Pumping action of blood / way of blood circulation/ contraction and relaxation of heart/beating of heart/Double circulation

Heart is really a double pump. The left side of heart (left atrium and left ventricle) acts as one pump which pumps blood into the whole body, except the lungs.

 The right side of heart (right atrium and right ventricle) acts as another pump which pumps blood only into the lungs Left atrium receives oxygenated (blood containing oxygen) blood from the lungs via pulmonary vein.

 When left atrium receives the blood it relaxes or expands. 

Then Left atrium contracts so it can transfer the blood to the left ventricle. 

When left ventricle receives the blood it relaxes or expands so it can collect the blood. 

 Then left ventricle contracts so that it can pump the blood out through aorta for the circulation in the body. 

 Right atrium receives the deoxygenated (blood containing carbon di oxide) blood from the body via vena cava. 

 When right atrium receives the blood it relaxes or expands . 

 Then the right atrium contracts so that it can transfer the blood to the right ventricle.

 When the right ventricle receives the blood it relaxes or expands so that it can receive the blood or dilates.

Then the right ventricle contract so that it can pump the blood out through pulmonary artery to lungs.