Artery

One type of vessel could not perform all three jobs efficiently, so the circulatory system contains arteries, capillaries and veins. Their structures differ because the pressure and purpose of the blood flow differ in each part of the system.

Arteries carry blood away from the heart, capillaries bring it close to cells for exchange, and veins carry it back towards the heart. These vessels have different jobs, so their walls and the space inside them—the lumen—are different.

Arteries carry blood away from the heart. They divide into progressively smaller vessels and eventually lead to networks of capillaries within organs. Capillaries are the exchange vessels: dissolved substances move between the blood and nearby cells across very short distances.

Arteries take blood away from the heart, capillaries carry it through tissues where substances are exchanged, and veins return it to the heart. Each vessel therefore faces different conditions and has a structure suited to its job.