Topic:Capillaries
Capillaries in horses are small blood vessels that form a network between arterioles and venules, facilitating the exchange of oxygen, nutrients, and waste products between blood and tissues. These microvessels are integral to the circulatory system, ensuring that tissues receive adequate blood supply and metabolic exchange. The structure and function of capillaries can vary depending on the specific tissue or organ they serve, influencing factors such as permeability and flow rate. Research in equine capillary function often focuses on topics such as blood flow regulation, the impact of exercise on capillary density and function, and the role of capillaries in various equine health conditions. This page compiles peer-reviewed research studies and scholarly articles that explore the anatomy, physiology, and clinical implications of capillary function in horses.
Pulmonary vascular pressures of strenuously exercising thoroughbreds after administration of flunixin meglumine and furosemide. High-intensity exercise results in a dramatic increase in mean pulmonary capillary blood pressure of horses, and administration of furosemide 4 hours before exertion significantly attenuates this exercise-induced increment. To test whether this effect of furosemide is mediated via release of prostaglandins, right atrial and pulmonary vascular pressures were measured in 8 healthy, sound, exercise-trained Thoroughbreds at rest and during incremental-step exercise on a treadmill. Horses were studied on 3 separate occasions: after i.v. administration of saline (0.9% NaCl) solution, after administr...
Capillary blood transit time in muscles in relation to body size and aerobic capacity. The mean minimal transit time for blood in muscle capillaries (tc) was estimated in six species, spanning two orders of magnitude in body mass and aerobic capacity: horse, steer, dog, goat, fox and agouti. Arterial (CaO2) and mixed venous (CvO2) blood O2 concentrations, blood hemoglobin concentrations ([Hb]) and oxygen uptake rates were measured while the animals ran on a treadmill at a speed that elicited the maximal oxygen consumption rate (VO2max) from each animal. Blood flow to the muscles (Qm) was assumed to be 85% of cardiac output, which was calculated using the Fick relationship. Total...
Heterogeneity of pericyte populations in equine skeletal muscle and dermal microvessels: a quantitative study. The objective of this ultrastructural investigation was to determine if populations of pericytes in equine dermal and skeletal muscle capillaries increase in a head-to-foot direction, as has been reported in human skeletal muscles. Samples of equine microvessels were obtained from the longissimus dorsi skeletal muscle 150 cm. from the ground, from the dermis above this muscle, from the extensor carpi radiali muscle at 55 cm. from the ground, from the dermis adjacent to that muscle, and from dermis 15 cm. from the ground, just above the hoof wall. Tissues were processed for transmission electro...
Comparative aspects of the strength of pulmonary capillaries in rabbit, dog, and horse. In previous studies of rabbit and dog lung, we demonstrated stress failure of pulmonary capillaries at high transmural pressures (Ptm). The Ptm necessary to elicit stress failure was 40 cmH2O higher in dog than rabbit, and the total blood-gas barrier (BGB) thickness was greater in dog than rabbit. This suggests that stress failure may be related to BGB thickness, and is consistent with the Laplace relationship which states that wall stress is proportional to capillary radius but inversely proportional to wall thickness. In the present studies, we compared BGB thickness and an index of capillar...
Evaluation of laser Doppler flowmetry for measuring coronary band and laminar microcirculatory blood flow in clinically normal horses. Once daily for 3 days, laser Doppler flowmetry was used in 5 healthy, nonsedated adult horses to evaluate coronary band and laminar microcirculatory blood flow (MBF) in both forelimbs. The coronary band had significantly (P < 0.05) higher MBF than did the laminae on the days evaluated. Significant variation in MBF was not found over the 3-day measurement period in any one site. Significant (P < 0.05) variation was found in coronary band MBF among horses. This variation was not observed in laminar MBF. On occlusion of the digital arteries at the level of the fetlock, marked decrease in coronary...
[The oxygen pathway: how well-built is the respiratory system?]. The question is explored of how the structural systems that constitute the pathway for oxygen from the air through the lung and the circulation of blood to the mitochondria in muscle cells affect the O2 flow rate, and whether their quantitative structural characteristics conform to the hypothesis of symmorphosis which postulates a match of structural design to overall functional demand. By combining physiological and morphometric studies, it is first demonstrated that the diffusing capacity of the human lung is slightly larger than that needed for maximal O2 uptake. Comparative studies of smal...
