The circulatory system shares many of the features of a house heating system, with a pump, pipes, valves, movement of fluid, and an energy source.

Circulatory systems vary from primitive diffusion-based systems to the complex double circulation of birds and mammals, in which a four-chambered heart pumps blood to the lungs and body organs in a sequence that allows pressure to be maintained in blood passing both to the lungs and to the organs.

The movement of blood through the body occurs through arteries, capillaries, and veins. Form and function are closely linked in the structure of these different types of blood vessels. Blood consists of a fluid called plasma that contains cells important in carrying oxygen and in fighting disease and foreign matter in the body. Plasma itself carries nutrients and hormones around the body.

The human heart beat is controlled by electrical impulses starting at the SA node in the right atrium. A series of valves within the heart prevent the blood from moving in the wrong direction when contractions take place.

Cardiovascular disease is the major cause of death in most developed nations. Heart attacks and strokes are both produced by a build up of tissue inside arteries, which interferes with blood flow to the heart itself, producing a heart attack, or to the brain, resulting in a stroke.

Although cardiovascular disease is a cause of dying of ‘old age’, millions of people die needlessly each year because they fail to reduce their risk factors.

It isn’t difficult to reduce your risk of dying from heart disease. Check your blood pressure and cholesterol level regularly. Maintain an active lifestyle, watch your weight and don’t smoke.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education