The Arterial System
The blood vessels of the body are responsible for circulating blood to and from every major organ and structure in the body. The vascular system (the network of blood vessels) consists of a single major artery, the aorta, into which oxygenated blood is pumped by the heart (left ventricle) with each heartbeat. The aorta divides into many major branches, conducting arterial blood to the head, body, limbs and all internal organs. Within each major organ or limb, the major artery of supply divides into multiple smaller arteries and eventually into tiny arterioles which transport the blood deep within the tissues. Arterioles then divide into many capillaries, each approximately the size of one of the red blood cells. These cannot be seen with the naked eye. Within the capillaries the red cells release oxygen to the tissues.

The Venous System
The red blood cells then take up carbon dioxide and other toxic materials which require removal from the tissues, in order to keep the organs healthy. From the capillaries the blood drains first into tiny venules, then into larger veins, finally being returned via the major veins to the right side of the heart.

The Pulmonary Circulation System
Blood returning to the right side of the heart contains an excess of toxic gases and little available oxygen. The right side of the heart, with every heartbeat, pumps blood into the pulmonary or lung blood vessels. Within the lungs, the pulmonary arteries branch many times, eventually into the pulmonary capillaries, which are in close contact with the air sacs (alveoli) of the lung. Here, fresh oxygen is obtained from the air that is breathed in (inhalation) and carbon dioxide is released and expelled by the body when a person breathes out (exhalation). The oxygenated blood is then transported a short distance from the lungs back to the left heart where it once again recycles into the arterial system.

It can therefore be appreciated that the heart and heart muscle is responsible for providing the motive, power and energy for ensuring that the blood moves actively through the arterial system, both in the body as a whole (systemic circulation) and in the lungs (pulmonary circulation). Venous blood in the upper half of the body tends to return to the heart by gravity whereas venous blood in the lower limbs, particularly in the standing position, requires some assistance to return to the heart against gravity. The muscles of the lower limbs pump the venous blood towards the heart and this flow is augmented by the piston action of the diaphragm during breathing, as well as the negative intrathoracic (chest cavity) pressure.
Adapted from: Know Your Blood Vessels. Levien LJ et al. KYB Pocket Books. © Ronin-Do (Pty) Ltd.
Used with permission from the publisher.