Mammal hearts In mammals the four-chambered heart separates the blood circulation into pulmonary and systemic branches.and all the blood flows through each in turn.The hearts of different mammals vary only in external form: their construction remains the same.In the pulmonary circulation the blood is saturated with oxygen and carbon—dioxide is released.In the systemic circulation the arterial blood releases oxygen to the tissues and absorbs carbon—dioxide.This venous blood flows through the superior(1)and inferior(2) vena cava to the right atrium f3).During the relaxation of the ventricular muscle (diastole)it flows through a tricuspid valve into the right ventricle f41.The contraction of this chamber(systole) forces the blood into the pulmonary artery(5).which has the pulmonic valve at its origin.then on through the two branches of the pulmonary artery (6+7)into the lungs.From there the arterial blood is returned through the pulmonary veins(8)to the left atrium(9).During the diastole the blood flows through amitral valve into the left ventricle r10).During the systole it is forced through the aorta(11)with the aortic valve at its origin.All valves prevent a backflow of the blood into the preceding chamber when the cardiac muscle is contracting.From the aorta arise the vessels supplying the heart(12+13)and large arteries supplying the head.neck and superior extremity with blood: right brachiocephalic artery(14).left carotid artery(15).and left subclavian artery f 16).The venous cardiac blood flows through the great cardiac vein(17)directly into the right atrium.In the dog heart only two large arteries f14+15)arise from the arch of aorta.