in Body Fluids and Circulation (Cardiovascular System, Circulatory system) by

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Cardiac cycle is defined as the series of events taking place through one heart beat. Heart rate depends on the frequency of cardiac cycle.

Following are the events that take place in a cardiac cycle:

  1. Early diastole – in this stage the heart is relaxed, semilunar valves (valves of aorta and pulmonary) close and atrioventricular/AV valves (bicuspid and tricuspid valves) open.
  2. Atrial systole – in this stage the atrium contracts and blood flows from atrium to ventricles. The position of the valves remains the same as that in early diastole.
  3. Isovolumic ventricular contraction – in this stage the ventricles are full and are ready to contract. Both the AV valves and semilunar valves remain closed.
  4. Ventricular ejection – ventricles empty the blood by contracting. Blood is pumped into aorta and pulmonary artery, and thereby to all body parts. During this phase the semilunar valves open and AV valves remain closed.
  5. Isovolumic ventricular relaxation – the ventricles stop contracting and begin to relax. Both the AV valves and semilunar valves remain closed.

The complete cardiac cycle is coordinated by the electrical impulse generated by the SA node and AV node.

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The Cardiac Event that take place from the beginning of One Heart Beat to the beginning of the next, are called as the Cardiac Cycle. The Cycle is initiated by the generation of the action potential at the Sinus node. The Sinus node is present at the opening of the superior vena cava, and the action potential travels from here to both the atria and then through the A-V bundles and finally to the ventricles. Because of this conducting system of the Atria to the Ventricles, there is delay of 0.1 second during the passage of cardiac impulse from the atria into the ventricles. Because of this the Atria contracts before the ventricular contraction, and hence it sends blood of pump the blood into the ventricles before the ventricular contraction begins. Hence, the atria act as the primer Pumps for the ventricles, and in turn ventricles provide the energy for moving blood through the body’s vascular system.

The Cardiac Cycle consist of; 1. Period of relaxation called as diastole. 2. Period of the contraction called as systole.

Relationship of the Cardiac Cycle and the ECG (Electrocardiogram): The electrocardiogram shows the P, Q, R, S, T waves. These waves are the electrical voltages generated by the heart and recorded on electrocardiograph from the surface of the body.

The P wave occurs due to the depolarization of the atria, which causes the atrial contraction.

After the 0.16 second of the onset of P wave, the Q, R, S waves appear as a result of the electrical depolarization of the ventricles. This causes the ventricular contraction and the ventricular pressure begins to rise. Hence, Q, R, S complex comes before the onset of the ventricular systole.

Finally the ventricular T wave appears. This stage is the stage of the ventricular repolarization. Hence, T wave occurs slightly before the end of ventricular contraction.

Functions of Atria as the Primer Pumps: The blood continuously flows in to the atria. After the atrial contraction about 80% of the blood flows in to the ventricles. And the addition 20% blood filling is done by further atrial contraction. And hence we can say that atria functions as the primer pump thereby increasing the ventricular pumping effectiveness as much as 20%. However, heart can even work properly without this extra 20% effectiveness as it has capacity to pump more than needed amount of the blood by the resting body.

Therefore, if atria fail to contract, still the patient may not be able to recognize it, as there may be no signs and symptoms. The failing of atria to contract can only be got when the patient is exercise, or doing so heavy physical work.

Function of the Ventricles as the Pump: During the ventricular systole, the huge amount of blood gets accumulated in the right and the left atria due to closure of A-V valve (Atrio-Ventricular valve). After the systole of ventricles get over, the pressure in the ventricles gradually drops to low diastole. Some amount of the pressure created in the atria due to ventricular systole, causes the opening of the A-V valve and there will be rush of blood in to the ventricles. This is called as the Rapid Filling of The Ventricles.

The rapid filling stage last for one third of the diastole. During middle third only small amount of blood flows in to the ventricles. During the last third of diastole, the atria contract and give an additional thrust to the inflow of blood into the ventricles; this accounts for about 20 per cent of the filling of the ventricles during each heart cycle.

Ventricular Systole: There are three stages in ventricular systole; 1. Period of Isovolumic (Isometric) Contraction, 2. Period of Ejection, 3. Period of Isovolumic (Isometric) Relaxation.

1. Period of Isovolumic (Isometric) Contraction: The pressure in the ventricles rises high just after the start of the ventricular contraction, which causes the closure of the A-V valve. The pressure in the ventricles build up so as to push opens the semilunar valves. This pressure is build up in about 0.02 to 0.03 second. Hence, in this period we can see that there is no emptying of the ventricles in spite of contraction. And hence, this called as the period of ISOVOLUMIC or ISOMETRIC CONTRACTION.

Period of Ejection: There are two periods in it, 1. Period of rapid emptying, 2. Period of slow ejection.

As soon as the pressure in the left ventricle rises slight above the 80mm Hg, the semilunar valves are pushed to opened against the pressure in the aorta and pulmonary artery. This causes immediate poring of the blood out of the ventricles. During first third of the period about 70% per cent of the blood is emptied, n remaining 30% is emptied in the next two third. Hence, first third is called the period of rapid ejection, and the last two thirds, the period of slow ejection.

Period of isovolumic (isometric) Relaxation: After ventricular systole, suddenly the ventricular relaxation begins. Hence, there is decrease in right and left ventricular pressure. There is closure of pulmonary and aortic valves. For next 0.03 to 0.06 second, the ventricular muscle continues to relax, and the volume of ventricles does not change, giving rise to the period of isovolumic or isometric relaxation. Hence, during this period there rapid decrease in ventricular pressure due to low diastolic levels. And hence, the A-V valves open to begin a new cycle of ventricular pumping.