Let us follow a single red blood cell (RBC) through one full cycle along the circulatory pathway Remember that RBCs carry oxygen throughout the body. Since the blood travels endlessly, an arbitrary choice must be made of a starting point to describe the RBC's route. We will begin at the point where the RBC has delivered its oxygen to a cell in need and is on its return back to the heart.
1. Once the deoxygenated red blood cell (RBC) returns to the heart, it enters either through the superior vana cava or the inferior vena cava. The superior vena cava returns deoxygenated blood from the upper part of the body to the heart. The inferior vena cava returns deoxygenated blood from the lower part of the body to the heart. These large veins lead into the right atrium.
2. The RBC passes through the tricuspid valve into the right ventricle.
3. The RBC is then pumped through the pulmonary valve into the pulmonary artery and on to the lungs. There the RBC gives off carbon dioxide and picks up oxygen.
4. The RBC returns to the heart through a pulmonary vein, enters the left atrium, passes through the mitral valve, and flows into the left ventricle.
5. The left ventricle pumps the fully oxygenated RBC through the aortic valve, into the aorta, the body's main artery, and out to the body.
6. From the aorta, the RBC flows into one of the many arteries of the body, through the arterioles, and then to the capillaries, where the RBC will deliver oxygen and nutrients to the cells and remove wastes and carbon dioxide. Next it moves through the venules, veins, and on to the vena cava in a deoxygenated state, and returns to the heart, only to begin its repetitive journey once again. This whole process has taken approximately 20 seconds!