Circulatory system, or cardiovascular system, in humans,
includes the combined functions of the heart, blood, and blood vessels. It transports
oxygen and nutrients to organs and tissues throughout the body and carry away
waste products. Among its vital functions, the circulatory system increases the
flow of blood to meet increased energy demands during exercise and regulates
body temperature. In addition, when foreign substances or organisms invade the
body, the circulatory system swiftly conveys disease-fighting elements of the
immune system, such as white blood cells and antibodies, to regions under
attack. Also, in the case of injury or bleeding, the circulatory system sends
clotting cells and proteins to the affected site, which quickly stop bleeding
and promote healing.
The heart, blood, and blood vessels are the three structural
elements that make up the circulatory system. The heart is the engine of the
circulatory system. It is divided into four chambers: the right atrium, the
right ventricle, the left atrium, and the left ventricle. The walls of these
chambers are made of a special muscle called myocardium, which contracts
continuously and rhythmically to pump blood. The pumping action of the heart
occurs in two stages for each heart beat: diastole, when the heart is at rest;
and systole, when the heart contracts to pump deoxygenated blood toward the
lungs and oxygenated blood to the body. During each heartbeat, typically about
60 to 90 ml (about 2 to 3 oz) of blood are pumped out of the heart. If the
heart stops pumping, death usually occurs within four to five minutes.
Blood consists of
three types of cells: oxygen-bearing red blood cells, disease-fighting white
blood cells, and blood-clotting platelets, all of which are carried through
blood vessels in a liquid called plasma. Plasma is yellowish and consists of
water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats.
(Encarta Encyclopedia)