Define systole.

Hint: During a heartbeat, there is contraction and relaxation of auricles and ventricles in specific sequence. Various series of events that occur during a heartbeat is known as cardiac cycle.

Complete Answer:
The contraction of heart is known as systole while its relaxation phase is known as diastole.

The two types of systole are:
1. Auricular systole: When the ventricles and auricles are filled with blood a wave of contraction. This impulse stimulates the auricular walls to contract. It is the second rapid filling stage. The blood from the respective auricles is pushed into the ventricles via the bicuspid and tricuspid valves. This process takes nearly 0.1 seconds and is responsible for the production of fourth heart sound.

2. Ventricular systole: After the atrial systole, the muscles of the auricles relax and it enters into the diastolic phase. During the diastolic phase, the blood vessels are again filled with blood.
- The impulse is then transferred to the AV node which then conducts the impulse to the ventricles via the Bundle of His to Purkinje fibres to Ventricular wall muscles. This stimulates the ventricular contraction.
- The ventricular systole begins along with the auricular diastolic phase. This results in an increased pressure of blood in the ventricle and it rises more than the pressure of the blood within the atria. Soon the AV valves close, producing the sound “Lubb…” and this prevents the backflow of blood from ventricles to auricles.
- When the blood pressure in the ventricles rises and goes beyond pressure of aorta then the semilunar valves open and blood flows into the respective aortae and its main branches.

Note: There is an alternate cyclic contraction i.e. systole and relaxation i.e. diastole constituting the heart beat of 0.8 second. Atrium takes 0.1 sec for systole and 0.7 sec for diastole while ventricles take 0.3 sec in systole and 0.5 sec for diastole.