Pulmonary and Systemic Circulations

The cardiovascular system comprises two circuits operating in series: the pulmonary circulation and the systemic circulation. Total blood volume can be estimated at approximately 70 mL per kilogram of body weight, giving most adults 4-6 litres of circulating blood.

The pulmonary circulation carries deoxygenated blood from the right heart to the lungs and returns oxygenated blood to the left heart. The pathway is: right ventricle → pulmonary arteries → pulmonary capillaries → pulmonary veins → left atrium. Gas exchange — oxygen loading and carbon dioxide unloading — occurs in the pulmonary capillaries. A separate pulmonary circuit exists so that the lungs do not compete with systemic organs for blood flow. Crucially, all deoxygenated blood must pass through the lungs. The pulmonary vasculature has very low resistance, creating a low-energy circuit that slows blood velocity through the capillaries and allows more time for gas exchange. Note that the lung parenchyma itself also receives oxygenated blood supply via the bronchial arteries from the systemic circulation.

The systemic circulation delivers oxygenated blood from the left heart to all body organs and returns deoxygenated blood to the right heart. The pathway is: left ventricle → aorta and branches → tissue capillaries → venae cavae → right atrium.

Vascular beds supplying different organs in the systemic circulation are arranged in parallel. This is physiologically advantageous: the total resistance of parallel resistors is lower than any individual resistance, maximising overall flow, and blood flow to each organ can be independently regulated by its arterioles. Exceptions to this parallel arrangement include the splanchnic portal circulation (gut → portal vein → liver) and the renal portal-like arrangement of glomerular and peritubular capillaries.

Blood flow to different organs does not always correlate with metabolic rate. Skeletal muscle, liver, gut, and brain receive flow proportional to oxygen consumption. The skin receives high flow for thermoregulation rather than metabolic need. The kidneys receive approximately 20-25% of cardiac output primarily for filtration. Cerebral blood flow remains remarkably constant at about 0.75 L/min due to tight autoregulation.

The distribution of blood volume across the circulation is unequal. Veins and venules — the capacitance vessels — contain the largest volume (~65%). Pressure is highest in the aorta and falls progressively, with the steepest drop occurring across the arterioles (the resistance vessels). Blood velocity and cross-sectional area are inversely related (v1A1 = v2A2): as the total cross-sectional area of capillaries is very large, blood velocity there is very slow, maximising contact time for exchange.