Discuss the capillary bed

A capillary is a microscopic channel that supplies blood to the tissues, through a process called perfusion. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5–10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation.

The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. Some variation in wall structure is seen depending on the size of the capillary. In a large capillary, several endothelial cells bordering each other may line the lumen, while in a small capillary, there may be only a single cell layer that wraps around to contact itself.

For capillaries to function, their walls must be leaky, or permeable, allowing some substances to pass through. There are three major types of capillaries, which differ according to their degree of permeability: continuous, fenestrated, and sinusoid capillaries (Figure 18.2.5
).

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