Cardiac cycle

The sequential events occuring from the initiation of one heartbeat to
the commencement of the next is called as one cardiac cycle. In this cycle,
the contraction phase is called systole. The relaxation phase is the diastole.

A single heart beat comprises a systole and diastole in both atria and
ventricles.

Atrial systole

There is a continuous flow of blood into the right atrium
through superior and inferior vena cava and coronary sinus. Simultaneously
the left atrium receives blood from 4 plulmonary veins. There is a passive
movement of nearly 70% of the blood. The remaining 30% is pumped into
the ventricles by atrial contraction.

Ventricular filling

When the valves in between atria and ventricles open
nearly two-third of the ventricle is filled. Remaining space gets filled up by
atrial contraction. (Cardiac Cycle)

Ventricular systole

As the atrial systole ends, the action potential
generated by the SA node reaches the AV node and rest of the fibrous
system. It causes contraction of the ventricular wall. Thus ventricular
pressure results. The very strong ventricular pressure pumps the blood into
respective arteries by causing the semilunar valves to open.

Ventricular diastole

Soon after the blood leaves the ventricles there is a
fall in the ventricular pressure. The semilunar valves close and the atrial
valves open to begin the next cycle. (Cardiac Cycle)

Heart sound

The heart sound felt by a stethescope is caused due to the
closure and opening of the valves. The generation of sound is rhythmic. The
first sound is louder (lubb) and of longer duration (0.16-0.90sec). It is due
to closure of the atrioventricular valves at the beginning of the ventricular
systole. The second sound is of shorter duration (dubb) (0.10sec). It is caused
at the end of the ventricular systole by the closure of semilunar valve. The
heart beats at the rate of about 72-80 times per minute in adults. The ventricular
systole causes a wave of distension due to blood flow. It is called as arterial
pulse. It can be felt on the wrist. The pulse rate corresponds to rate of
heartbeat. (Cardiac Cycle)