THE CARDIAC CYCLE

The human heart, like that of all mammals and birds, is really two separate pumping system operating within a single organ. The right pump sends blood to the lungs, and the left pump sends blood to the rest of the body.

The heart has two pairs of valves. One pair, the atrioventricular(AV) valves, guards the opening between the atria and ventricles. The AV valve on the right side is the tricuspid valve, and the AV valve on the left is the bicuspid, or mitral,valve. Another pair of valve, together called the semilunar valves, guard the exits from the ventricles to the arterial system; the pulmonary valve is located at the exit of the right ventricle, and the aortic valve is located at the exit of the left ventricle.These valves open and close at the heart goes through its cardiac cycle of rest (diastole) and contraction(systole). The closing of these valves produces the "lub-dub" sounds heard with a stethoscope.

Blood returns to the resting heart through veins that empty into the right and left atria. As the atria fill and the pressure in them rises, the AV valves open to admit the blood into the ventricles. The ventricles become about 80% filled during this time. Contraction of the atria wrings out the final 20% of the 80 milliliters of blood the ventricles will receive, on average, in a resting person. These events occur while the ventricles are relaxing, a period called ventricular diastole. After a slight dealy, the ventricles contract, a period called ventricles systole. Contraction of each ventricle increases the pressure within each chamber,causing the AV valve to forcefully close(the "lub" sound), thereby preventing blood from backing up into the atria. immediately after the AV valves close, the pressure in the ventricles forces the semilunar valves open so that blood can be pushed out into the arterial system. As the ventricles relax closing of the semilunar valves prevents backflow (the "dub" sound).

The right and left pulmonary arteries deliver oxygendepleted blood from the right ventricle to the right and left lungs. As previously mentioned, these return blood to the left atrium of the heart via the pulmonary veins. The aorta and all its branches are systemic arteries, carrying oxygen-rich blood from the right ventricle to all parts of the body. The coronary arteries are the first branches off the aorta; these supply the heart muscle itself. Other systemic arteries branch from the aorta as it makes an arch above the heart, and as it descends and traverses the thoracic and abdominal cavities. These branches provide all body organs with oxygenated blood. The blood from the body organs, now lower in oxygen, return to the heart in the systemic veins. These eventually empty into two major veins: the superior vena cava, which drains the upper body, and the inferior vena cava, which drains the lower body. These veins empty into the right atrium and thereby complete the systemic circulstion.