The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, encompassing both contraction and relaxation of the heart muscles. This cycle is crucial for the effective pumping of blood throughout the body and consists of two primary phases: diastole (relaxation) and systole (contraction).
Phases of the Cardiac Cycle
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Cardiac Diastole:
- During diastole, the heart relaxes, and both the atrial myocardium (the muscle tissue of the atria) and the ventricular myocardium (the muscle tissue of the ventricles) are in a relaxed state.
- The atria fill with blood coming from the veins (superior and inferior vena cava into the right atrium, and pulmonary veins into the left atrium). This filling causes an increase in atrial pressure, which leads to the opening of the atrioventricular (AV) valves (the tricuspid valve on the right and the mitral valve on the left).
- When the AV valves open, blood flows from the atria into the ventricles, effectively filling the ventricles with blood in preparation for the next contraction. This passive filling mainly occurs during the early part of diastole.
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Atrial Systole:
- Toward the end of diastole, the atrial myocardium contracts (atrial systole), pushing the remaining blood from the atria into the ventricles. This contraction is essential to ensure that the ventricles are as full as possible before they contract.
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Ventricular Systole:
- Following atrial systole, the ventricles begin to contract (ventricular systole). As the ventricular myocardium contracts, the pressure within the ventricles increases, causing the AV valves to close to prevent backflow of blood into the atria.
- The increased pressure also opens the semilunar valves (the pulmonary valve on the right and the aortic valve on the left), allowing blood to flow out of the heart: the right ventricle pumps blood into the pulmonary artery and into the lungs for oxygenation, while the left ventricle pumps blood into the aorta and throughout the body.
Role of Heart Valves During the Cardiac Cycle
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Atrioventricular (AV) Valves:
- These valves (tricuspid and mitral) play a crucial role during the transition from diastole to systole. When the ventricles fill with blood and pressure rises, the AV valves close to prevent backflow into the atria.
- During diastole, the AV valves are open, allowing blood to flow into the ventricles from the atria.
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Semilunar Valves:
- The semilunar valves (pulmonary and aortic) open during ventricular systole when the pressure in the ventricles exceeds the pressure in the arteries. This allows blood to be ejected from the heart.
- After the ventricles empty and begin to relax, the semilunar valves close to prevent backflow of blood from the arteries into the ventricles, initiating the transition back to diastole.
Summary
In summary, the cardiac cycle is a continuous process of filling (during diastole) and emptying (during systole) the heart chambers. The atrial myocardium contracts to fill the ventricles, while the AV and semilunar valves regulate blood flow and prevent backflows, ensuring the heart functions efficiently in circulating blood through the body. Understanding this cycle is vital for recognizing how the heart works to maintain proper hemodynamics and overall cardiovascular health.