Capillaries

Structure

Capillary walls are made of a single layer of endothelial cells resting on a basement membrane, with no smooth muscle. Their diameter is roughly 5 to 10 micrometers, just wide enough to allow red blood cells to pass in single file.

Function

The thin walls and slow blood flow maximize diffusion. Oxygen and nutrients move outward to tissues, while carbon dioxide and waste products move inward to the blood.

Types

Continuous capillaries are found in muscle and the nervous system. Fenestrated capillaries, with small pores, occur in the kidneys and intestines. Sinusoidal capillaries with larger gaps appear in the liver, spleen, and bone marrow.