34. Describe the flow of blood through the heart during different phases of the cardiac cycle.

The sequence of events, which occur from the beginning of one heartbeat to the beginning of the next is called cardiac cycle.

The phase of contraction is called systole and the phase of relaxation is diastole. The pumping action during single cardiac cycle involves the following steps:

(i) Atrial systole : Under normal conditions, blood continuously enters into right atrium through superior vena cava, as well as inferior vena cava and coronary sinus. The pulmonary veins bring blood to the left atrium from lungs. From the atria, nearly 70% of the blood passively flows into the ventricles. Rest of the blood is pumped into the ventricle by the contraction of atria. The arrangement of valve is such that it allows the flow of blood from atria to ventricles. Atrial systole lasts for about 0.15 second.

(ii) Ventricular filling : As soon as the atrioventricular valve opens, nearly one-third of the ventricle is filled. Rest of the ventricular filling takes place during the contraction of the atria.

(iii) Ventricular systole : When the atrial systole approaches its end, the action potential passes from SA node to the AV node, as well as the ventricles. Consequently, the ventricles contract which results in the rise of ventricular pressure and both the atrioventricular valves are closed to prevent the backflow of blood. The contraction of ventricle is so strong that the ventricular pressure exceeds the aortic pressure, as a result of which, the semilunar valve opens. The blood is pumped into the major arteries. 

(iv) Ventricular diastole : The ventricular systole is followed by ventricular diastole. The semilunar valves close to prevent backflow of blood from aortas. At the same time, the AV valve opens to allow the flow of blood from both the atria to the respective ventricles.