May 6, 1998

The human heart consists of four chambers. The left and right atrium and ventricles. The atrium chambers receive blood returning to the heart and the ventricle chambers pump blood out of the heart. Arteries carry blood away from the heart to organs throughout the body. Veins return blood to the heart.

The ventricle pumps blood into a forked artery that directs the blood into two different circuits. One circuit leads to the lungs and skin where the blood picks up oxygen while flowing through the capillaries, microscopic blood vessels penetrating the tissues and consisting of a single layer of cells that allows exchange between the blood and tissue fluids. A second circuit leads to all the organs except the lungs and then returns to the heart through veins.

Each heartbeat is a sequence of muscle contraction and relaxation called the cardiac cycle. The four chambers of the heart go through phases of contraction, or systole, and relaxation, or diastole in each cycle. The timing of these phases is not the same for all the chambers; the atria contract slightly before the ventricles.

The major artery leading from the heart and supplying blood to the body is called the aorta. The major arteries that carry blood to the lungs from the heart are the pulmonary arteries. The Superior Vena Cava artery transports blood from the head and arms into the right atrium. The Inferior Vena Cava transports blood from the legs into the right atrium also.

Both the atria and ventricles are relaxed when they are filling with blood. As pressure rises in the atria, the valves between the atria and ventricles are forced open, and the ventricles start to fill. As the atria contract, additional blood is forced into the ventricles. The ventricles then contract, causing the AV valves, the valves between the atria and ventricles including the Tricuspid valve connecting the right atrium to the right ventricle and the Bicuspid valve connecting the left atrium to the left ventricle, to snap shut, and the pressure inside the ventricles rises sharply. With the increased pressure, the valves leading to the aorta and the pulmonary artery open, and blood flows out of the heart. The valve that is connected to the right ventricle to the pulmonary artery is the pulmonary semi-lunar valve and the valve that is connected to the right atrium to the aorta is the aortic semi-lunar valve. After the blood has been ejected, the ventricles relax, and the cardiac cycle starts again.