Capillary

A capillary is a microscopic blood vessel linking arterioles and venules through tissue beds. Its thin wall enables exchange of gases, nutrients, water, and waste between blood and surrounding tissues.

Capillary wall: The thin boundary between capillary blood and surrounding tissue, formed mainly by endothelial cells and a basement membrane. Its selectively permeable layers enable exchange while keeping blood cells and most proteins inside.
Arteriole: A small artery that carries blood toward capillary beds and regulates their blood supply. Arterioles deliver blood to the capillaries and control inflow through constriction and dilation.
Blood–brain barrier: A selective barrier formed mainly by brain endothelial cells that limits movement of substances from blood into brain tissue. Brain capillaries have unusually tight junctions that restrict exchange and protect neural tissue.
Artery: A blood vessel that carries blood away from the heart, usually through a thick, elastic wall. Arteries carry blood under higher pressure and have walls unlike the thin capillary barrier.
Capillary exchange: The movement of water, gases, nutrients, and wastes between blood in capillaries and surrounding tissues. This is the central function made possible by capillaries' thin walls and extensive surface area.
Venule: A small vessel that collects blood from capillaries and carries it toward larger veins. Venules receive blood as it leaves capillary networks.
Glomerular filtration barrier: The layered kidney barrier that filters water and small solutes from blood into the nephron while retaining cells and most proteins. Specialized glomerular capillaries perform high-volume filtration rather than ordinary tissue exchange.
Vein: A blood vessel that carries blood toward the heart, often with valves that limit backward flow. Veins return blood from tissues but are not the primary sites of exchange.
Starling equation: An equation describing fluid movement across capillary walls from hydrostatic and oncotic pressure differences. It relates pressure forces within capillaries to filtration into or absorption from tissue.
Endothelium: A layer of cells lining blood and lymphatic vessels and forming a selective interface with surrounding tissues. Capillary walls are chiefly endothelial, with specialized structures varying by organ.

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