According to The Franklin Institute, there are three types of blood vessels, each with its own function. Veins carry blood back to the heart, arteries carry blood away from the heart and capillaries connect arteries to veins.
Veins consist of three layers: a layer of tissue on the outside, a layer of smooth epithelial cells on the inside and a layer of muscle in between. Veins transport blood to the heart and lungs at low pressure after receiving it from the capillaries. Valves inside the veins keep blood moving in one direction.
The human body's arterial system branches out from one main artery, the dorsal aorta. Like veins, arteries have three layers: an outer layer of tissue, an inner layer of epithelial cells and a layer of muscle in between. Arteries deliver oxygenated blood to the capillaries, where the exchange of oxygen and carbon dioxide takes place. An artery's muscle helps it expand and contract in rhythm with the heart beating to keep blood moving through the system.
Capillaries connect veins and arteries to exchange oxygen and carbon dioxide. Thin and weak, capillaries are only as thick as one epithelial cell. Blood passes through capillaries one cell at a time, single file. The blood cells release oxygen, which passes through the capillary walls into nearby tissue. Tissue then releases carbon dioxide through the capillary walls into the red blood cells.