Question

how a heart function

Answer 1

At first blood reaches to the left atrium from the lungs and passes to the left ventricles then to the aorta and to all body vessels to supply the tissues.
And then it is Drained from these tissues by venules which grow larger and larger till major veins ;svc and ivc  those pass blood into right atrium then to right ventricle into pulmonary artery to reach the lungs where gas exchange occurs then the oxygenated blood is transported via pulmonary veins to the left atruim and so on