The two main components to the circulatory system work simultaneously: one is systemic circulation, in which the left side of the heart pumps oxygenated blood throughout the body, except for the lungs. In the other, blood is pumped by the right side of the heart to the lungs by means of pulmonary circulation. It is designed to collect oxygen from the lungs, so it can be distributed throughout the body by systemic circulation. It also rids the body of carbon dioxide, the waste product of cellular metabolism. The manner in which systemic circulation is performed is, as follows:
After oxygen has been distributed by way of systemic circulation to all of the body’s cells, the deoxygenated blood is returned to the right atrium of the heart by the two largest veins: the superior vena cava, and the inferior vena cava. This blood is pumped by the right atrium, through the tricuspid valve, and into the right ventricle. The right ventricle contracts, forcing the blood past the pulmonary valve, and to the lungs by way of the pulmonary artery. Arteries always transport blood from the heart to various parts of the body, while veins always transport blood back to the heart.
As blood flows through the pulmonary artery, it branches into smaller blood vessels called arterioles, which continue to branch off until they become capillaries. The capillaries expose the blood to oxygen in the lungs tiny air sacs, which are called alveoli. The blood transports carbon dioxide produced by the cells to the alveoli while it picks up oxygen. The carbon dioxide is then expelled from the lungs every time we exhale. The oxygen binds to the hemoglobin in red blood cells; turning blood into a bright red color, and is then transported back to the heart by the pulmonary vein to the left atrium of the heart, where systemic circulation then takes place.
Pulmonary circulation requires proper functioning of both heart and lungs. Lung diseases such as emphysema and pneumonia can reduce the amount of oxygen available to the blood, while heart disease can disrupt the flow of oxygen from the lungs to the rest of the body. Blood diseases can also adversely affect pulmonary circulation. For instance, a low red blood cell count, called anemia, will reduce the blood’s capacity to carry oxygen.
Maintaining proper pulmonary circulation is essential for sustaining the life of the organism; whether it is a hummingbird, or a human being.