Capillaries and Their Role in Sha
"Capillaries are the smallest of a body’s blood vessels, measuring 5-10 μm [micrometres (1/1000 of a millimeter) in diameter], which connect arterioles and venules, and enable the interchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissues."
Capillaries, which are made of single layers of epithelial cells, expand or contract in diameter as an indirect function of the circulation of potent vasoconstricting hormones such as adrenaline, angiotensin II, and endothelin or the circulation of potent vasodilating hormones including prostacyclin and nitric oxide.
"These hormones directly affect the metarterioles (or arterial capillaries) which are short vessels that link arterioles and capillaries. Instead of a continuous tunica media, they have individual muscle cells placed a short distance apart, each forming a precapillary sphincter that encircles the entrance to one capillary. Constriction of these sphincters reduces or shuts off blood flow through their respective capillaries and diverts blood to tissues or organs elsewhere.*"