The tricuspid valve forms the boundary between the right ventricle and the right atrium. Deoxygenated blood enters the right side of the heart via the inferior and superior vena cava. These are large veins that transport deoxygenated blood from the body back to the heart. Blood collects within the right atrium, and it must flow through the tricuspid valve in order to enter the right ventricle. Then, blood exits the heart via the pulmonary artery, which transmits blood to the lungs for oxygenation. The term 'tricuspid' refers to how the valve is constructed. It contains three flap-like cusps that, when closed, keep blood from regressing back into the right atrium. This regression is known as tricuspid regurgitation, and it is common in diseased hearts, often as a result of drug abuse. The tricuspid valve is not the only cardiac valve that is 'tricuspid' in nature. On the left side of the heart, the aortic valve also features three cusps. This valve serves a boundary between the left ventricle and the aorta. There is some congenital variation, however; some people are born with two-cusp-only aortic valve.