Swamps of Capillaries

Capillaries are the exchange vessels and have the smallest inside diameters of all the vessels. However, there are so many capillaries that their combined inside diameters are greater than the inside diameters of the arteries, so all arterial blood can pass through capillaries without pooling. Arterioles reduce the perfusion pressure of arterial blood to the lower blood pressures of capillaries.

Blood flow in capillaries is very slow and is not uniform. In one capillary, flow may slow down, halt and then reverse like water surges in a swamp. Oxygen, carbon dioxide and metabolites exchange through the walls of capillaries between the slowly moving blood and the stationary tissues. The differences between the ratios of the resistances to flow in the arterioles and the post-capillary vessels determine intra-capillary blood pressures and thus how much blood flows through the capillary beds to perfuse the tissues. Because capillary walls are thin, individual capillaries may swell or collapse depending on the differences in pressures across the capillary walls and flows within and outside them. High external pressures can collapse capillaries preventing adequate perfusion of tissues. A single capillary or bed may open or close depending on the needs of nearby cells. Capillary walls often dilate or constrict responding to locally produced, vaso-active chemicals. The small diameters of capillaries permit their thin walls to exert leverage upon high internal pressures (Laplace's Law). Large capillary surface to volume ratios facilitate rapid exchange.

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