The pulmonary circulatory system delivers deoxygenated blood from the right ventricle of the heart to the lungs, and returns oxygenated blood from the lungs to the left atrium of the heart. At its most minute level, the alveolar capillary bed, the pulmonary circulatory system is the principle point of gas exchange between blood and air that moves in and out of the lungs during respiration.

The pulmonary vascular circuit begins with pulmonary arteries that branch from the pulmonary trunk leaving the right ventricle of the heart. Venous blood collected from the systemic and coronary circulation collects in the left atrium and during the diastolic portion of the cardiac cycle, flows into the right ventricle. As the heart contracts during systole, the semilunar valves that comprise the pulmonary valve separating the pulmonary trunk from the right ventricle open to allow blood to be rapidly pumped into the...