Mechanisms of exercise-induced pulmonary hemorrhage in the equine athlete. This article discusses exercise-induced pulmonary hemorrhage (EIPH), a pathophysiological syndrome which occurs worldwide in the equine athlete. It reviews the history of EIPH, the incidence in performance horses, the etiology, studies performed on the treadmill to determine the mechanisms of EIPH, and the most likely causes of stress failure of the pulmonary capillaries.
Muscular changes in Venezuelan wild horses naturally infected with Trypanosoma evansi. Skeletal muscle biopsy specimens were taken from 10 male horses naturally infected with Trypanosoma evansi and from 10 uninfected males. An indirect fluorescent antibody test was used to provide a rapid and reliable indication of infection. Histological, histochemical and transmission electron microscopical techniques were used to examine skeletal muscle. The ultrastructural features in muscle fibres were those usually seen in autoimmune disease, namely fibre and capillary necrosis and mononuclear cell infiltration, consisting of macrophages. Changes in fibre-type percentages did not occur in ...
Fibre type distribution, capillarization and enzymatic profile of locomotor and nonlocomotor muscles of horses and steers. Samples were taken at slaughter from heart and both locomotor and nonlocomotor muscles from animals of similar body weight but adapted to different levels of activity: three horses and three steers. All samples were analyzed biochemically to measure the activity of key metabolic enzymes. The skeletal muscles were analyzed histochemically for fibre type composition, fibre area and capillary supply. The general pattern of differences in fibre type composition and metabolic profile between muscle groups was similar in both horses and steers. The hearts of both species had high citrate synthase (C...
Frusemide attenuates the exercise-induced rise in pulmonary capillary blood pressure in horses. Catheter mounted micro-tip-manometers (the signals from which were matched with fluid-filled pressure signals from same cardiovascular sites and zeroed at the point of the shoulder), were used to study pulmonary haemodynamics in 8 healthy sound horses at rest and during exercise performed at 8, 10, 12 and 14 m/s on a treadmill. Measurements were made without frusemide (control) and 4 h after iv administration of 250 mg frusemide. Post-frusemide data were also obtained on a separate day, and these observations were not significantly different from those made on the same day as controls. Pre-fru...
Ischemia/reperfusion injury of the ascending colon in ponies: a correlative study utilizing microvascular histopathology and corrosion casting. Volvulus of the ascending colon (ACV) in the horse results in microvascular injury and necrosis of the intestinal mucosa. This study investigated the site and type of microvascular injury which occurs within the mucosa and submucosa following ACV. Histopathology of volvulus treated ponies demonstrated mucosal necrosis with microvascular hemorrhage and thrombosis. Thrombi occurred within the subepithelial capillaries and edema and hemorrhage developed throughout the mucosa and submucosa. Vascular casts allowed 3-D viewing of samples obtained from the entire pelvic flexure and demonstrated two d...
Evaluation of the microcirculation of the equine small intestine after intraluminal distention and subsequent decompression. Effects of intraluminal distention (25 cm of H2O, 120 minutes) and subsequent decompression (60 minutes) on intramural vascular patterns of the small intestine was evaluated in 7 anesthetized horses. Intraluminal distention (25 cm of H2O, 120 minutes) was created in 2 jejunal segments in each horse. Experimental and control segments were removed either immediately after the experimental period or after 60 minutes of decompression. The vascular system of experimental and control jejunal segments was lavaged with NaCl, then was injected with a blue-colored radiopaque medium for microangiography ...
Evaluation of the microcirculation of the equine jejunum and ascending colon after ischemia and reperfusion. Intramural vascular patterns of the jejunum and colon were evaluated during ischemic strangulation obstruction (ISO, 70 minutes) and subsequent reperfusion (60 minutes) in 7 adult anesthetized horses. Microvasculature of experimental and control segments was described by comparison of results from microangiography, light microscopy, and scanning electron microscopy of vascular replicas. Experimental and control segments with isolated vascular arcades were removed either immediately after the experimental period or after 60 minutes of reperfusion. Blood was flushed from the vascular system by u...
