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The structure of the vasa vasorum varies with the size, function and location of the vessels. Cells need to be within a few cell-widths of a capillary to stay alive. In the largest vessels, the vasa vasorum penetrates the outer (tunica adventitia) layer and middle (tunica media) layer almost to the inner (tunica intima) layer.
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.
In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. [1] [2] It includes the cardiovascular system, or vascular system, that consists of the heart and blood vessels (from Greek kardia meaning heart, and Latin vascula meaning vessels).
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.
Heat transfer is monitored by thermoreceptors which regulate the amount of capillary beds open for heat dissipation. The surface chemistry of the endothelial cell lining also dictates fluid flow. A charged surface will acquire a layer of stagnant diffuse ions that hinder the flow of ions in the lumen.
However, later studies suggest a more conservative figure of 9,000–19,000 kilometres (5,600–11,800 mi) taking into account updated capillary density and average muscle mass in adults. [11] Despite these later studies, many textbooks and other types of media include Krogh's estimates as a fun fact as opposed to the more recent studies. [12] [13]
In a healthy vascular system, the endothelium lines all blood-contacting surfaces, including arteries, arterioles, veins, venules, capillaries, and heart chambers. This healthy condition is promoted by the ample production of nitric oxide by the endothelium, which requires a biochemical reaction regulated by a complex balance of polyphenols, various nitric oxide synthase enzymes and L-arginine.
The morphology, distribution, density and molecular fingerprints of pericytes vary between organs and vascular beds. [ 4 ] [ 5 ] Pericytes help in the maintainenance of homeostatic and hemostatic functions in the brain , where one of the organs is characterized with a higher pericyte coverage, and also sustain the blood–brain barrier . [ 6 ]