Blood circulates in two linked circuits: the pulmonary, which carries blood to the lungs to be oxygenated, and the systemic, which supplies oxygenated blood to the body. Arteries carrying blood from the heart divide into smaller vessels called arterioles and then into capillaries, where nutrient and waste exchange occurs. Capillaries join up to form venules, which in turn join to form veins that carry blood back to the heart. The portal vein does not return blood to the heart but carries it to the liver.
The heart powers the pulmonary and the systemic circulations. In the pulmonary circulation, deoxygenated blood (blue) travels to the lungs, where it absorbs oxygen before returning to the heart. This oxygenated blood (red) is pumped around the body in the systemic circulation. Body tissues absorb oxygen, and deoxygenated blood returns to the heart to be pumped to the lungs again.
The blood pressure in the veins is about a tenth of that in the arteries. Various physical mechanisms ensure that there is adequate venous return (blood flow back to the heart). Many deep veins lie within muscles. When the muscles contract, they squeeze the veins and force blood back to the heart. The action of inhalation during breathing also draws blood to the heart. In addition, venous return from the upper body is assisted by gravity.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.