Capillary blood sampling

A capillary is a small blood vessel from 5 to 10 micrometres (µm) in diameter, and having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules. These microvessels are the site of exchange of many substances with the interstitial fluid surrounding them. Substances which exit include water (proximal portion), oxygen, and glucose; substances which enter include water (distal portion), carbon dioxide, uric acid, lactic acid, urea and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in the microcirculation. A capillary is a small blood vessel from 5 to 10 micrometres (µm) in diameter, and having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules. These microvessels are the site of exchange of many substances with the interstitial fluid surrounding them. Substances which exit include water (proximal portion), oxygen, and glucose; substances which enter include water (distal portion), carbon dioxide, uric acid, lactic acid, urea and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in the microcirculation. During early embryonic development new capillaries are formed through vasculogenesis, the process of blood vessel formation that occurs through a de novo production of endothelial cells which then form vascular tubes. The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already present endothelium which divides. 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. Individual capillaries are part of the capillary bed, an interweaving network of capillaries supplying tissues and organs. The more metabolically active a tissue is, the more capillaries are required to supply nutrients and carry away waste products. There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, and metarterioles, found only in the mesenteric circulation. They are short vessels that directly connect the arterioles and venules at opposite ends of the beds. Metarterioles are found primarily in the mesenteric microcirculation. The physiological mechanisms underlying precapillary resistance is no longer considered to be a result of precapillary sphincters outside of the mesentery organ. Lymphatic capillaries are slightly larger in diameter than blood capillaries, and have closed ends (unlike the blood capillaries open at one end to the arterioles and open at the other end to the venules). This structure permits interstitial fluid to flow into them but not out. Lymph capillaries have a greater internal oncotic pressure than blood capillaries, due to the greater concentration of plasma proteins in the lymph.