CARDIAC CYCLE

It is the sequence of events which take place during the completion of one heart beat. Heart beat involves three distinct stages. Relaxation phase – diastole. Blood entry into atria. The deoxygenated blood enters right atrium through vena cava, and oxygenated blood enters left atrium through pulmonary veins.

Relaxation of Walls.

The walls of the atria and that of ventricles are relaxed. As the atria are filled with blood, they become distended and have more pressure than the ventricles. This relaxed period of heart chambers is called diastole.

2. Atria Contract

– atrial systole The muscles of atria simultaneously contract, when the atria are filled and distended with blood, this is called atrial systole. The blood passes through tricuspid and bicuspid valves, into the two ventricles which are relaxed.

3. Ventricles contract – ventricular systole.

When the ventricles receive blood from atria, both ventricles contract simultaneously and the blood is pumped to pulmonary arteries and aorta. Lubb Sound. The tricuspid and bicuspid valves close, and „lubb‟ sound is made.

Dubb Sound.

Ventricular systole ends, and ventricles relax at the same time semilunar valves at the base of pulmonary artery and aorta close simultaneously, and „dubb‟ sound is made. (Lubb, dubb can be heard with the help of a stethoscope).

Duration of beat.

One complete heart beat consists of one systole and one diastole, and lasts for about 0.8 seconds. Beating. In one‟s life, heart contracts about 2.5 billion times, without stopping.