The Circulatory System

The body of a typical 12-lb (5-kg) cat contains about 12 fl oz (330 ml) of blood, about the same amount as in a soft-drink can. Pressure is highest in the arteries, which carry blood from me heart, and lowest in the veins returning it to the heart.

The muscular, elastic walls of arteries expand and contract as the heart pushes blood through them: this is the pulse. The thinner walls of the veins are more easily damaged; they nave no pulse, and contain valves to ensure that the blood in them moves only one way.

Different parts of the body need different amounts of blood. The brain is a small part of me body's weight, but takes 15 to 20 percent of its blood.

Resting muscles get about twice this, but during pursuits or in escape situations, up to 90 percent of the blood can be diverted into me muscles from the internal organs and even from the brain.

The amount of blood that each part of the body receives is controlled by nerves and hormones that cause arterioles to dilate in response to local activity, dramatically increasing the blood supply to that region.