with a special clamp fixing the
FIG. 1. A schematic diagram of a cinema-television complex with a contact epiobjective. (1) Mercury-quartz lamp, (2) condenser, (3) heat-protecting filter, (4) blue light filter, (5) ring diaphragm, (6) object of filming, (7) epiobjective, (8) elliptic mirror, (9) eyepiece, (10) prism, (11) and (12) camera lenses, (l3) camera “Rodina” (14) view-finder, (15) TV lens, (16) vidicon target, 17 TV kinescope.
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animal’s head to the required position. To observe vessels of cerebral cortex, a rectangular window approximately 6 mm wide and 8 mm long was made in the parietal bones. The dura mater at this site was removed without damaging the pia mater. The animal’s head was oriented in a horizontal plane. An exposed surface of cerebral hemispheres was brought into touch with a contact objective. To study muscle microvessels, the temporalis was used. An incision in the skin was made, the aponeurosis was removed, and a muscle area approximately 8 x 10 mm in size was exposed. The animal’s head was turned around the long axis of the body so that the surface of the temporalis could be oriented in a horizontal plane and then the head was fixed in this position. An exposed area of the muscle was brought into touch with a contact objective. Surgery was performed under light ether anesthesia and novocaine applied locally about 1 hr prior to the beginning of the experiment; therefore, all observations were carried out without anesthesia. To maintain the temperature of brain and muscle at 35-37” the contact objective was heated and cerebral and muscle surfaces were superfused with McIllwain and Ringer’s solutions, respectively, at 37” (pH 7.35). The stage was also heated, which permitted maintenance of rectal temperature at 37.5 + 0.5”. Throughout the experiment the value of the systemic arterial blood pressure was continually monitored via a cannula in the femoral artery. All the experiments were performed at blood pressure in the limits of 130-80 mm Hg. The most difficult problem in developing methods for direct measurement of the blood flow velocity through capillaries was the search for a “mark” by which blood motion along the vessel could be observed. Our idea was as follows: Erythrocytes in capillaries move in one row one after another. Observational data indicate that at normal arterial pressure there are gaps, spaces filled with plasma, in the continuous erythrocyte flow in different capillaries of brain and muscles. Such gaps appear and pass irregularly through a vessel from time to time. On a film a gap appears as a light space clearly seen on the dark background of erythrocyte flow. Figure 2 shows one of the cerebral cortex capillaries at the moment when such a gap passes. In the figure, two subsequent frames are given. In the second picture the gap is seen to shift, due to blood flow. As the interval between two frames is 0.025 set we can calculate easily the rate of the gapmotion and, consequently, the blood flow velocity through the capillary. In this case the shift was 72 pm per 0.1 set (4 frames). The average velocity for the vessel involved during the period of measurement was 0.72 mm/set. To obtain a more precise value of the shift, the picture was magnified several times and a special calibrated rule with the scale spacing of 1 pm was used for measurements. The reasons for errors in measurements may be as follows: 1. Inaccuracy in measuring the gap shift due to a washing out of its contours (l-2%). 2. Changes in the shape and length of the gap itself resulting from the nonuniformity of the vessel diameter along its length and deflection of a vessel from the observer or towards him. A special checking out and calculations have shown that this error is small and amounts to not more than l-2%. 3. Unevenness in the functioning of a mechanical part of the camera. This error is the smallest and according to the technical characteristics of the camera
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FIG. 2. Brain capillaries at the depth of 40 pm. The interval between frames presented is 0.1 sec. The shift of the plasma gap in a stream of erythrocytes is seen (arrows).
is not more than 1%. The overall error of measurement was calculated to be 3-5%. The ,method described above needs no injection of foreign substances into the blood stream to obtain a tracer and can be used on unanesthetized animals under physiological conditions. Gaps in the continuous erythrocyte flow are observed practically in all capillaries in steady state and at normal blood pressures studied so far, and they are of physiological origin. The disadvantage of this method is
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that measurement of the blood flow velocity can occur only when erythrocytes move through a vessel in one row. The procedural sequellae were as follows: A vessel (or a field of vision with two or three vessels) was filmed continually for lo-20 set; this was repeated once or twice within several minutes. In so doing we managed to film a minimum 10 to 20 gaps in every capillary. While measuring the image, the velocity of flow was estimated, on average, 10 times for each capillary, the mean velocity through a given vessel being inferred from these measurements. The linear rate of blood flow was measured in 100 microvessels (from 2 to 5 km in diameter) of surface layers of cerebral cortex of 28 rats at the depth of 15-70 pm; and in 157 microvessels of the temporalis of 30 other rats, the diameter of temporalis microvessels was also 2-5 km. It should be noted that by the diameter we mean the borders of flowing erythrocytes which are clearly seen on the film. Results are presented as means *SE. RESULTS The microvessels of the surface layers of rats’ cerebral cortex studied had the following morphophysiological features: 1. The vessels investigated had the smallest diameter of all the vessels which could be observed on the cerebral cortex. 2. The walls of these vessels were not visible whereas they could be seen in larger vessels. 3. The vessels of the indicated diameter formed a complicated network, their length between network branchings amounting to 60-200 km. Some vessels joined venules. 4. In all the vessels studied the erythrocytes moved in one row one after another, except for the gap filled with plasma. These features allowed us to regard these microvessels as capillaries. Observations of capillary blood flow and measurements using the technique described indicated that the velocity of a capillary blood flow varies continuously. At constant blood pressure the flow velocity in an individual capillary may vary over a wide range around some average value during lo-20 set (time of continual filming). An average of 10 separate measurements of blood flow velocity have been performed for each capillary. The mean velocity for a given capillary was deduced from these measurements. One such capillary is demonstrated above in Fig. 2. The average velocity was determined for 100 capillaries of cerebral cortex. The distribution of mean velocities of blood flow in these capillaries is shown by a histogram in Fig. 3. From this figure it is seen that for a major part of capillaries (63%) the average velocity is in the range 0.5-1.5 mm/set. The blood flow velocity below 0.3 and above 1.5 mm/set has been registered only in 5% and 3% of the capillaries studied, respectively. The average velocity for all capillaries amounted to 0.79 2 0.03 mmlsec. Muscles microvessels of a similar luminal diameter (2-5 km) were, as a rule, -located along muscle fibers. Their length from the arterial end to the venous end was 800-2000 Frn, though at intervals of 200-600 km they were connected by anastomoses. On the basis of the morphophysiological features listed we class-
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y 251
wehcify, FIG.
