A: The events that occur from the beginning of one heart beat to the beginning of next constitute a cardiac cycle. It can be divided into three phases (contraction of heart is called systole and relaxation is known as diastole).
1. Atrial Systole (0.1 sec): When the atria are filled with blood, action potential generates in sinoatrial node. Thus a wave of systole spreads over the atria. Hence atria contract.
It increases the flow of blood into ventricles by 30%. Normally blood flows into ventricles from atria without any systole. During atrial systole, deoxygenated blood
from right atrium flows into right ventricle. The oxygenated blood from left atrium flows into left ventricle.
2. Ventricular Systole (0.3 sec): The action potentials of sino-atrial node reaches the
atrio ventricular node, from where, they are conducted into walls of ventricles through
bundle of His and Purkinje fibres. Thus ventricles contract (at the same time, atria
undergo relaxation). When ventricles contract, the bicuspid and tricuspid valves are
closed (to prevent backward flow of blood into atria) causing the first heart sound lub.
At the same time, deoxygenated blood from right ventricle flows into pulmonary arch and
oxygenated blood from left ventricle flows into the systemic arch.
3. Cardiac Diastole (0.4 sec): Now the ventricles relax. Thus pressure falls, resulting in the closure of semilunar valves at the bases of aortic arches causing second heart sound dup. When the pressure falls further, the tricuspid and bicuspid valves open and blood from atria flows into ventricles freely. At this phase all the four chambers of heart are in relaxed state. Soon another cardiac cycle sets in.
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