Cardiac cycle

Introduction

A cardiac cycle is the graphic representation of the series of events while the blood is being circulated through all parts of the body, however, it is closely linked with the cardiac events that occur within the heart. More precisely, cardiac cycle is the consequence of opening and closing of various chambers and valves of the heart. It is very clear that the direction and force of blood flow in the heart is correlated with the behaviours of  4 chambers of the heart . In general, cardiac cycle is the response to systole and diastole as a result of  contraction and relaxation of chambers. The whole idea of the cardiac cycle lies in the contraction and relaxation of chambers due to the electrical impulses formed by nodes of the heart( SA node and AV node) .  Ventricles are responsible for systemic arterial circulation whereas the atrium helps in receiving the blood from the body cells. A cycle lasts for an approximate  0.8 seconds and each phase of the cycle can be assessed through ECG( electrocardiogram) - A procedure to monitor cardiac health. This post is intended to describe the phases of the cardiac cycle and the changes that occur while the heart is active.

 

Cardiac cycle - animation( courtesy to DrJanaOfficial / CC BY-SA  https://creativecommons.org/licenses/by-sa/4.0))

 

Systole

Starting from the systole( a contraction state), the ventricular pressure of the heart gradually rises due to the contraction of the 2 ventricles. At the same time, atrioventricular valves on either side gets shut which help to create a temporary pause of blood flow from the ventricles into the corresponding major arteries. Accordingly, the first heart sound LUB is produced. During the valve closure, blood is pushed into the aorta and pulmonary arteries by the left and right ventricles respectively. The rapid increase in the pressure within the right and left ventricles forces both the pulmonic and aortic valves to open simultaneously when blood has moved forward from the corresponding ventricles. There is no chance of regurgitation (backflow) of blood into the atrium because  the 2 valves, (AV valve and the pulmonic valves) are closed immediately after the blood is pushed forward. At the beginning of contraction, there seems to be a rapid rise in the pressure, however, gradually the arterial pressure gets distributed across all the arteries which result in normal closure.  As a result, there is a fall in the pressures of pulmonary artery and aorta which leads to the closure of the semilunar valves. Following this, diastole begins

 

Diastole

At this point, the 2 atria contracts so that blood is pushed to ventricles. To prevent the backflow of blood from the ventricles into the atrium, the semilunar valve is shut resulting in the second heart sound DUB. In general, diastole is a state of relaxation of major chambers of heart because at this phase ,the ventricles are relaxed while simultaneously, the atrioventricular valves open. The opening of AV valves eventually push the blood from atrium to ventricles followed by closure of both the AV valves. . 

 

Electrical activity of the heart

Atrial systole is a response to the electrical impulse initiated by the SA node. SA node generates a systematic contraction of the atrial muscles causing increased pressure within the atria. Consequently, the blood is ejected into the ventricles. Soon after this, the electrical impulse starts at the junction between the 2 atria and ventricles. The electrical activity in the AV junction is managed by AV node( Atrioventricular node). Ventricular systole begins as a result of AV node action  From the AV node, the bundle of His or bundle branches receive the electrical impulses. Bundle of hid help in constantly spreading the effect of electrical impulse throughout the ventricular region. The combination of SA node, AV node, bundle of his along with the lub-dub sounds results in a cycle called the cardiac cycle. 

 

 Phases of the cardiac cycle

  1. Atrial contraction or atrial systole (mitral valve closes)

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

  3. Rapid ventricular ejection( Maximum  blood enters into the aorta and  pulmonary artery)

  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

 

Time duration of  Cardiac Cycle

Cardiac cycle typically lasts for 0.6-0.8 seconds. The whole cycle is divided into many events called Ventricular systole and Ventricular Diastole. Duration for  each event is  calculated as shown below.

  1. Ventricular systole = 0.27 (0.3) seconds 

  2. Ventricular diastole = 0.53 (0.5) seconds

  3. Isometric contraction = 0.05 seconds

  4.  Ejection period = 0.22 seconds

 

So total duration of Ventricular Systole  is 0.05 + 0.22 = 0.27 Seconds 

Similarly, ventricular Diastole is further divided into 

1. Protodiastole = 0.04 seconds 
2. Isometric relaxation = 0.08 seconds
3. Rapid filling = 0.11 seconds
4. Slow filling = 0.19 seconds 
5. Last rapid filling = 0.11 seconds 


So the total duration of Ventricular diastole is 0.04+ 0.08 +0.11+0.19+0.11= 0.53 seconds 

The net value for a complete cycle is calculated as ventricular systole events+ Ventricular Diastole events

i,e 0.27+0.53=0.8 seconds

Chamber Pressures

During the ventricular systole, the pressure generated on 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 ( from the body into the atrium) because of the higher pressure in the superior and inferior venacava than the atria ( 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. 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. Consequently, 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 major 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  low at this point. At the end of diastole, pressure in the atrium and ventricle gets to 4 to 12 mm Hg.

 

Cardiac Output

Cardiac output is the net amount of blood pumped out by the heart in one minute. It is approximately 5 Liters per minute in a normal adult. Cardiac output is the product of stroke volume and the heart rate. Stroke volume is the total amount of blood ejected by each heartbeat. Under stable conditions, the average resting stroke volume is 70 mL, and the heart rate is 60 to 100 beats per minute (bpm) which makes approximate 5 liters per minute, however it can vary slightly depending upon factors like age, weight, health condition among many others.

Therefore, cardiac output is calculated by CO=SVxHR

Where CO=Cardiac output, SV=Stroke volume and HR=Heart rate

 

Read More

1.

Cardiac muscles

2.

Anatomy of human heart

3.  

WBCs                                                          

4.

Blood pressure

 

Check your understanding

  1. What is systole?

  2. Describe how the heart sounds lub-dub is produced.

  3. Mention the phases of cardiac cycle.

  4. What is cardiac output ? Explain with the formula.

  5. Why is cardiac cycle essential?

  6. How can we examine each phase of the cardiac cycle?

 

 



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