3’. The frequency distribution
mm/s
of mean blood velocities in brain capillaries.
ified these vessels as capillaries. Among 157 microvessels of muscles studied 123 microvessels were classified as capillaries. Figure 4 shows several capillaries. In Figure SA the distribution of average velociti.es of blood flow in muscle capillaries is shown by a histogram. As one can see in the majority of muscle capillaries (75%) the flow velocity ranges from 0.6 to 1.5 mm/set. In 17% of capillaries the velocity is above 1.5 mm/set. The average velocity of blood flow is 1.14 + 0.04 mm/set, which is 42% larger than that in the cerebral capillaries. In muscles blood flow velocity was also measured in minute arterioles and precapillaries. Some vessels with the luminal diameter of 2-5 km and one-row erythrocyte flow had obvious morphological difference from capillaries. They branched from arterial stems and then gave rise to several (two or three) vessels with similar diameter. Moreover, these vessels were not located strictly along muscle fibers, and they were shorter than microvessels which were judged as capillaries. We examined 34 such vessels. The average velocity of blood flow was 2.43 ? 0.08 mm/set, or twice as high as that observed in capillaries (Fig. 5B). One vessel is shown in Fig. 6. DISCUSSION It is commonly known that the blood flow velocity in normal microcirculatory beds v;aries continuously dependent upon the functional state of organs (Zweifach, 1977). In our brain experiments we always observed blood redistribution on the arteriole and precapillary level which resulted from the intermittent flow in the arteriolar anastomoses or from the alterations of flow direction, as reported by Rosenblum and Zweifach (1963). We also observed the continuous variations of blood flow velocity in capillaries. The increase of flow velocity could take place in some capillaries simultaneously with the transitory slowing or stopping of RBC flow in adjacent capillaries that often results from passing of white blood cells. Our goal was a determination of capillary velocity in order to estimate the efficiency of the oxygen transport in tissue. The mean linear flow velocity was calculated from 1100 separate measurements for 100 capillaries (2-5 Frn i.d.) of parietal cortex. This value (0.79 & 0.03 mm/ set) is similar to data reported by Ma er al. (1974) for the smallest pial vessels, using the two-slit photometric method. They found the mean velocity of 0.75 + 0.15 mm/set and 0.79 + 0.18 mm/set for venous capillaries 3-5 p.m i.d. and minute venules 5-12 p,m i.d. The authors also reported an average velocity of
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FIG. 4. Capillaries of temporalis at the depth of 30 pm. The thin arrows indicate the Iihil ‘t of gap during 0.15 set (6 frames). Blood flow velocity in this capillary is 1.34 mm/se C. The thick a.rrow shows the border of a muscle fiber. plasma
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UL-
FIG. 5. The frequency distribution of the mean blood flow velocities in capillaries (A) and terminal arterioles (B) of temporalis.
FIG. 6. Terminal arterioles in rat temporalis at the depth of 50 pm. (a) An arteriola with diameter of 10 km; (b) a branch with the lumen of 5 pm, which is subdivided into two capillaries; (c). The arrow shlows the border of muscle fiber.
4.6 mmlsec (from 0.83 to 10.5 mmlsec) in minute arterial branches, with diameters ranging from 3 to 8 km (arterioles and arterial capillaries according to their terms). We have not succeeded in observing the arteriole branching into capillaries in brain up to the depth of 70 Km but we frequently observed terminal arterioles and arteriolar anastomoses with lumen diameters of 10 or 5 pm. We have no quantitative data concerning the blood flow velocity in these vessels. Rosenblum (1969) measured mean velocity of 3.5 mm/set in pial arterioles with the diameter
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in the range IO-30 Frn using high-speed cinematography. This value is similar to data reported by Ma et al. (1974) for arterial vessels of first and second orders. Our method can be applied only for capillaries with lumens that are approximately equal to the size of red cells. It was impossible to record the gaps between RBC in larger vessels where they do not move in one row when the framing rate is 40 frames/set. Rosenblum (1969) stated that rhythmic alteration in velocity in pial arterioles of 30 km i.d. was in accordance with cardiac cycle. This velocity pulse was also seen occasionally in venules but was not recognizable in vessels 5 to 10 p,rn wide. We were able to observe gap movement on 8 to 12 frames in sequence, that is, during 0.2 to 0.3 set (two or three consecutive heart beats) but we did not observe any regularity of blood velocity change in capillaries that was coincidental to the cardiac cycle. There are a few reports dealing with the direct measurements of linear RBC velocity in muscle capillaries. An average velocity of 0.38 + 0.02 mm/set was found in capillaries of the cat sartorius muscle (Burton and Johnson, 1972; Johnson et al., 1976). For the cat tenuissimus muscle Eriksson and Myrhage (1972) observed a variation of flow velocity from 0 to 1.5 mm/set and the average velocity was measured to be 0.5 mm/set, and Intaglietta et al. (1975) measured velocities up to 10 mm/set. Tuma et al. (1975) reported a mean capillary flow velocity of 0.38 mm/set in rabbit tenuissimus. Klitzman and Duling (1979) found that in hamster cremaster muscle, capillary flow averaged 0.21 mm/set. The only available RBC velocity data for rat striated muscle capillaries (0.26 & 0.05 mm/set) was obtained by Henrich and Hecke (1978) for gracilis muscle capillaries. All these measurements were performed on dissected and isolated muscles in narcotized animals. For the rat temporalis vessels, we obtained a higher value of the RBC velocity than reported by other investigators. An average velocity of 1.14 ? 