Stress failure of pulmonary capillaries in racehorses with exercise-induced pulmonary hemorrhage. Bleeding into the lungs in thoroughbreds is extremely common; there is evidence that it occurs in essentially all horses in training. However, the mechanism is unknown. We tested the hypothesis that exercise-induced pulmonary hemorrhage (EIPH) is caused by stress failure of pulmonary capillaries. Three thoroughbreds with known EIPH were galloped on a treadmill, and after the horses were killed with intravenous barbiturate the lungs were removed, inflated, and fixed for electron microscopy. Ultrastructural studies showed evidence of stress failure of pulmonary capillaries, including disruptions...
Pulmonary haemodynamics in the exercising horse and their relationship to exercise-induced pulmonary haemorrhage. Exercise-induced pulmonary haemorrhage (EIPH) is a common occurrence in race horses. Although blood in cases of EIPH has been suspected to originate from the bronchial circulation, which receives approximately 1% of the left ventricular output, physiological evidence has recently emerged to indicate that the pulmonary circulation, which receives the entire output of the right ventricle, is a more likely source. High transmural pulmonary capillary pressures have been shown to cause breaks in the capillary endothelium, basement membrane as well as in the alveolar epithelium. Blood constituents e...
Localization of xanthine dehydrogenase mRNA in horse skeletal muscle by in situ hybridization with digoxigenin-labelled probe. In situ hybridization was used to localize xanthine dehydrogenase (XDH) mRNA in horse skeletal muscle. Capillary endothelial cells were found to express XDH, but muscle cells did not give any signal. The digoxigenin-labelled probe was produced by PCR with primers based on the cDNA sequence of mouse XDH and horse lung cDNAs. A 4.3 kb mRNA was detected in a Northern blot.
Effect of constant load training on skeletal muscle histochemistry of thoroughbred horses. Skeletal muscle adaptations to training of differing intensities were examined in 10 thoroughbred horses that underwent six weeks of treadmill training followed by six weeks of detraining. The horses were randomly assigned to either a slow group exercised at 40 per cent maximum oxygen uptake (VO2max) or a fast group at 80 per cent VO2max. Resting muscle biopsies were taken before training, after six weeks of training and after six weeks of detraining, from m gluteus medius and m biceps femoris. Muscle was analysed histochemically for fibre type composition (myosin ATPase) and capillary supply ...
Pulmonary artery wedge pressure increases with high-intensity exercise in horses. Using catheter mounted microtip manometers, right atrial, pulmonary artery, and pulmonary artery wedge pressures were studied in 8 horses while they were standing quietly (rest), and during galloping at treadmill speeds of 8, 10, and 13 m/s. At rest, mean (+/- SEM) heart rate, mean right atrial pressure, mean pulmonary artery pressure, and mean pulmonary artery wedge pressure were 37 (+/- 2) beats/min, 8 (+/- 2) mm of Hg, 31 (+/- 2) mm of Hg, and 18 (+/- 2) mm of Hg, respectively. Exercise at treadmill belt speed of 8 m/s resulted in significant (P < 0.05) increments in heart rate, right at...
Changes in cross-sectional area and capillary supply of the muscle fiber population in equine gluteus medius muscle as a function of sampling depth. The right and left gluteus medius muscles of 4 mature Andalusian stallions were examined by repeated needle biopsy over a specific area and depth, as well as at different depths, to determine whether the cross-sectional area and capillary supply of the various fiber types are homogeneous throughout the muscle. The muscle biopsy specimens were histochemically analyzed for fiber types (myofibrillar adenosine triphosphatase) and capillaries (amylase-periodic acid-Schiff method). Differences between contralateral sites were not identifiable for any of the analyzed variables. Differences between sa...
Specialized veins in the submucosa of the equine ileocecal junction. Spirally arranged bundles of sub-endothelial smooth muscle enfold the small to medium-sized submucosal veins in the equine ileocecal junction. The muscle bundles, accompanied by the endothelial lining, bulge into the lumen of the vessels, partly occluding the latter. Transmission electron microscopy of the muscle cells reveals features consistent with vascular smooth muscle ultrastructure. It is proposed that the throttling effect of the muscle bundles causes engorgement of the submucosal venous plexus, which then assists in the closing of the ileocecal orifice.
