smallcirculation

Smallcirculation, commonly called the pulmonary circulation, is the part of the cardiovascular system that carries blood between the heart and the lungs. It is distinct from the systemic (or large) circulation, which supplies the rest of the body with oxygenated blood. The primary function of the small circulation is gas exchange: it routes deoxygenated blood from the heart to the lungs to be oxygenated and then returns it to the heart for systemic distribution.

Pathway: Blood leaves the right ventricle through the pulmonary trunk, which divides into the right and left pulmonary arteries. These vessels branch throughout the lungs and lead to a network of capillaries surrounding the alveoli. In the alveolar walls, carbon dioxide is released from the blood and oxygen is absorbed. The oxygenated blood then collects in pulmonary venules and veins, which merge to form the four pulmonary veins returning to the left atrium.

Physiology: The pulmonary circuit operates at a much lower pressure and resistance compared with the systemic circuit. This low-pressure design protects the fragile pulmonary capillaries. In healthy adults, mean pulmonary arterial pressure is lower than systemic arterial pressure. Regulation involves autonomic input and local factors, including hypoxic vasoconstriction, which helps optimize gas exchange by directing blood toward well-ventilated regions of the lung.

Clinical relevance: Disorders of the small circulation, such as pulmonary hypertension, pulmonary embolism, or edema resulting from left heart failure, can impair gas exchange and raise right-heart workload. Knowledge of pulmonary circulation is essential for interpreting respiratory and cardiovascular assessments and managing related diseases.