0.08 mm/ set was calculated from 1300 separate measurings with 123 capillaries in 30 rats. The difference may be due to the fact that in our procedure a minimum surgical procedure was applied on the in situ temporalis. The intact muscle was exposed by a skin incision and careful removal of the fascia. An animal was discarded if the smallest damage to the muscle surface occurred. Another positive feature of our procedure is that the experiments were carried out on unanesthetized or lightly anesthetized animals, which helps maintain muscle tone. The method presented here did not allow quantitation of RBCfilled working capillaries and capillaries filled by plasma or closed. However, the number of working capillaries was counted in a few layers of the temporalis muscle by analysis of the films (Kalinina et al., 1979). The average distance between capillaries was calculated to be 50 t 3.5 km; the capillary density was found to be about 330 capillaries/mm2. This result agrees with the data of direct in vivo measurements by Fronek and Zweifach (1977). They found that only about 30-35% of the cat tenuissimus capillaries were actively perfused under resting conditions of the muscle. The mean velocity of 2.43 ~fr 0.8 mm/set, measured in the temporalis terminal arterioles and precapillaries (lumen of 3-6 km) is similar to the mean velocity of 3.8 + 1.0 mm/set in cat tenuissimus precapillaries (9 km) reported by Fronek and Zweifach (1977). Blood circulation through capillaries performs many physiological functions,
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and oxygen supply to tissues through capillaries can be considered one of the most important. Because oxygen is not stored in tissues, oxygen transport must meet the oxygen demand of tissues at every moment of time. The agreement between demand and supply is especially important for the brain, the functions of which are immediately disturbed in the absence of oxygen. Several years ago Ivanov and Kislyakov (1974) and Kislyakov and Ivanov (1974) improved previously known spatial mathematical models for diffusion of oxygen in cerebral tissue (Griinewald, 1968, 1969, 1973; Gtinewald and Sowa, 1977). A complicated mathematical analysis of oxygen transport from capillaries to cerebral tissue and mechanisms controlling this transport was proposed. The calculations were based on experimental data related to capillary network density in cerebral cortex, diameter and length of separate capillaries, overall oxygen consumption by cerebral tissue, oxyhemoglobin dissociation curve, oxygen capacity of the blood, and so on. Under these conditions it was estimated that blood flow velocity through capillaries of cerebral cortex should range between 0.3-0.5 and 1.5-1.7 mm/set. It cannot be less than 0.3-0.5 mm/set, since neurons will not receive an adequate supply of oxygen. It cannot be more than 1.5-1.7 mm/set because at this velocity the oxygen supply to nerve cells will not be improved. At this velocity the ~0, at the venous end of a capillary is close to that at the arterial end. In other words, at the velocity of capillary blood flow of 1.5-1.7 mm/set the physiological reserve for improved delivery of oxygen at the expense of increased velocities of blood flow turns out to be practically exhausted. Blood flow changes in capillaries from 0.5 to 1.0-1.2 mm/set regulate the oxygen diffusion from capillaries to tissue most efficiently. Even small changes in the velocities within this range affect oxygen transport to tissues. Acco.rding to the experimental data described, in the vast majority of capillaries flow velocities indeed lie within the range 0.5-1.5 mm/set, the velocity being less tha.n 0.3 and more than 1.5 mm/set only in 5 and 3% of capillaries, respectively. Thus the experimental data are consistent with the calculated values. For mufscles the relationships are somewhat different. The difference in capillary length is probably responsible for a different capillary blood flow velocity and efficiency. The mean capillary length in skeletal muscle is about three times that of brain capillaries, which may explain the higher capillary flow velocity in resting and working muscle when compared to the brain. With a similar velocity tissue oxygen supply would be unsatisfactory at the venous end of a muscle capillary. To determine oxygen pressure distribution in a volume of muscle tissue the mathematical calculation was based on capillary density and blood velocity data (Lyabach and Ivanov, 1979). The calculation gave ~0, of 40 torr at the point of lowest oxygen supply. This indicates that temporalis muscle is saturated sufficiently with oxygen when the capillary blood velocity is 1.14 & 0.08 mm/ set witlh a density of 330 working capillaries/mm*. A considerable increase in the oxygen consumption during transition of a resting muscle into actively contracting muscle leads to a sharp fall of ~0, in muscle. However, an increase in the capillary velocity to 2.0-2.5 mm/set as was cited (Lyabach and Ivanov, 1979) allows tissue oxygen pressure to be maintained at a sufficiently high level to compensate completely for the oxygen deficiency. The increased oxygen consumption of the working muscle was fourfold. This observation suggests that a variation in the velocity of capillary blood flow in muscles in physiological limits
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is a powerful physiological mechanism to compensate for the oxygen deficiency, which can be compared with the opening of reserve capillaries. REFERENCES BARKROFT, J. (1934). “Features in the Architecture of Physiological Function.” Cambridge Univ. Press, Cambridge. BR~NEMARK, P. L., AND ERIKSSON,E. (1972). Method for studying qualitative and quantitative change of blood flow in skeletal muscle. Acta Physiol. Stand. 84, 284-288. BURTON, K. S., AND JOHNSON,P. C. (1972). Reactive hyperemia in individual capillaries of skeletal muscle. Amer. J. Physiol. 223, 517-524. ERIKSSON,E., AND LISANDER, B. (1972). Low flow states in the microvessels of skeletal muscle in the cat. Acta Physiol. Stand. 86, 202-210. ERIKSSON,E., AND MYRHAGE, R. (1972). Microvascular dimensions and blood flow in skeletal muscle. Acta Physiol. Stand. 