The Cardiac Cycle

Introduction

The cardiac cycle is a rhythmic sequence of events that describes the complete heartbeat, including the contraction and relaxation of the heart's chambers. This cycle is essential for maintaining blood circulation and ensuring that oxygen-rich blood is pumped to the body's tissues while deoxygenated blood is sent to the lungs for oxygenation. The cardiac cycle involves specific phases, each with its unique functions and pressure changes, primarily regulated by the heart's valves.

Functions of the Cardiac Cycle

The cardiac cycle is divided into several phases, each with its distinct functions and pressure changes

1. Diastole

• Function: Diastole is the phase of relaxation in which the heart chambers (atria and ventricles) expand and fill with blood.
• Blood Flow: During diastole, blood returning from the body (systemic circulation) and lungs (pulmonary circulation) flows into the atria.
• Atrial Filling: As the atria expand, they act as reservoirs for blood, and passive filling occurs. This phase allows the ventricles to be adequately filled with blood before the next contraction.

2. Atrial Systole

• Function: Atrial systole is the contraction of the atria, which forces the remaining blood into the ventricles.
• Blood Transfer: During this phase, the atria contract, pushing the remaining blood through the atrioventricular (AV) valves and into the ventricles.
• Ventricular Filling: Atrial systole ensures that the ventricles receive the maximum amount of blood possible before they contract.

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The Cardiac Cycle

3. Ventricular Systole

• Function: Ventricular systole is the contraction of the ventricles, leading to the ejection of blood into the aorta and pulmonary artery.
• Valve Closure: As the ventricles contract, the pressure inside them rises, closing the AV valves (tricuspid and mitral valves) to prevent the backflow of blood into the atria.
• Ejection of Blood: Pressure in the ventricles exceeds the pressure in the aorta and pulmonary artery, forcing the semilunar (SL) valves to open, allowing blood to be ejected into these arteries.

Effect of Pressure Changes on Valves

AV Valve Closure

During ventricular systole, when the ventricular pressure rises, it exceeds the atrial pressure, causing the AV valves (tricuspid and mitral valves) to close. This prevents the backflow of blood into the atria.

SL Valve Closure

During diastole, when the ventricles relax, their pressure decreases. Simultaneously, the pressure in the aorta and pulmonary artery becomes higher than in the ventricles, causing the semilunar (SL) valves to close. This prevents blood from flowing back into the ventricles.

Heart Sounds and Valves

Heart Sounds

The opening and closing of the heart's valves are responsible for the characteristic "lub-dub" sounds heard with a stethoscope. The "lub" sound corresponds to the closure of the AV valves at the beginning of ventricular systole, while the "dub" sound corresponds to the closure of the SL valves at the end of ventricular systole.

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The Cardiac Cycle

Summary

The cardiac cycle is a rhythmic sequence of events that describes the complete heartbeat. It involves the contraction and relaxation of the heart's chambers, each with specific functions. Diastole allows the atria and ventricles to fill with blood, atrial systole transfers the remaining blood to the ventricles, and ventricular systole ejects blood into the aorta and pulmonary artery. The opening and closing of the atrioventricular (AV) and semilunar (SL) valves are responsible for heart sounds. These valves play a crucial role in ensuring the one-way flow of blood through the heart, maintaining efficient circulation, and preventing backflow. Understanding the cardiac cycle is essential for comprehending heart function and circulation in the human body.