The atrium is the upper region of the heart consisting of two chambers in humans.
It connects the blood vessels (which pump blood into the heart) and the ventricles. The right atrium receives blood from the rest of the body (systemic circulation) via the superior vena cava and the inferior vena cava (venous circulation). Blood in the right atrium is always deoxygenated. The left atrium receives blood from the lungs via the pulmonary veins. Blood in the left atrium is always oxygenated. Both atria are connected to ventricles via atrioventricular valves; the ones connecting the right atrium to the right ventricle are tricuspid and the ones connecting the left atrium to the left ventricles are mitral.