In birds, the system is very complex, with sphincters around the metarterioles themselves. In fishes and salamanders, due to the lack of metarterioles, all the blood passes through the capillaries. However, in frogs and amniotes, most of the blood usually passes through the metarterioles instead, although it can still be diverted through the capillary networks if need be. The system is completely abandoned in mammals, with the result that all the blood from the tail (if present), groin, pelvic area, and hind legs is forced to pass directly into the posterior vena cava, and the only blood passing out of the kidney through the renal vein is that that entered it through the renal artery.
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Metarterioles connect arterioles and capillaries. A tributary to the venules is known as a thoroughfare channel. The microcirculation has three major components: pre-capillary, capillary, and post-capillary. In the pre-capillary sector, arterioles, and precapillary sphincters participate.
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Metarterioles exist in the mesenteric microcirculation, and the name was originally conceived only to define the "thoroughfare channels " between arterioles and venules. In recent times the term has often been used instead to describe the smallest arterioles directly prior to the capillaries.
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The microcirculation is the circulation of the blood in the smallest blood vessels, the microvessels of the microvasculature present within organ tissues. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. Arterioles carry oxygenated blood to the capillaries, and blood flows out of the capillaries through venules into veins. In addition to these blood vessels, the microcirculation also includes lymphatic capillaries and collecting ducts.
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Diagram of a capillary Blood flows from the heart through arteries, which branch and narrow into arterioles, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to become venules, which in turn widen and converge to become veins, which then return blood back to the heart through the venae cavae. In the mesentery, metarterioles form an additional stage between arterioles and capillaries. Individual capillaries are part of the capillary bed, an interweaving network of capillaries supplying tissues and organs.
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The more metabolically active a tissue is, the more capillaries are required to supply nutrients and carry away products of metabolism. There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, and sinusoids, a type of open-pore capillary found in the liver, bone marrow, anterior pituitary gland, and brain circumventricular organs. Capillaries and sinusoids are short vessels that directly connect the arterioles and venules at opposite ends of the beds. Metarterioles are found primarily in the mesenteric microcirculation.
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In lungfish and tetrapods, the renal portal vein is joined by a vein traveling upwards from the abdominal vein,Pet Plaec.com Retrieved November 20, 2015 which can bring venous blood from the hind limbs and ventral body wall into the renal portal system, or alternatively, enable blood from the tail and groin to pass into the hepatic portal system, already served by blood from the gut, via the hepatic portal vein, and from the hind legs and ventral body wall, via the abdominal vein. In fishes and salamanders, the renal portal vein branches and enters a capillary network very similar to the ones in the nephric portal system. In frogs and amniotes, metarterioles appear, with capillary networks connected to them, and sphincter muscles around the entrances to the capillaries.
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