pulmonary circulation
What is the primary function of pulmonary circulation?
How does pulmonary circulation differ from systemic circulation?
Where does gas exchange occur in pulmonary circulation?
What are some disorders affecting pulmonary circulation?
pulmonary circulation, portion of the circulatory system that carries blood between the heart and the lungs. Its primary function is to replenish the blood’s oxygen supply and remove carbon dioxide, a waste product of cellular metabolism. Together with systemic circulation, which delivers blood to the rest of the body, pulmonary circulation forms one of the two major circuits of the cardiovascular system.
In mammals, birds, and other animals with four-chambered hearts, pulmonary circulation begins when the right ventricle pumps oxygen-poor blood into the pulmonary artery. Unlike most arteries, which carry oxygenated blood, the pulmonary arteries transport deoxygenated blood to the lungs. The pulmonary artery divides into right and left branches, which repeatedly subdivide into smaller vessels until they reach the capillary networks surrounding the alveoli, the tiny air sacs where gas exchange occurs.
Within the alveolar capillaries, carbon dioxide diffuses from the blood into the lungs to be exhaled, while oxygen moves from inhaled air into the bloodstream. The oxygen-rich blood then flows into progressively larger vessels that form the pulmonary veins. These veins return the blood to the left atrium of the heart, where it enters systemic circulation and is distributed throughout the body.
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Pulmonary circulation evolved in early vertebrates as lungs became important respiratory organs. Lungfishes and amphibians were among the first animals to possess a partial separation between pulmonary and systemic blood flow through a three-chambered heart. In crocodilians, birds, and mammals, the complete division of the ventricle into right and left chambers produced a four-chambered heart with separate pulmonary and systemic circuits. This, increased the efficiency of oxygen transport and supported higher metabolic demands.
Because pulmonary circulation operates at much lower pressures than systemic circulation, it allows efficient gas exchange without damaging the delicate tissues of the lungs. Disorders affecting this pathway include pulmonary hypertension, which increases pressure within the pulmonary arteries, and pulmonary embolism, in which a blood clot blocks blood flow to part of the lung. Such conditions can reduce oxygen uptake and place additional strain on the heart. Chronic lung diseases and certain congenital heart defects can also impair pulmonary circulation.