What happens to the blood when it enters the heart?
The heart has four chambers; the right ventricle and atrium, and the left ventricle and atrium. Deoxygenated blood, once it has travelled the entire body, enters the heart from the vena cava into the right atrium. There, it passes through the tricuspid/atrioventricular valve into the right ventricle when the atrium contracts. The valves snap shut to prevent backflow as the ventricle fills. The ventricle then pumps the blood out through the semi-lunar/pulmonary valve, into the lungs via the pulmonary artery (the only artery where the blood is deoxygenated). In the lungs, the blood is reoxygenated by releasing CO2 and picking up O2.
The blood them travels back into the heart, entering through the pulmonary vein into the right atrium. The atrium contracts and pushes the blood past the bicuspid/atrioventricular valve, into the right ventricle. This ventricle is more muscular than the left one, because its job is to push the blood back into the circulatory system with enough pressure to continue the flow of blood through the whole system. The blood is pumped out through the aortic/semilunar valve into the aorta, and will start its journey around the body once again.
