Two circuits, or subdivisions, for blood flow with respect to gas exchange:
Pulmonary circuit : blood flow between the heart and lungs
Systemic circuit : blood flow between heart and body tissues
Oxygen-poor blood returns to the right atrium via the superioir and inferior venae cava and coronary sinus
The right atrium contracts, forcing blood through the tricuspid valve into the rigt ventricle
The right ventricle contracts, closing the tricuspid valve, and forcing blood through the pulmonary semilunar valve into the pulmonary trunk and arteries
The pulmonary arteries carry blood to the lungs, where it enters alveolar capillaries, the site of gas exchange with the alveoli of the lungs; here the blood drops off carbon dioxide and picks up oxygen
Oxygen-rich blood flows back to the left atrium of the heart via pulmonary veins
The left atrium pumps blood through the mitral (bicuspid) valve into the left ventricle
The left ventricle contracts, closing the mitral valve, opening the aortic semilunar valve, and pumping blood into the aorta for distribution to the systemic circuit of the body