ATRIAL CONTRACTIONS
The atria are thin-walled sacs that function as collection chambers for blood returning to the heart. They have relatively thin walls because they only pump blood the short distance to the ventricles. The right atrium receives deoxygenated blood returning from the systemic circuit through the superior and inferior venae cavae. The left atrium receives oxygenated blood returning from the pulmonary circuit through the pulmonary veins.

Both atria contract simultaneously, squeezing the blood into the ventricles through a pair of one-way atrioventricular valves that separate the atria from the ventricles. The valve that separates the right atrium and ventricle has three flaps, and is called the tricuspid valve. The valve that separates the left atrium and ventricle has two flaps and is called the bicuspid, or mitral, valve. Both of these valves prevent backflow into the atria upon ventricular contraction.