The rhythmic contraction and relaxation of atria and ventricles are known as the heartbeat. Doctors use the stethoscope to hear this. When the ventricles contract and cuspid valves close, a 'lub' sound is produced. When the pulmonary and aortic valves close, a 'dub' sound is produced. Doctors use stethoscope to hear this lub-dub sound in our heart. A normal heart repeats these sounds about 72 times per minute.

Human beings have special types of proteins called antigens, on the surface of RBCs. There are two types of antigens found on human RBCs which are antigen-A and antigen-B. Based on these antigens human blood falls into four groups. They are A, B, AB, and O.

Human beings have special types of proteins called antibody that is produced to attack foreign substances in our body. Our blood plasma contains two types of antibodies which are anti-A and anti-B. Anti-A antibody is present in the 'blood group-B' and anti-B antibody is present in 'blood group A' whereas, 'blood group O' contains both of them. 'Blood group AB' do not have any antibody.