86, 211-223. FRONEK, K., AND ZWEIFACH, B. W. (1977). Microvascular blood flow in cat tenuissimus muscle. Microvasc. Res. 14, 181-189. FULTON, Y. (1955). “Textbook of Physiology.” Saunders, Philadelphia/London. GR~NEWALD, W. (1968). Theoretical analysis of the oxygen supply in tissue. In “Oxygen Transport in Blood and Tissue” (D. W. Lubbers, U. C. Luft, J. Thews, and E. Witzleb, eds.), pp. 100-114. Thieme, Stuttgart. GR~NEWALD, W. (1969). Digitale Simulation eines raumlichen Diffusionsmodelles der 0, - Versorgung biologischer Gewebe. Pjliiegers Arch. 309, 266-284. GR~NEWALD, W. (1973). Computer calculation for tissue oxygenation and the meaningful presentation of the results. In “Advances in Experimental Medicine and Biology,” Vol. 37B, “Oxygen Transport to Tissue” (H. I. Bicher and D. F. Bruley, eds.), pp. 783-792. Plenum, New York/London. GR~NEWALD, W., AND SOWA, W. (1977). Capillary structures and O2 supply to tissue. Rev. Physiol. Biochem. Pharmacol. 77, 149-209. HENRICH,H. N., AND HECKE, A. (1978). A gracilis muscle preparation for quantitative microcirculatory studies in the rat. Microvasc. Res. 15, 349-356. INTAGLIETTA, M., SILVERMAN, N. R., AND TOMPKINS, W. R. (1975). Capillary flow velocity measurements in vivo and in situ by television method. Microvasc. Res. 10, 165-179. IVANOV, K. P., AND KISLYAKOV, Yu. YA. (1974). The oxygen available in a neuron and surrounding tissue. Setchenov Physiol. J. USSR 60, 900-905. JOHNSON,P. C., BURTON, K. S., HENRICH, H., AND HENRICH, U. (1976). Effect of occlusion duration on reactive hyperemia in sartorius muscle capillaries. Amer. J. Physiol. 230, 715-719. KALININA, M. K., LEVKOVICH, Yu. I., IVANOV, K. P., AND MICKAILOVA, G. P. (1979). Blood flow in the skeletal muscle microvessels in normoxia and arterial hypoxemia. Setchenov Physiol. /. USSR 65,
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KISLYAKOV. Yu. YA., AND IVANOV, K. P. (1974). Distribution of p0, in neurons and brain capillaries as a function of blood flow velocity at normoxia and hypoxemia. Setchenov Physiol. J. USSR 60, 1216-1222. KLITZMAN, B., AND DULING, B. R. (1979). Microvascular hematocrit and red cell flow in resting and contracting striated muscle. Amer. J. Physiol. 237(4), H481-H490; Amer. J. Physiol. Heart Circ. Physiol. 6(4), H481-H490. KOO, A., AND CHENG, K. K. (1976). Vascular escape in the cerebral microcirculation in the rat. Microvasc.
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KROGH, A. (1924). “The Anatomy and Physiology of Capillaries.” Elliots Books, Northford, Corm. IYABACH, E. G.. AND IVANOV, K. P. (1979). On the physiological regulation of oxygen transport iu muscles. Dokl. Akad. Nauk USSR 248, N2, 488-491. MA, Y. P., KOO, A., KWAN, H. C., AND CHENG, K. K. (1974). On-line measurement of the dynamic velocity of erythrocytes in the cerebral microvessels in the rat. Microvasc. Res. 8, l-13. OPITZ, E., AND SCHNEIDER,M. (1950). Uber gie Sauerstoffversorguug des Cehiru. ,?rgeb. Physio/, 46,
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W. I. (1975). Effect of pial arteriolar constriction on red cell velocity in pial venules and on venular diameter. Microvasc. Res. 9, 38-42. ROSENBLUM, W. I., AND ZWEIFACH, B. W. (1963). Cerebral microcirculation in the mouse brain. Arch.
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M. (1950). Z. Nervenheilkunde, 162, 113-135. F., CHILDS, C. M., INTAGLIETTA, M., AND ARFORS, K. -E. (1975). Microvascular flow pattern in the t.enuissimus muscle. Bib/. Amt. 13, 151-152. ZWEIFACHI, B. W. (1977). Perspectives in microcirculation. In “Microcirculation,” Vol. 1, pp. l-19. Univ. Park Press, Baltimore/London/Tokyo. SCHNEIDER,
TUMA,
R.
Blood
RESEARCH
22, 143-155 (1981)
Flow Velocity
in Capillaries of Brain and Muscles Physiological Significance K. P.
and Its
M. K. KALININA, Yu. I. LEVKOVICH
IVANOV,
AND
Pavlov Institute of Physiology of the Academy of Sciences Leningrad 199164, USSR
of
the USSR,
Received April I, 1980
Intravital microfilming by means of a dark-field contact epiobjective was used for measuring capillary blood flow velocity in the brain and skeletal muscles of the rat. The linear flow rate in capillaries was determined by measuring the rate of motion of plasma-filled “gaps” in the continuous erythrocyte flow. The mean linear red cell velocity for 100 cerebral capillaries 2-5 pm in diameter was found to be 0.79 t 0.03 mm/set. In the temporalis muscle the velocity was equal to 1.14 f 0.04 mmisec in 123 capillaries and 2.43 2 0.08 mm/set in 34 arterioles and precapillaries not more than 5 pm in luminal diameter. The experimentally obtained average values of blood flow velocities in cerebral capillaries indicate that these velocities vary mainly from 0.5 to 1.5 mm/set. This agrees with previously performed calculations based on the mathematical model which suggests that this range of velocities is optimal to adequately supply neurons with oxygen.
INTRODUCTION Oxygen pressure (PO,) in living tissues depends on many factors such as the oxygen capacity and actual saturation of the blood with oxygen, density of the capillary network, shape and location of the oxyhemoglobin dissociation curve, the blood flow velocity through capillaries, and so on. Barcroft (1934) was the first to develop a theoretical analysis of the role of blood flow velocity in the regulation of tissue ~0,. Studies of blood flow velocity through capillaries and its physiological variations are of interest for the brain where there are few or no reserve capillaries (Fulton, 195.5; Opitz and Schneider, 1950; Schneider, 1950). An increase in blood hemoglobin concentration or a shift of the dissociation curve to the right takes time. That is why for the brain a change in the velocity of blood flow through capillaries is supposed to be the most important physiological mechanism to improve oxygen transport as the oxygen content in the blood falls or as the demand of nerve cells for oxygen increases. It is well known that reserve capillaries do exist in muscle and their mobilization takes place during transition of muscles from a resting to a contractive state when oxygen demand increases considerably. This powerful physiological reaction 1.0 increase oxygen delivery was discovered by Krogh (1924). A detailed 143 0026-2862/81/050143-1)$02.00/0 Copyright 0 1981 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in U.S.A.
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by Grtinewald and Sowa (1977). Thus, the oxygen supply to muscle tissue is regulated in a different manner than that to cerebral tissue. The question naturally arises as to whether the oxygen supply to muscles can be regulated not only by opening up of reserve capillaries but also by changes in the velocity of capillary blood flow. Up to now direct measurements of the blood flow velocity through capillaries of various organs have been extremely difficult. There are few articles on this subject, which was reviewed several years ago by Fronek and Zweifach (1977). Rosenblum (1969, 1971, 1975) and Koo and Cheng (1976) measured the blood flow velocity in pial arterioles and venules, and Ma et al. (1974) performed similar measurements in capillaries of subarachnoid space of anesthetized animals. A few attempts to measure directly the microvessel flow velocity were carried out on dissected skeletal muscles (Fronek and Zweifach, 1977; Branemark and Eriksson, 1972; Eriksson and Lisander, 1972; Eriksson and Myrhage, 1972; Burton and Johnson, 1972; Johnson et al., 1976; Henrich and Hecke, 1978; Intaglietta et al., 1975; , Tuma et al., 1975). The purpose of the present study was to measure directly the red cell velocity in capillaries of cerebral cortex and skeletal muscles of unanesthetized animals by a specially developed technique. MATERIALS
AND METHODS
The ,studies were carried out using a cinema-TV complex for observing and microfilming minute blood vessels of brain and muscles in situ with the help of a contact dark-field epiobjective (20 x 0.60). The objective was brought into contact with the object under investigation (surface of brain or muscle). The depth of vision was controlled by varying the effective length of the microscope body tube, i.e., the distance between the objective and the eyepiece. With such focusing the distance between the objective and the object studied did not change and the objective merely touched the object without damaging it. With the effective magnification of 300 x we could observe and film blood vessels 2-5 pm in diameter and more at the tissue depth to 70 pm. The best picture contrast at maximum actinic brightness was obtained while illuminating the object by an ultra-high-pressure mercury-quartz lamp ( 11P Ill250-2 or HBO-500 type) without contrast-light filters. Two zones of line emission of these lamps (405-436 and 546-577 nm) coincide with those parts of the spectrum where the absorption of light by hemoglobin is the largest. The light flux emitted by this type of lamp is absorbed completely by erythrocytes whereas cerebral and muscle tissues absorbing light nonselectively reflect the incident light. A movie camera was used for filming blood vessels at the rate of 40 frames/ sec. Image focusing throughout the experiment was performed on a television screen with weak blue light, since the TV camera iconoscope is highly sensitive in the short-wave region of the spectrum (about 400 nm). The blue filter was removed for the period of filming, approximately lo-20 sec. Figure 1 is a schematic diagram of a cinema-TV complex with a contact epiobjective. From a lamp (1) the light flux is directed by a condenser (2) to a ring diaphragm (5) through a heat-protective filter (3) and a blue light filter (4). An elliptical mirror (8) turns a ring-shaped light flux to the periphery of the
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epiobjective (7), so that only the preparation surface which contacts with a frontal lens of the objective is illuminated. Nonilluminated underlying layers of the preparation produce the effect of dark field, thus greatly increasing the picture contrast. Focusing is achieved by moving an eyepiece (9), which focuses the image on the vidicon target (16) with the help of an objective (15); visual control is performed on a TV kinescope (17). For filming, the camera (13) is set against the eyepiece by moving along the AA-A’AI axis (9), and the control of field of vision is accomplished through a view-finder (14) of the camera. An animal was fixed on a special stage of the installation equipped with a mechanical device for vertical and horizontal displacement of the stage. Using this device the surface of brain or muscle of the animal was brought into contact with a frontal lens. The site for filming was selected by a horizontal motion of the stage with the animal. The depth of vision made possible by the relative transparency of cerebral and muscle tissue structures was regulated optically, as noted above, by changing the effective length of the microscope body tube (1) (Fig. 1). A special scale on the body tube was provided for measuring the depth at which filming was carried out by a known expression: 1 = V& * Au/n, where 1 is the magnitude of variation of the effective length of the body tube, mm; A (T is the change in the depth of vision, mm; V,, is the objective ratio; n is the refractive index of a tissue. Surface structures of a tissue served as a reference point for counting 1. As mentioned above, the limiting depth of visual observation and filming of vessels was equal to 0.070 mm (1 being 21 mm) for brain and muscles. Structures lying closer to the surface disappeared from the field of vision, since the depth of focus was not more than lo-15 pm. The experiments were performed on white Wistar rats (female) weighing 260-280 g. Animals were mounted on the stage with a special clamp fixing the
FIG. 1. A schematic diagram of a cinema-television complex with a contact epiobjective. (1) Mercury-quartz lamp, (2) condenser, (3) heat-protecting filter, (4) blue light filter, (5) ring diaphragm, (6) object of filming, (7) epiobjective, (8) elliptic mirror, (9) eyepiece, (10) prism, (11) and (12) camera lenses, (l3) camera “Rodina” (14) view-finder, (15) TV lens, (16) vidicon target, 17 TV kinescope.
