1.7—Blood vessel structure and function

Arteries carry blood away from the heart under high pressure; veins return it at lower pressure; capillaries bring blood close to cells. The wall and lumen of each vessel suit its job.

Arteries have thick elastic and muscular walls to withstand and smooth pressure pulses. Veins have thinner walls, a wider lumen and valves to prevent backflow. Capillaries have a one-cell-thick wall and narrow lumen, slowing cells so exchange distances stay short.

A pulse can be felt in an artery because ventricular pressure stretches its elastic wall. A leg vein relies on valves and skeletal-muscle contractions to move blood back towards the heart against gravity.

Capillary branching creates a large exchange area; slow flow and a thin barrier allow oxygen, glucose and wastes to diffuse between blood and tissue fluid.

Arteries and veins are named by direction relative to the heart, not by oxygen content: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.