The small (pulmonary) circulation serves to enrich the blood with oxygen in the lungs. It begins in the right ventricle, where all the venous blood that enters the right atrium passes through the right atrioventricular (atrioventricular) opening. From the right ventricle comes the pulmonary trunk, which, approaching the lungs, is divided into the right and left pulmonary arteries.
The latter branch into the lungs into arteries, arterioles, precapillaries, and capillaries. In capillary nets that interweave pulmonary vesicles, the blood gives up carbon dioxide and receives in exchange a new supply of oxygen (pulmonary respiration). Oxidized blood becomes scarlet again and becomes arterial. The oxygen-rich arterial blood flows from the capillaries into the venules and the veins, which, merging into four pulmonary veins (but two on each side), flow into the left atrium.
In the left atrium, the small (pulmonary) circulatory circuit ends, and the arterial blood that enters the atrium passes through the left atrioventricular orifice into the left ventricle, where the great circulation begins.