Explain the Cardiac Cycle of human body?

The different stages of a heart pumping cycle are referred to as follows: contraction of the ventricles is called systole, and relaxation is diastole. Blood pressure rises to a maximum during systole, and falls to a minimum during diastole. Normal blood pressure for an adult is 120 mm Hg during systole, and 80 mm Hg during diastole, commonly recorded as 120/80.

During the relaxation phase, blood flows from the atria into the ventricles. Contraction of the two atria followed by contraction of the ventricles is referred to as one cardiac cycle. The "lub-dub" sounds you hear through a stethoscope are actually caused by this complete cycle.

Contraction of the heart is governed by natural pacemakers, which are assemblies of specialized cells producing electrical signals to the heart muscles at a regular rate. Contraction of the stimulated cells excites neighboring cells. Although the pacemaker cells produce signals on their own, their rate can be altered by nerve impulses from the autonomic nervous system.

Pacemaker cells are found in the sinoatrial (SA) node in the wall of the right atrium, and a smaller number in the atrioventricular (AV) node in the floor of the right atrium. A stimulus starts at the SA node, which causes the atria to contract. This signal is then carried to the AV node, which initiates contraction of the ventricles.

When coordination of the contractions is not properly synchronized, a condition called fibrillation develops. Atrial fibrillation is not immediately life-threatening, but it can lead to fatal blood clotting. Ventricular fibrillation causes an inadequate blood supply to the brain and vital organs and is quickly fatal.