O2 delivery at VO2max and oxidative capacity in muscles of standardbred horses. The purpose of this study was to describe the relationships between 16 physiological, biochemical, and morphological variables presumed to relate to the oxidative capacity in quadriceps muscles or muscle parts in Standardbred horses. The variables included O2 delivery (blood flow) and mean capillary transit time (MTT) during treadmill locomotion at whole animal maximal O2 consumption (VO2max, 134 +/- 2 ml.min-1 x kg-1), capillary density and capillary-to-fiber ratio, myoglobin concentration, oxidative enzyme activities, glycolytic enzyme activities, fiber type populations, and fiber size. Thes...
Measurement of muscle surface capillary blood flow by laser Doppler flowmetry. Muscle surface capillary blood flow was measured in the biceps femoris and lateral head of the triceps brachii muscles in six horses before and during halothane anesthesia by using laser Doppler flowmetry. During 90 minutes of anesthesia, muscle surface capillary blood flow was reduced to 20% to 40% of preanesthetic values. Muscle surface capillary blood flow tended to be lower in dependent muscles than in nondependent muscles, and this disparity was greater in the forelimbs than in the hind limbs.
Microvascular permeability changes in ischemia/reperfusion injury in the ascending colon of horses. The normal microvascular permeability of the ascending colon in horses and the microvascular permeability of that segment after ischemia and reperfusion were investigated. Microvascular permeability was estimated by the ratio of lymphatic protein to plasma protein concentration (Cl/Cp) at high lymph flow rates in 8 adult horses in 2 equal groups: normal and ischemic (2-hour period). Lymphatic flow rates and lymph and plasma protein concentrations were determined. Intestinal biopsy specimens were obtained at the end of each experiment. Flow independent values were selected and compared by one-w...
Distribution of coronary collateral blood flow at different levels of collateral growth in conscious ponies. Coronary collateral growth was stimulated in chronically instrumented conscious ponies by a previously validated intermittent coronary occlusion method. Changes in regional myocardial function (sonomicrometry) and reactive hyperemia (Doppler method) were used to monitor collateral growth and to program measurements of regional myocardial blood flow (microsphere method). A serial analysis of the transmural and lateral distributions of collateral blood flow was performed at the native and three superimposed levels of collateral growth. Results in nine animals undergoing an average of 553 +/- 188...
Regional brain blood flow during prolonged submaximal exercise in ponies. Experiments were carried out on 8 healthy ponies to examine the effects of prolonged submaximal exercise on regional distribution of brain blood flow. Brain blood flow was ascertained by use of 15-microns-diameter radionuclide-labeled microspheres injected into the left ventricle. The reference blood was withdrawn from the thoracic aorta at a constant rate of 21.0 ml/min. Hemodynamic data were obtained with the ponies at rest (control), and at 5, 15, and 26 minutes of exercise performed at a speed setting of 13 mph on a treadmill with a fixed incline of 7%. Exercise lasted for 30 minutes and w...
Estimating transit time for capillary blood in selected muscles of exercising animals. The mean minimal capillary transit time was estimated in muscles of various animals using a combination of physiological and morphometric methods. Radioactive microspheres were injected intravascularly in various animals running on a treadmill at maximum oxygen consumption rate (VO2,max) to label blood flow to individual muscles. The muscles were then removed and preserved by standard methods for electron microscopy. The volume density of mitochondria was measured to assess muscle oxidative capacity. Capillary densities in muscle cross-sections, capillary diameters and tortuosities were incorp...
Microvascular circulation of the descending colon in horses. The microvascular circulation of the descending colon was studied in 5 adult horses, using microangiography and light microscopy combined with gross studies and scanning electron microscopy of vascular replicas. After heparinization, horses were euthanatized, and 3 segments of the descending colon and its mesentery containing 1 vascular arcade were removed from each horse. The fecal balls were gently massaged from the lumen, and the blood was flushed free of the circulation with isotonic NaCl. In 5 segments, the vascular system was injected with a modified radiopaque medium and evaluated radio...