The division of heart and separation of systemic and pulmonary circulation is complete in birds and mammals. The structure of mammalian heart and also how the volume of fluid pumped by the heart is adjusted according to the oxygen needs of the body.
Figure: Mammalian Heart
The mammalian heart is a muscular organ covered by a tough fibrous sheath the pericardium. It consists of four chambers. Each half consists of a thin walled atrium and a thick walled ventricle. There are two sets of valves. Atrioventricular valve separates the cavities of the atrium and ventricle and prevents backflow of blood. Semilunar valves are present where the pulmonary artery leaves the right ventricle and the aorta leaves the left ventricle, preventing backflow. The contraction of heart is called systole and the relaxation, diastole. A heartbeat consists of a rhythmic contraction and relaxation of the whole muscle mass. The heart rests only during the short interval between contractions.