Riding on a Drop of Blood

1. From the right atrium the blood passes to the right ventricle through the tricuspid valve, which consists of three flaps of tissue. The tricuspid valve remains open during diastole, or ventricular filling. When the ventricle contracts, the valve closes, sealing the opening and preventing backflow into the right atrium. Five cords attached to small muscles, called papillary muscles, on the ventricles' inner surface prevent the valves' flaps from being forced backward.
2. From the right ventricle blood is pumped through the semilunar valve, which has three half-moon-shaped flaps, into the pulmonary artery. This valve prevents backflow from the artery into the right ventricle. From the pulmonary artery blood is pumped to the lungs where it releases carbon dioxide and picks up oxygen.
3. From the lungs, the blood is returned to the heart through pulmonary veins, two from each lung. From the pulmonary veins the blood enters the left atrium and then passes through the mitral valve to the left ventricle. As the ventricles contract, the mitral valve prevents backflow of blood into the left atrium, and blood is driven through the aortic valve into the aorta, the major artery that supplies blood to the entire body.