Cardiac Cycle: The Rhythmic Heartbeat Explained

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:

1. Atrial Diastole: Relaxation of atria; blood flows into the ventricles.

2. Atrial Systole: Atria contract, filling ventricles.

3. Isovolumic Contraction: Ventricles contract, valves close.

4. Ventricular Ejection: Blood is pumped from the ventricles to the lungs and body.

5. Isovolumic Relaxation: Ventricles relax, valves close to prevent backflow.

6. Ventricular Filling: Blood fills the ventricles from the atria.

7. 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.

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.