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animal’s head to the required position. To observe vessels of cerebral cortex, a rectangular window approximately 6 mm wide and 8 mm long was made in the parietal bones. The dura mater at this site was removed without damaging the pia mater. The animal’s head was oriented in a horizontal plane. An exposed surface of cerebral hemispheres was brought into touch with a contact objective. To study muscle microvessels, the temporalis was used. An incision in the skin was made, the aponeurosis was removed, and a muscle area approximately 8 x 10 mm in size was exposed. The animal’s head was turned around the long axis of the body so that the surface of the temporalis could be oriented in a horizontal plane and then the head was fixed in this position. An exposed area of the muscle was brought into touch with a contact objective. Surgery was performed under light ether anesthesia and novocaine applied locally about 1 hr prior to the beginning of the experiment; therefore, all observations were carried out without anesthesia. To maintain the temperature of brain and muscle at 35-37” the contact objective was heated and cerebral and muscle surfaces were superfused with McIllwain and Ringer’s solutions, respectively, at 37” (pH 7.35). The stage was also heated, which permitted maintenance of rectal temperature at 37.5 + 0.5”. Throughout the experiment the value of the systemic arterial blood pressure was continually monitored via a cannula in the femoral artery. All the experiments were performed at blood pressure in the limits of 130-80 mm Hg. The most difficult problem in developing methods for direct measurement of the blood flow velocity through capillaries was the search for a “mark” by which blood motion along the vessel could be observed. Our idea was as follows: Erythrocytes in capillaries move in one row one after another. Observational data indicate that at normal arterial pressure there are gaps, spaces filled with plasma, in the continuous erythrocyte flow in different capillaries of brain and muscles. Such gaps appear and pass irregularly through a vessel from time to time. On a film a gap appears as a light space clearly seen on the dark background of erythrocyte flow. Figure 2 shows one of the cerebral cortex capillaries at the moment when such a gap passes. In the figure, two subsequent frames are given. In the second picture the gap is seen to shift, due to blood flow. As the interval between two frames is 0.025 set we can calculate easily the rate of the gapmotion and, consequently, the blood flow velocity through the capillary. In this case the shift was 72 pm per 0.1 set (4 frames). The average velocity for the vessel involved during the period of measurement was 0.72 mm/set. To obtain a more precise value of the shift, the picture was magnified several times and a special calibrated rule with the scale spacing of 1 pm was used for measurements. The reasons for errors in measurements may be as follows: 1. Inaccuracy in measuring the gap shift due to a washing out of its contours (l-2%). 2. Changes in the shape and length of the gap itself resulting from the nonuniformity of the vessel diameter along its length and deflection of a vessel from the observer or towards him. A special checking out and calculations have shown that this error is small and amounts to not more than l-2%. 3. Unevenness in the functioning of a mechanical part of the camera. This error is the smallest and according to the technical characteristics of the camera
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FIG. 2. Brain capillaries at the depth of 40 pm. The interval between frames presented is 0.1 sec. The shift of the plasma gap in a stream of erythrocytes is seen (arrows).
is not more than 1%. The overall error of measurement was calculated to be 3-5%. The ,method described above needs no injection of foreign substances into the blood stream to obtain a tracer and can be used on unanesthetized animals under physiological conditions. Gaps in the continuous erythrocyte flow are observed practically in all capillaries in steady state and at normal blood pressures studied so far, and they are of physiological origin. The disadvantage of this method is
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that measurement of the blood flow velocity can occur only when erythrocytes move through a vessel in one row. The procedural sequellae were as follows: A vessel (or a field of vision with two or three vessels) was filmed continually for lo-20 set; this was repeated once or twice within several minutes. In so doing we managed to film a minimum 10 to 20 gaps in every capillary. While measuring the image, the velocity of flow was estimated, on average, 10 times for each capillary, the mean velocity through a given vessel being inferred from these measurements. The linear rate of blood flow was measured in 100 microvessels (from 2 to 5 km in diameter) of surface layers of cerebral cortex of 28 rats at the depth of 15-70 pm; and in 157 microvessels of the temporalis of 30 other rats, the diameter of temporalis microvessels was also 2-5 km. It should be noted that by the diameter we mean the borders of flowing erythrocytes which are clearly seen on the film. Results are presented as means *SE. RESULTS The microvessels of the surface layers of rats’ cerebral cortex studied had the following morphophysiological features: 1. The vessels investigated had the smallest diameter of all the vessels which could be observed on the cerebral cortex. 2. The walls of these vessels were not visible whereas they could be seen in larger vessels. 3. The vessels of the indicated diameter formed a complicated network, their length between network branchings amounting to 60-200 km. Some vessels joined venules. 4. In all the vessels studied the erythrocytes moved in one row one after another, except for the gap filled with plasma. These features allowed us to regard these microvessels as capillaries. Observations of capillary blood flow and measurements using the technique described indicated that the velocity of a capillary blood flow varies continuously. At constant blood pressure the flow velocity in an individual capillary may vary over a wide range around some average value during lo-20 set (time of continual filming). An average of 10 separate measurements of blood flow velocity have been performed for each capillary. The mean velocity for a given capillary was deduced from these measurements. One such capillary is demonstrated above in Fig. 2. The average velocity was determined for 100 capillaries of cerebral cortex. The distribution of mean velocities of blood flow in these capillaries is shown by a histogram in Fig. 3. From this figure it is seen that for a major part of capillaries (63%) the average velocity is in the range 0.5-1.5 mm/set. The blood flow velocity below 0.3 and above 1.5 mm/set has been registered only in 5% and 3% of the capillaries studied, respectively. The average velocity for all capillaries amounted to 0.79 2 0.03 mmlsec. Muscles microvessels of a similar luminal diameter (2-5 km) were, as a rule, -located along muscle fibers. Their length from the arterial end to the venous end was 800-2000 Frn, though at intervals of 200-600 km they were connected by anastomoses. On the basis of the morphophysiological features listed we class-
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y 251
wehcify, FIG.
3’. The frequency distribution
mm/s
of mean blood velocities in brain capillaries.
ified these vessels as capillaries. Among 157 microvessels of muscles studied 123 microvessels were classified as capillaries. Figure 4 shows several capillaries. In Figure SA the distribution of average velociti.es of blood flow in muscle capillaries is shown by a histogram. As one can see in the majority of muscle capillaries (75%) the flow velocity ranges from 0.6 to 1.5 mm/set. In 17% of capillaries the velocity is above 1.5 mm/set. The average velocity of blood flow is 1.14 + 0.04 mm/set, which is 42% larger than that in the cerebral capillaries. In muscles blood flow velocity was also measured in minute arterioles and precapillaries. Some vessels with the luminal diameter of 2-5 km and one-row erythrocyte flow had obvious morphological difference from capillaries. They branched from arterial stems and then gave rise to several (two or three) vessels with similar diameter. Moreover, these vessels were not located strictly along muscle fibers, and they were shorter than microvessels which were judged as capillaries. We examined 34 such vessels. The average velocity of blood flow was 2.43 ? 0.08 mm/set, or twice as high as that observed in capillaries (Fig. 5B). One vessel is shown in Fig. 6. DISCUSSION It is commonly known that the blood flow velocity in normal microcirculatory beds v;aries continuously dependent upon the functional state of organs (Zweifach, 1977). In our brain experiments we always observed blood redistribution on the arteriole and precapillary level which resulted from the intermittent flow in the arteriolar anastomoses or from the alterations of flow direction, as reported by Rosenblum and Zweifach (1963). We also observed the continuous variations of blood flow velocity in capillaries. The increase of flow velocity could take place in some capillaries simultaneously with the transitory slowing or stopping of RBC flow in adjacent capillaries that often results from passing of white blood cells. Our goal was a determination of capillary velocity in order to estimate the efficiency of the oxygen transport in tissue. The mean linear flow velocity was calculated from 1100 separate measurements for 100 capillaries (2-5 Frn i.d.) of parietal cortex. This value (0.79 & 0.03 mm/ set) is similar to data reported by Ma er al. (1974) for the smallest pial vessels, using the two-slit photometric method. They found the mean velocity of 0.75 + 0.15 mm/set and 0.79 + 0.18 mm/set for venous capillaries 3-5 p.m i.d. and minute venules 5-12 p,m i.d. The authors also reported an average velocity of
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FIG. 4. Capillaries of temporalis at the depth of 30 pm. The thin arrows indicate the Iihil ‘t of gap during 0.15 set (6 frames). Blood flow velocity in this capillary is 1.34 mm/se C. The thick a.rrow shows the border of a muscle fiber. plasma
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UL-
FIG. 5. The frequency distribution of the mean blood flow velocities in capillaries (A) and terminal arterioles (B) of temporalis.
