The pulmonary circulation is the uninterrupted flow of blood from the heart to the lungs, and from the lungs back into the heart. Also known as lesser circulation, this is the detailed circuit of the pulmonary circulation:
The deoxygenated (unoxygenated) blood that flows from the vena cava into the right atrium runs down through the tricuspid valve into the right ventricle. The right ventricle contracts during the cardiac systole and pumps the oxygen-poor blood out of the heart. Thus, this deoxygenated blood runs through the pulmonary valve into the left and right pulmonary artery, flowing all the way into the left and right lung respectively.
In the lungs, the pulmonary arteries fork into smaller branches that keep giving off even smaller offshoots until they become capillaries in the lung alveoli (tiny sac-like cavities). The deoxygenated blood flows through the millions of alveoli, releasing the carbon dioxide from the body cells metabolic respiration and simultaneously picking up the oxygen from the atmospheric air we breathe in. Then we exhale the carbon dioxide that has been conveyed in the deoxygenated blood.
Once the hemoglobins in the red-blood cells have picked up oxygen in the alveoli, what used to be deoxygenated blood becomes oxygenated blood, which immediately flows back to the heart through the left and right pulmonary veins. This oxygen-rich blood flows into the heart left atrium; then it runs down through the mitral valve into the heart left ventricle, which in turn pumps the oxygenated blood into the aorta, which is the main artery of the circulatory system. Next, this arterial blood flows through the arteries to the rest of the body to supply all the tissues and organs with oxygen, which is necessary for the production of ATP by the cell mitochondria.
The pulmonary circulation was discovered and explained by the Spanish physician, anatomist, mathematician, and philosopher Miguel Servet (Servetus) in the mid 16th century.
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| Above, schematic picture of heart and pulmonary circulation |