Circulation of a Red Blood Cell Through The Heart and Body

Circulation path of a red blood cell through the body

Blood flows in these two circuits in humans:

a. A pulmonary circuit loops between the heart and lungs. Oxygen-poor blood from systemic veins enters the heart’s right atrium, is pumped through pulmonary arteries to the two lungs, picks up oxygen, then flows through pulmonary veins to the heart’s left atrium.

b. A systemic circuit loops between the heart and all tissues. Oxygenated blood in the left atrium flows into the left ventricle, is pumped into the aorta, then is distributed to capillary beds. There, the blood gives up oxygen and picks up carbon dioxide. Systemic veins return the blood to the heart’s right atrium.

Circulation path of a red blood cell through the heart

Each time the heart beats, its four chambers go through phases of systole (contraction) and diastole (relaxation). The sequence of contraction and relaxation is a cardiac cycle. First, relaxed atria fill with blood. Increasing fluid pressure forces both AV valves to open. Blood flows into the ventricles, which fill when the atria contract. As the filled ventricles contract, the rising fluid pressure forces the AV valves shut. It rises so sharply above the pressure in the great arteries that it forces the semilunar valves to open- and blood leaves the heart. The ventricles relax, semilunar valves close, and the already filling atria are ready to repeat the cycle. Contraction of the ventricles is the driving force for blood circulation.