Cardiovascular System: In order for the body to stay alive, each of its cells must receive a continuous supply of food and oxygen. At the same time, carbon dioxide and other materials produced by the cells must be picked up for removal from the body. This process is continually maintained by the body's circulatory system. The primary circulatory system consists of the heart and blood vessels, which together maintain a continuous flow of blood through the body delivering oxygen and nutrients to and removing carbon dioxide and waste products from peripheral tissues. A subsystem of the circulatory system, the lymphatic system, collects interstitial fluid and returns it to the blood. The heart pumps oxygen-rich blood from the lungs to all parts of the body through a network of arteries, and smaller branches called arterioles. Blood returns to the heart via small venules, which lead to the larger veins. Arterioles and venules are linked even smaller vessels called met arterioles. Capillaries, blood vessels a single cell thick, branch off from the met arterioles and then rejoin them. The network of tiny capillaries is where the exchange of oxygen and carbon dioxide between blood and body cells takes place. The average adult has over 60,000 miles of blood vessels in their body.

Blood: a vital fluid found in humans and other animals that provide important nourishment to all body organs and tissues and carries away waste materials. Sometimes referred to as “the river of life,” blood is pumped from the heart through a network of blood vessels collectively known as the circulatory system.

An adult human has about 5 to 6 liters (1 to 2 gal) of blood, which is roughly 7 to 8 percent of total body weight. Infants and children have comparably lower volumes of blood, roughly proportionate to their smaller size. The volume of blood in an individual fluctuates. During dehydration, for example while running a marathon, blood volume decreases. Blood volume increases in circumstances such as pregnancy, when the mother’s blood needs to carry extra oxygen and nutrients to the baby.

Circulatory System: or cardiovascular system, in humans, the combined function of the heart, blood, and blood vessels to transport 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.