Vein

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.

The capillaries then join into increasingly larger veins, which carry blood back towards the heart.

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.