The upper chambers of the heart are called the left and right atria. The primary function of both the atria is to receive blood that is coming into the heart.
In addition to two upper chambers, the heart also has two lower chambers. The lower chambers are known as the left and right ventricles, which have a primary responsibility of pumping blood out of the heart. Separating the upper and lower chambers are different valves, which control the direction and rate of blood flow. A complete circulating cycle is defined as each instance the two atria bring in blood and the ventricles pump out blood.