Cardiac cycle


The cardiac cycle is the series of events in the pumping of blood across the body. Blood flow is correlated with the behaviours of how the 4 chambers of the heart act and how well the body responds to the contraction and relaxation. The whole idea of the cardiac cycle lies in the contraction and relaxation of chambers of the heat generated by the electrical impulses. The cardiac cycle is a very systematic process directly influenced by the ventricular and atrial action. Each phase of the cycle can be assessed through ECG( electrocardiogram) - A procedure to monitor cardiac health.

Table Of Contents

1. Introduction

2. Systole

3. Diastole

4. Chamber Pressures

5. Cardiac cycle and cardiac Output



Cardiac cycle - animation( courtesy to DrJanaOfficial / CC BY-SA



Starting from the systole( a contraction state), the ventricular pressure rises. As a result of the contraction of the 2 ventricles, atrioventricular valves get closed. As a consequence of the closure of valves, the first heart sound LUB is produced. During the valve closure, blood is pushed into the aorta and pulmonary arteries. The rapid increase in the pressure within the right and left ventricles forces both the pulmonic and aortic valves to open. The blood starts moving into the pulmonary artery and aorta, simultaneously. There is no chance of regurgitation (backflow) of blood into the atrium becasue of the AV valve closure. In the beginning, there seems to be a rapid rise in the pressure but slowly the arterial pressure gets distributed evenly as the blood is completely flown into the target sites.  As a result, there is a reduction in the pulmonary artery and aortic pressure, causing the closure of the semilunar valves.



During diastole (a state of relaxation), the ventricles are relaxed. As a consequence, the atrioventricular valves open resulting in the returning of blood from the systemic circulation into the atrium. From the atrium, blood is pushed to ventricles as a result of atrial systole.  Atrial systole is a response to the electrical impulse initiated by the SA node so that the atrial muscles contract causing increased pressure within the atria which inturn result inthe ejection of blood into the ventricles. Soon after this, ventricular systole begins as a result of SA node action in the ventricular area. Phases of the cardiac cycle and the corresponding events are:

  1. Atrial contraction (mitral valve closes)

  2. Biventricular isovolumetric contraction when both valves are closed (aortic valve opens).

  3. Rapid ventricular ejection( Maximum of the blood gets out of ventricles)

  4. Slow ventricular ejection (rest of the blood gets emptied resulting in aortic valve closure).

  5. Ventricular isovolumetric relaxation occurs when both valves are closed (mitral valve opens.

  6. Ventricular filling and diastasis

Phrases of the cardiac cycle


Chamber Pressures

During the ventricular systole, the pressure generated in the right side of the heart is 15 to 25 mm Hg which is more than the pulmonary artery diastolic pressure of 8 to 15 mm Hg. As a result, the blood is ejected into the pulmonary circulation from the pulmonary arteries. During the diastole, there is an inflow of venous blood into the atrium because of the higher pressure in the superior and inferior vena cava ( approximately 8 to 10 mm Hg which is more than that of the atrium). At his stage, the blood flows through the open tricuspid valve towards the right ventricle until the  2 right chamber pressures get back to the range of 0 to 8 mm Hg. During the systole, there is high-pressure in the ventricles (110 to 130 mm Hg) which is way higher than the resting aortic pressure of 80 mm Hg. The pressure gradient help to eject blood into the aorta. During left ventricular ejection, the resultant aortic pressure is 110 to 130 mm Hg that eventually pushes the blood through the pulmonary arteries. The forward directional movement of blood flow into the aorta ceases as soon as the ventricle relaxes. During the diastole, oxygenated blood returns from the pulmonary circulation into the left atrium. Blood readily flows into the left ventricle because ventricular pressure is also low. At the end of diastole, pressure in the atrium and ventricle gets to 4 to 12 mm Hg.


Cardiac cycle and cardiac Output

Cardiac output is the net amount of blood pumped out by the ventricle for a brief 1 minute period. It is approximately 5 Liters per minute in a normal adult. On the other side,  Stroke volume is the total amount of blood ejected by each individual heartbeat. Under stable conditions, in an adult, the average resting stroke volume is 70 mL, and the heart rate is 60 to 100 beats per minute (bpm). Cardiac output complements the cardiac cycle. CO=SVxHR, Where CO=Cardiac output, SV=Stroke volume and HR=Heart rate




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