FIG. 6. Terminal arterioles in rat temporalis at the depth of 50 pm. (a) An arteriola with diameter of 10 km; (b) a branch with the lumen of 5 pm, which is subdivided into two capillaries; (c). The arrow shlows the border of muscle fiber.
4.6 mmlsec (from 0.83 to 10.5 mmlsec) in minute arterial branches, with diameters ranging from 3 to 8 km (arterioles and arterial capillaries according to their terms). We have not succeeded in observing the arteriole branching into capillaries in brain up to the depth of 70 Km but we frequently observed terminal arterioles and arteriolar anastomoses with lumen diameters of 10 or 5 pm. We have no quantitative data concerning the blood flow velocity in these vessels. Rosenblum (1969) measured mean velocity of 3.5 mm/set in pial arterioles with the diameter
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in the range IO-30 Frn using high-speed cinematography. This value is similar to data reported by Ma et al. (1974) for arterial vessels of first and second orders. Our method can be applied only for capillaries with lumens that are approximately equal to the size of red cells. It was impossible to record the gaps between RBC in larger vessels where they do not move in one row when the framing rate is 40 frames/set. Rosenblum (1969) stated that rhythmic alteration in velocity in pial arterioles of 30 km i.d. was in accordance with cardiac cycle. This velocity pulse was also seen occasionally in venules but was not recognizable in vessels 5 to 10 p,rn wide. We were able to observe gap movement on 8 to 12 frames in sequence, that is, during 0.2 to 0.3 set (two or three consecutive heart beats) but we did not observe any regularity of blood velocity change in capillaries that was coincidental to the cardiac cycle. There are a few reports dealing with the direct measurements of linear RBC velocity in muscle capillaries. An average velocity of 0.38 + 0.02 mm/set was found in capillaries of the cat sartorius muscle (Burton and Johnson, 1972; Johnson et al., 1976). For the cat tenuissimus muscle Eriksson and Myrhage (1972) observed a variation of flow velocity from 0 to 1.5 mm/set and the average velocity was measured to be 0.5 mm/set, and Intaglietta et al. (1975) measured velocities up to 10 mm/set. Tuma et al. (1975) reported a mean capillary flow velocity of 0.38 mm/set in rabbit tenuissimus. Klitzman and Duling (1979) found that in hamster cremaster muscle, capillary flow averaged 0.21 mm/set. The only available RBC velocity data for rat striated muscle capillaries (0.26 & 0.05 mm/set) was obtained by Henrich and Hecke (1978) for gracilis muscle capillaries. All these measurements were performed on dissected and isolated muscles in narcotized animals. For the rat temporalis vessels, we obtained a higher value of the RBC velocity than reported by other investigators. An average velocity of 1.14 ? 0.08 mm/ set was calculated from 1300 separate measurings with 123 capillaries in 30 rats. The difference may be due to the fact that in our procedure a minimum surgical procedure was applied on the in situ temporalis. The intact muscle was exposed by a skin incision and careful removal of the fascia. An animal was discarded if the smallest damage to the muscle surface occurred. Another positive feature of our procedure is that the experiments were carried out on unanesthetized or lightly anesthetized animals, which helps maintain muscle tone. The method presented here did not allow quantitation of RBCfilled working capillaries and capillaries filled by plasma or closed. However, the number of working capillaries was counted in a few layers of the temporalis muscle by analysis of the films (Kalinina et al., 1979). The average distance between capillaries was calculated to be 50 t 3.5 km; the capillary density was found to be about 330 capillaries/mm2. This result agrees with the data of direct in vivo measurements by Fronek and Zweifach (1977). They found that only about 30-35% of the cat tenuissimus capillaries were actively perfused under resting conditions of the muscle. The mean velocity of 2.43 ~fr 0.8 mm/set, measured in the temporalis terminal arterioles and precapillaries (lumen of 3-6 km) is similar to the mean velocity of 3.8 + 1.0 mm/set in cat tenuissimus precapillaries (9 km) reported by Fronek and Zweifach (1977). Blood circulation through capillaries performs many physiological functions,
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and oxygen supply to tissues through capillaries can be considered one of the most important. Because oxygen is not stored in tissues, oxygen transport must meet the oxygen demand of tissues at every moment of time. The agreement between demand and supply is especially important for the brain, the functions of which are immediately disturbed in the absence of oxygen. Several years ago Ivanov and Kislyakov (1974) and Kislyakov and Ivanov (1974) improved previously known spatial mathematical models for diffusion of oxygen in cerebral tissue (Griinewald, 1968, 1969, 1973; Gtinewald and Sowa, 1977). A complicated mathematical analysis of oxygen transport from capillaries to cerebral tissue and mechanisms controlling this transport was proposed. The calculations were based on experimental data related to capillary network density in cerebral cortex, diameter and length of separate capillaries, overall oxygen consumption by cerebral tissue, oxyhemoglobin dissociation curve, oxygen capacity of the blood, and so on. Under these conditions it was estimated that blood flow velocity through capillaries of cerebral cortex should range between 0.3-0.5 and 1.5-1.7 mm/set. It cannot be less than 0.3-0.5 mm/set, since neurons will not receive an adequate supply of oxygen. It cannot be more than 1.5-1.7 mm/set because at this velocity the oxygen supply to nerve cells