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Pulmonary circulation is the system through which oxygen is added to the blood. Deoxygenated blood is sent from the heart to the lungs, where it gathers oxygen and leaves carbon dioxide behind, and then is sent back to the heart to be distributed to the rest of the body. It is a part of the larger circulatory system, which can be though of as the transport system of the body: It transports blood around the body through a vast system of blood vessels, carrying everything from nutrients to hormones to water, before cycling back through the heart.
In a simpler sense, the circulatory system is composed of two loops: the pulmonary circulation loop and the systemic circulation loop, although some sources also include the coronary loop as well. Systemic circulation refers to the system through which oxygenated blood — blood that has oxygen in it — is sent out into the body and then returns to the heart after delivering its nutrients to distant cells. In the pulmonary circulation loop, the blood is oxygenated by the lungs in preparation for entering the systemic loop again. The coronary loop moves blood through the heart, which is vital for pulmonary circulation to occur.
Blood cells enter pulmonary circulation after returning from a trip around the body and enter the right atrium of the heart through two major veins, the superior and inferior vena cava. At this point in the journey, the blood cells do not contain any oxygen. From the atrium, the heart pushes the blood through the tricuspid valve into the right ventricle and then through the pulmonary valve into the pulmonary artery. The pulmonary artery splits in two and carries the blood to both lungs where it will receive oxygen.
In the lungs, the arteries branch into ever smaller tubes, eventually pushing the blood through tiny vessels called capillaries. The average diameter of a capillary is about eight microns, or roughly the size of one blood cell. Capillaries are spread over the walls of the minute air sacs in the lungs, called alveoli, where oxygen diffuses through the walls of the capillaries and is picked up by the blood cells. The blood cells, in turn, drop waste carbon dioxide into the lungs, allowing it to be exhaled.
Now laden with oxygen, the blood cells are sent back towards the heart. The tiny capillaries segue into pulmonary veins, which merge into ever larger ones until there are two from each lung. These are referred to as the right superior and inferior pulmonary veins and the left superior and inferior pulmonary veins. All of them, however, empty into the left atrium of the heart. With a contraction of the heart muscle, the blood will be forced through the mitral valve into the left ventricle, and then through the aortic valve and out the aorta, where it enters the systemic loop to deliver oxygen-rich blood to the body.
The following summarizes each step in the circulation path: