Step-by-Step Guide to the Cardiac Cycle for Students
The human heart is a muscular organ, roughly the size of a fist, responsible for pumping blood through the body. It plays a vital role in delivering oxygen and nutrients, while also removing waste products from tissues. The movement of blood within the cardiovascular system is made possible by the intricate processes of the cardiac cycle.
Also Read: Structure and Function of the Human Heart
What is the Cardiac Cycle?
The cardiac cycle refers to the complete sequence of events that occur during one heartbeat, including both the contraction and relaxation phases of the heart chambers. The cycle ensures that blood is pumped effectively to the lungs and the rest of the body. A typical cardiac cycle is divided into several phases, where the heart muscles contract and relax in a coordinated way, allowing blood to flow through the body efficiently.
Read More: Blood Cells
Cardiac Cycle in Numbers: A healthy heart beats around 72 times per minute, meaning that one full cardiac cycle occurs approximately 72 times in a minute. This is equivalent to around 0.8 seconds per cycle. The cycle consists of both systolic (contraction) and diastolic (relaxation) phases.
Step-by-Step Cardiac Cycle Flow Chart
A step-by-step cardiac cycle flow chart outlines each phase in sequence, making it easier for students to understand how the heart operates. Here's a simplified flow:
Atrial Diastole: Relaxation of atria; blood flows into the ventricles.
Atrial Systole: Atria contract, filling ventricles.
Isovolumic Contraction: Ventricles contract, valves close.
Ventricular Ejection: Blood is pumped from the ventricles to the lungs and body.
Isovolumic Relaxation: Ventricles relax, valves close to prevent backflow.
Ventricular Filling: Blood fills the ventricles from the atria.
Atrial Diastole: Rest phase before the cycle repeats.
Cardiac Cycle Diagram
A cardiac cycle diagram visually demonstrates the relationship between the phases, illustrating how blood moves through the heart’s chambers and valves. The diagram can also help show how the heart responds to electrical signals that initiate contraction and relaxation.
Phases of a Cardiac Cycle
There are 7 Phases of a Cardiac Cycle. Understanding these phases is essential for comprehending how the heart functions and maintains circulation throughout the body.
Atrial Diastole: The heart chambers are in a relaxed state. During this phase, the atrioventricular valves (AV valves) are open, allowing blood to flow from the atria to the ventricles. Both the aortic and pulmonary valves are closed during this time.
Atrial Systole: In this phase, the atria contract, pushing the remaining blood into the ventricles. This phase helps fill the ventricles to their maximum capacity.
Isovolumic Contraction: The ventricles start contracting, but there is no change in the volume of blood inside them yet. The AV valves close, and the aortic and pulmonary valves remain shut.
Ventricular Ejection: During this phase, the ventricles contract forcefully, and blood is pumped into the pulmonary trunk and aorta. The aortic and pulmonary valves open to allow the ejection of blood.
Isovolumic Relaxation: After the blood is ejected, the ventricles begin to relax, but no blood flows into them yet. The aortic and pulmonary valves close, preventing backflow of blood.
Ventricular Filling: Blood flows from the atria into the ventricles as the AV valves open. This is the phase where the ventricles refill with blood, ready to begin the cycle again.
Atrial Diastole: The heart enters a brief period of relaxation before the next cycle begins.
Also Read: Cardiac Output
The Function of the Cardiac Cycle
The cardiac cycle is essential for maintaining an efficient and continuous flow of blood throughout the body. It ensures that oxygen-rich blood is delivered to the organs and tissues while removing carbon dioxide and waste products. Through the rhythmic contraction and relaxation of the heart's chambers, the body maintains proper circulation, thus enabling normal bodily function.
The function of the cardiac cycle can be understood through these key tasks:
Oxygen delivery: The left ventricle pumps oxygen-rich blood through the aorta to the body.
Waste removal: The right ventricle pumps deoxygenated blood to the lungs for gas exchange (removal of carbon dioxide and replenishment of oxygen).
Blood pressure regulation: Through the heart’s pumping action, blood pressure is maintained, ensuring the proper flow to all body parts.
Duration of the Cardiac Cycle
In a healthy adult, the cardiac cycle duration is about 0.8 seconds, with the heart beating at 72 beats per minute. The exact duration of each phase varies:
Atrial Systole: ~0.1 seconds
Ventricular Systole: ~0.3 seconds
Atrial Diastole: ~0.7 seconds
Ventricular Diastole: ~0.5 seconds
The overall cycle duration remains approximately constant but can adjust depending on factors like exercise or relaxation.
Read More: Regulation of Cardiac Activity
Key Facts About the Cardiac Cycle
The cardiac cycle involves two main phases: systole (contraction) and diastole (relaxation).
