Pulmonary circulation is the movement of blood from the heart to the lungs and back again. Unlike systemic circulation, which supplies oxygen to the entire body, the pulmonary system specifically targets gas exchange. Deoxygenated blood is pumped out of the right side of the heart, travels through the lungs to drop off carbon dioxide and pick up oxygen, and then returns to the left side of the heart. This low-pressure circuit ensures that delicate lung tissues are protected from fluid overload while maximizing respiratory efficiency.
The structures involved in pulmonary circulation consist of the right ventricle, the pulmonary trunk, the right and left pulmonary arteries, the lung capillary beds, and four pulmonary veins.
Blood flow follows a strict anatomical sequence starting in the right ventricle. When this chamber contracts, it opens the pulmonary semilunar valve and propels blood into the pulmonary trunk. The trunk divides into the right and left main pulmonary arteries, heading toward each respective lung. Inside the lungs, these arteries branch repeatedly into smaller lobar arteries, segmental arteries, and eventually microscopic pulmonary arterioles that feed the capillary beds surrounding the alveoli.
Gas exchange occurs across the ultra-thin alveolar-capillary membrane through simple diffusion. Deoxygenated blood arriving at the pulmonary capillaries has a high partial pressure of carbon dioxide and a low partial pressure of oxygen. Alveolar air has a high concentration of oxygen and a low concentration of carbon dioxide. Gases move down their concentration gradients: oxygen diffuses from the alveoli into the capillary blood, binding to hemoglobin, while carbon dioxide diffuses from the blood into the alveoli to be exhaled.
Once gas exchange is complete, oxygen-rich blood collects from the pulmonary capillaries into small venules. These venules merge to form larger pulmonary veins. Unlike the rest of the venous system, the four pulmonary veins carry oxygenated blood. Two veins emerge from the left lung and two from the right lung. All four empty directly into the posterior wall of the left atrium, ready to be pumped into systemic circulation via the left ventricle.
Pulmonary blood pressure is significantly lower than systemic blood pressure, with a normal mean pulmonary arterial pressure of about 15 mmHg. Low pressure is maintained because the pulmonary blood vessels have large diameters and thin walls that offer very little resistance to flow. Furthermore, a unique physiological response called hypoxic pulmonary vasoconstriction occurs when local oxygen levels drop: small pulmonary arterioles constrict to redirect blood flow away from poorly ventilated alveoli toward areas that are well-oxygenated.
Disruptions in pulmonary circulation can lead to severe cardiopulmonary compromise.