capillary

Capillaries are the tiniest blood vessels—so narrow that red cells often pass through single file—and they are where the whole point of circulation actually happens: the handoff of oxygen and nutrients to tissues, and the pickup of carbon dioxide and waste. If arteries and veins are highways, capillaries are the doorsteps where deliveries are made.

A capillary wall is just a single layer of endothelial cells on a thin basement membrane, which makes it thin enough for substances to diffuse across easily. Capillaries form dense networks (capillary beds) fed by arterioles and drained by venules. Exchange is governed by the balance between blood pressure pushing fluid out and osmotic forces pulling it back in—the so-called Starling forces—so a small net amount of fluid normally leaves at the arterial end and is reclaimed or carried off by lymphatics.

Capillary density and the fraction of capillaries open at any moment vary by tissue and by need: hard-working muscle recruits far more open capillaries during exercise. When this exchange or its fluid balance goes wrong—for example when capillary pressure rises in heart failure—fluid accumulates in tissues as edema.