The left ventricle pumps oxygen-rich blood to the body, while the right ventricle pumps deoxygenated blood to the lungs.
Electrical impulses from the sinoatrial (SA) node initiate the cycle, helping coordinate the heart's rhythm.
The cardiac cycle duration is essential in determining heart rate and overall cardiovascular health.
FAQs on Cardiac Cycle Simplified: Heartbeat Phases & Flow Explained
1. What is the cardiac cycle as per the CBSE Class 11 syllabus?
The cardiac cycle is the sequence of events that occurs in the heart during a single heartbeat. It involves the rhythmic contraction (systole) and relaxation (diastole) of the heart's chambers—the atria and ventricles. This entire cycle is completed in approximately 0.8 seconds and ensures the continuous pumping of blood throughout the body.
2. What are the main phases of a single cardiac cycle?
A single cardiac cycle consists of several key phases that ensure efficient blood flow. The primary phases are:
- Joint Diastole: Both atria and ventricles are relaxed, allowing blood to passively fill the ventricles.
- Atrial Systole: The atria contract, pushing the remaining blood into the ventricles.
- Ventricular Systole: The ventricles contract. This is further divided into Isovolumic Contraction (all valves are closed) and Ventricular Ejection (blood is forced into the aorta and pulmonary artery).
- Ventricular Diastole: The ventricles relax, starting with Isovolumic Relaxation, and the cycle begins again with passive ventricular filling.
3. How is the duration of one cardiac cycle calculated?
The duration of one cardiac cycle can be calculated by taking the reciprocal of the heart rate. For a normal resting heart rate of 75 beats per minute, the duration of a single cardiac cycle is 60 seconds divided by 75 beats, which equals 0.8 seconds per beat.
4. What is the fundamental difference between systole and diastole?
The fundamental difference lies in the state of the heart muscle. Systole is the phase of contraction, where the heart chambers pump blood out. For example, during ventricular systole, blood is ejected into the major arteries. In contrast, diastole is the phase of relaxation, where the heart chambers relax and fill with blood in preparation for the next contraction.
5. Why is the 'lub' sound of the heart produced before the 'dub' sound?
The sequence of heart sounds follows the mechanical events of the cardiac cycle. The first heart sound, 'lub', is produced by the closure of the atrioventricular (AV) valves (tricuspid and mitral) at the beginning of ventricular systole. The second heart sound, 'dub', is produced shortly after by the closure of the semilunar valves (aortic and pulmonary) at the end of ventricular systole. This timing ensures that blood flows forward and does not leak back into the chambers.
6. How do Stroke Volume (SV) and Cardiac Output (CO) describe the heart's efficiency?
These two measurements quantify the heart's pumping performance. Stroke Volume (SV) is the volume of blood pumped out by one ventricle with each beat, typically around 70 mL. Cardiac Output (CO) is the total volume of blood pumped by one ventricle per minute. It is calculated by multiplying the heart rate by the stroke volume (CO = HR × SV). It gives a comprehensive measure of blood circulation.
7. What is the importance of 'joint diastole' in the cardiac cycle?
Joint diastole is critically important because it is the primary period for ventricular filling. During this phase, both the atria and ventricles are relaxed, allowing blood to flow passively from the great veins, through the atria, and into the ventricles. About 80% of ventricular filling occurs during this phase, making it essential for ensuring the ventricles have an adequate volume of blood to pump during the subsequent systole.
8. How do the SA node and AV node regulate the sequence of the cardiac cycle?
The SA node (Sinoatrial node) acts as the heart's natural pacemaker, initiating the electrical impulse that causes the atria to contract (atrial systole). This impulse then travels to the AV node (Atrioventricular node), which introduces a slight delay before passing the signal to the ventricles. This delay is crucial as it allows the atria to finish contracting and empty their blood into the ventricles before the ventricles begin their contraction.
9. What would be the immediate consequence if the semilunar valves failed to close properly?
If the semilunar valves (aortic and pulmonary) failed to close properly, it would lead to a condition called regurgitation. After ventricular systole, blood that was just pumped into the aorta and pulmonary artery would leak back into the relaxing ventricles. This would decrease the overall cardiac output, reduce the efficiency of blood circulation, and force the heart to work harder to supply the body with enough oxygenated blood.
10. Can the atria and ventricles contract at the same time? Explain why this doesn't happen in a healthy heart.
No, in a healthy heart, the atria and ventricles do not contract at the same time. The cardiac cycle is designed for sequential contraction: first the atria contract to fill the ventricles, and then the ventricles contract to pump blood out of the heart. This coordinated sequence is ensured by the heart's electrical conduction system, specifically the delay at the AV node. If they contracted simultaneously, blood would not flow efficiently from the atria to the ventricles, severely compromising the heart's function as a pump.
