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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 products of metabolism.
General diagram of a portal venous system, for example, this occurs in the hypophyseal portal system between the hypothalamus and the anterior pituitary gland.. In the circulatory system of vertebrates, a portal venous system occurs when a capillary bed pools into another capillary bed through veins, without first going through the heart.
The microcirculation has three major components: pre-capillary, capillary, and post-capillary. In the pre-capillary sector, arterioles, and precapillary sphincters participate. Their function is to regulate blood flow before it enters the capillaries and venules by the contraction and relaxation of the smooth muscle found on their walls.
In humans, the only significant example is the hepatic portal vein which combines from capillaries around the gastrointestinal tract where the blood absorbs the various products of digestion; rather than leading directly back to the heart, the hepatic portal vein branches into a second capillary system in the liver.
Their unique structure permits interstitial fluid to flow into them but not out. The ends of the endothelial cells that make up the wall of a lymphatic capillary overlap. When pressure is greater in the interstitial fluid than in lymph, the cells separate slightly, like the opening of a one-way swinging door, and interstitial fluid enters the ...
An important example of such a condition is elevated blood pressure in the portal vein. This condition, called portal hypertension , is a major complication of cirrhosis . In abdominal obesity fats, inflammatory cytokines and other toxic substances are transported by the portal vein from visceral fat into the liver, leading to hepatic insulin ...
A metarteriole is a short microvessel in the microcirculation that links arterioles and capillaries. [1] Instead of a continuous tunica media, they have individual smooth muscle cells placed a short distance apart, each forming a precapillary sphincter that encircles the entrance to that capillary bed.
The smallest veins are the post-capillary venules. Veins have a similar three-layered structure to arteries. The layers known as tunicae have a concentric arrangement that forms the wall of the vessel. The outer layer, is a thick layer of connective tissue called the tunica externa or adventitia; this layer is absent in the post-capillary ...