Pulmonary Circulation and the Ventilation- Perfusion Ratio

The heart drives two separate and distinct circulatory systems in the body: the pulmonary circulation and the systemic circulation. The pulmonary circulation is analogous to the entire systemic circulation. Similar to the systemic circulation, the pulmonary circulation receives all of the cardiac output. Therefore, the pulmonary circulation is not a regional circulation like the renal, hepatic, or coronary circulations. A change in pulmonary vascular resistance has the same implications for the right ventricle as a change in systemic vascular resistance has for the left ventricle. The pulmonary arteries branch in the same tree-like manner as do the airways. Each time an airway branches, the arterial tree branches so that the two parallel each other (Fig. 20.1). More than 40% of lung weight is comprised of blood in the pulmonary blood vessels. The total blood volume of the pulmonary circulation (main pulmonary artery to left atrium) is approximately 500 mL or 10% of the total circulating blood volume (5,000 mL). The pulmonary veins contain more blood (270 mL) than the arteries (150 mL). The blood volume in the pulmonary capillaries is approximately equal to the stroke volume of the right ventricle (about 80 mL) under most physiological conditions.