As the ventricle is completely divided in birds, mammals and some reptiles (crocodiles, alligator), the left and right parts of the heart function as air tight conduits for pure and impure blood. The right part receives impure blood from whole body and sends it to the lungs for oxygenation. The left part receives purified blood from the lungs and supplies it to the whole body. Thus, the right and left parts of the heart respectively serve as completely separated pulmonary and systemic hearts. This is known as double heart circuit. In man, the rate of heart beat (double circulation) is about 75 times per minute