Capillaries (pronounced /ˈkæpɨlɛri/) are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissues.
Blood flows from the heart to the arteries, which branch and narrow into the arterioles, and then branch further still into the capillaries. After the tissue has been perfused, capillaries join and widen to become venules and then widen more to become veins, which return blood to the heart.
Capillaries do not function on their own. The "capillary bed" is an interweaving network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries they will require to supply nutrients and carry away waste products.
A capillary bed can consist of two types of vessels: true capillaries which branch mainly from metarterioles and provide exchange between cells and the circulation. Secondly, capillary beds also consists of a vascular shunt which is a short vessel that directly connects the arteriole and venule at opposite ends of the bed.
Metarterioles provide direct communication between arterioles and venules and are important in bypassing the bloodflow through the capillaries. The internal diameter of 8 μm forces the red blood cells to partially fold into bullet-like shapes and to go into single file in order for them to pass through.
Precapillary sphincters are rings of smooth muscles at the origin of true capillaries that regulate blood flow into true capillaries and thus control blood flow through a tissue.
There are two main types of capillaries:
- Continuous - They are continuous in the sense that the endothelial cells provide an uninterrupted lining, and only allow small molecules, like water and ions to diffuse through tight junctions which leave gaps of unjoined membrane which are called intercellular clefts. Tight junctions can be further divided into two subtypes:
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