The smallest arteries are known as arterioles. These supply the capillaries, a network of fine tubes just large enough to allow red blood cells to pass through them.

The walls of the capillaries are very thin and fluid containing oxygen and nutrients can easily cross them and enter the tissues. Fluid containing carbon dioxide and other waste products can also cross the capillary walls in the opposite direction, into the blood.

Blood in the capillaries drains into small veins, known as venules, that unite to form larger veins and eventually enter their contents into the vena cavae.