The right atrium is one of the four chambers of the heart. The heart is comprised of two atria and two ventricles. Blood enters the heart through the two atria and exits through the two ventricles. Deoxygenated blood enters the right atrium through the inferior and superior vena cava. The right side of the heart then pumps this deoxygenated blood into the pulmonary arteries around the lungs. There, fresh oxygen enters the blood stream, and the blood moves to the left side of the heart, where it is then pumped to the rest of the body. There is a major difference between the heart of a developing fetus and that of a fully mature adult: a fetus will have a hole in the right atrium. This allows blood to flow straight through to the left atrium. This is significantly important to a fetus' circulatory health. While in the womb, the fetus draws oxygenated blood from its mother. Once born, lungs become necessary and the connection between the two atria closes.