For most humans, blood takes a certain path to circulate throughout the body. The deoxygenated blood first enters through the vena cava—a major artery—into the right atrium of the heart. From there, it passes through a one-way valve called the tricuspid valve (right atrioventricular, or AV) into the right ventricle. Then, a strong muscular contraction forces it out through the pulmonary artery and into the lungs. The blood passes through capillaries in the lungs, where gas exchange oxygenates the blood; it then returns via the pulmonary veins to the heart, entering through the left atrium and on to the left ventricle, which then contracts, sending the oxygenated blood out of the heart to the body’s organs via the aorta.