Circulation of blood.The cardiovascular, or circulatory, system in humans is composed of the heart and the blood vessels—arteries, veins, and capillaries. Its purpose is to provide nutrients and oxygen to the tissues and to remove wastes from them. It is also where the body fights infections.
The human heart is a pear-shaped muscular organ about the size of a fist. A wall of muscle, the septum, divides the right side from the left. Each of these two sides is further divided into an atrium, or upper chamber, and a ventricle, or lower chamber. The human heart beats 60 to 80 times a minute while a person is at rest. The heart rests only about 0.4 second between beats.
Circuits of the circulatory system.
Circulation of blood consists of two main circuits. Pulmonary circulation carries blood from the heart to the lungs where waste gases, mostly carbon dioxide, are removed from the blood, and oxygen is taken on by hemoglobin in the erythrocytes, or red blood cells (see Blood). The blood then returns to the heart and is pumped to other parts of the body in the systemic circulation, which comprises the blood supply to the entire body except the lungs. Within the systemic circulation is the portal circulation, which supplies blood to the liver. Capillaries and veins carrying nutrient-rich blood from the digestive organs merge to form the portal vein, leading to the liver. Blood from the liver reenters the systemic circulation via the inferior vena cava.