Capillaries
Smallest of blood vessels
Form “U-turns” that connect arterial outflow to venous return Form an extensive network of short branched interconnecting vessels that course among individual cells of the body. Found near almost every cell in the body. Form an enormous surface area contact point(s) with tissue cells Flow of blood through capillaries is referred to as microcirculation. Main function is the exchange of substances between blood and interstitial fluid. Structure is well suited to an “exchange-type” vessel as they have no tunica media or tunica externa – only a single layer of endothelial cells and a basement membrane. Exchange of materials happens only through the walls of capillaries and the beginning of venules.
Anastomoses: Arterivenous anastomoses (AVAs) are direct connections between small arteries and small veins. AVAs function to shunt blood away from tissues of intermittent or decreased activity and are also important for thermoregulation. When open, AVAs provide a low-resistance pathway for blood flow allowing blood to bypass the capillary bed. With regards to thermoregulation, when the animal is overheated, AVAs open to allow for greater heat loss. AVAs are important in maintaining tissue metabolism by allowing perfusion of the feet when a horse is subjected to cold environments. AVAs close in cold environments to prevent over chilling of the extremities. Anastomoses can also be between arteries or between veins (brain, heart & feet).
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