will not be improved. At this velocity the ~0, at the venous end of a capillary is close to that at the arterial end. In other words, at the velocity of capillary blood flow of 1.5-1.7 mm/set the physiological reserve for improved delivery of oxygen at the expense of increased velocities of blood flow turns out to be practically exhausted. Blood flow changes in capillaries from 0.5 to 1.0-1.2 mm/set regulate the oxygen diffusion from capillaries to tissue most efficiently. Even small changes in the velocities within this range affect oxygen transport to tissues. Acco.rding to the experimental data described, in the vast majority of capillaries flow velocities indeed lie within the range 0.5-1.5 mm/set, the velocity being less tha.n 0.3 and more than 1.5 mm/set only in 5 and 3% of capillaries, respectively. Thus the experimental data are consistent with the calculated values. For mufscles the relationships are somewhat different. The difference in capillary length is probably responsible for a different capillary blood flow velocity and efficiency. The mean capillary length in skeletal muscle is about three times that of brain capillaries, which may explain the higher capillary flow velocity in resting and working muscle when compared to the brain. With a similar velocity tissue oxygen supply would be unsatisfactory at the venous end of a muscle capillary. To determine oxygen pressure distribution in a volume of muscle tissue the mathematical calculation was based on capillary density and blood velocity data (Lyabach and Ivanov, 1979). The calculation gave ~0, of 40 torr at the point of lowest oxygen supply. This indicates that temporalis muscle is saturated sufficiently with oxygen when the capillary blood velocity is 1.14 & 0.08 mm/ set witlh a density of 330 working capillaries/mm*. A considerable increase in the oxygen consumption during transition of a resting muscle into actively contracting muscle leads to a sharp fall of ~0, in muscle. However, an increase in the capillary velocity to 2.0-2.5 mm/set as was cited (Lyabach and Ivanov, 1979) allows tissue oxygen pressure to be maintained at a sufficiently high level to compensate completely for the oxygen deficiency. The increased oxygen consumption of the working muscle was fourfold. This observation suggests that a variation in the velocity of capillary blood flow in muscles in physiological limits
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is a powerful physiological mechanism to compensate for the oxygen deficiency, which can be compared with the opening of reserve capillaries. REFERENCES BARKROFT, J. (1934). “Features in the Architecture of Physiological Function.” Cambridge Univ. Press, Cambridge. BR~NEMARK, P. L., AND ERIKSSON,E. (1972). Method for studying qualitative and quantitative change of blood flow in skeletal muscle. Acta Physiol. Stand. 84, 284-288. BURTON, K. S., AND JOHNSON,P. C. (1972). Reactive hyperemia in individual capillaries of skeletal muscle. Amer. J. Physiol. 223, 517-524. ERIKSSON,E., AND LISANDER, B. (1972). Low flow states in the microvessels of skeletal muscle in the cat. Acta Physiol. Stand. 86, 202-210. ERIKSSON,E., AND MYRHAGE, R. (1972). Microvascular dimensions and blood flow in skeletal muscle. Acta Physiol. Stand. 86, 211-223. FRONEK, K., AND ZWEIFACH, B. W. (1977). Microvascular blood flow in cat tenuissimus muscle. Microvasc. Res. 14, 181-189. FULTON, Y. (1955). “Textbook of Physiology.” Saunders, Philadelphia/London. GR~NEWALD, W. (1968). Theoretical analysis of the oxygen supply in tissue. In “Oxygen Transport in Blood and Tissue” (D. W. Lubbers, U. C. Luft, J. Thews, and E. Witzleb, eds.), pp. 100-114. Thieme, Stuttgart. GR~NEWALD, W. (1969). Digitale Simulation eines raumlichen Diffusionsmodelles der 0, - Versorgung biologischer Gewebe. Pjliiegers Arch. 309, 266-284. GR~NEWALD, W. (1973). Computer calculation for tissue oxygenation and the meaningful presentation of the results. In “Advances in Experimental Medicine and Biology,” Vol. 37B, “Oxygen Transport to Tissue” (H. I. Bicher and D. F. Bruley, eds.), pp. 783-792. Plenum, New York/London. GR~NEWALD, W., AND SOWA, W. (1977). Capillary structures and O2 supply to tissue. Rev. Physiol. Biochem. Pharmacol. 77, 149-209. HENRICH,H. N., AND HECKE, A. (1978). A gracilis muscle preparation for quantitative microcirculatory studies in the rat. Microvasc. Res. 15, 349-356. INTAGLIETTA, M., SILVERMAN, N. R., AND TOMPKINS, W. R. (1975). Capillary flow velocity measurements in vivo and in situ by television method. Microvasc. Res. 10, 165-179. IVANOV, K. P., AND KISLYAKOV, Yu. YA. (1974). The oxygen available in a neuron and surrounding tissue. Setchenov Physiol. J. USSR 60, 900-905. JOHNSON,P. C., BURTON, K. S., HENRICH, H., AND HENRICH, U. (1976). Effect of occlusion duration on reactive hyperemia in sartorius muscle capillaries. Amer. J. Physiol. 230, 715-719. KALININA, M. K., LEVKOVICH, Yu. I., IVANOV, K. P., AND MICKAILOVA, G. P. (1979). Blood flow in the skeletal muscle microvessels in normoxia and arterial hypoxemia. Setchenov Physiol. /. USSR 65,
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W. I. (1975). Effect of pial arteriolar constriction on red cell velocity in pial venules and on venular diameter. Microvasc. Res. 9, 38-42. ROSENBLUM, W. I., AND ZWEIFACH, B. W. (1963). Cerebral microcirculation in the mouse brain. Arch.
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M. (1950). Z. Nervenheilkunde, 162, 113-135. F., CHILDS, C. M., INTAGLIETTA, M., AND ARFORS, K. -E. (1975). Microvascular flow pattern in the t.enuissimus muscle. Bib/. Amt. 13, 151-152. ZWEIFACHI, B. W. (1977). Perspectives in microcirculation. In “Microcirculation,” Vol. 1, pp. l-19. Univ. Park Press, Baltimore/London/Tokyo. SCHNEIDER,
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