Cardiac Cycle
The cardiac cycle refers to the sequence of events that occur during one complete heartbeat. It involves the contraction and relaxation of the heart muscle, resulting in the flow of blood through the heart's chambers and into the arteries. The cycle ensures that oxygenated blood is delivered to the body and deoxygenated blood is sent to the lungs.
Phases of the Cardiac Cycle:
The cardiac cycle is divided into two main phases: diastole (relaxation) and systole (contraction).
1. Diastole (Relaxation Phase)
This phase occurs when the heart muscle is relaxed, and the chambers fill with blood.
Ventricular Diastole (Relaxation of the Ventricles):
The ventricles relax and expand.
Blood from the atria flows into the ventricles through the open atrioventricular (AV) valves.
The semilunar valves are closed, preventing blood from flowing back into the ventricles from the aorta and pulmonary artery.
Atrial Systole (Contraction of the Atria):
The atria contract, pushing the remaining blood into the ventricles.
This completes the ventricular filling, which is crucial for the next phase.
2. Systole (Contraction Phase)
This phase involves the contraction of the ventricles, which pumps blood into the major arteries.
Ventricular Systole (Contraction of the Ventricles):
The ventricles contract, increasing the pressure inside them.
When the pressure in the ventricles exceeds the pressure in the atria, the AV valves close, preventing blood from flowing back into the atria.
The increasing pressure in the ventricle forces open the semilunar valves (aortic and pulmonary), allowing blood to be ejected into the aorta (from the left ventricle) and the pulmonary artery (from the right ventricle).
Isovolumetric Contraction:
This brief phase occurs at the start of ventricular systole when all valves (AV and semilunar) are closed.
The ventricles contract, but no blood is ejected yet because the pressure is not high enough to open the semilunar valves.
Blood volume remains constant during this phase.
Ejection Phase:
Once the semilunar valves open, blood is ejected from the ventricles into the arteries.
The left ventricle pumps oxygenated blood into the aorta to be distributed throughout the body, while the right ventricle pumps deoxygenated blood into the pulmonary artery for oxygenation in the lungs.
3. Relaxation and Refill
After the ventricles have ejected the blood, they begin to relax, and the heart prepares to fill again for the next cycle.
Isovolumetric Relaxation:
The ventricles relax, and the pressure inside them drops.
The semilunar valves close, preventing blood from flowing back into the ventricles from the arteries.
The AV valves remain closed until the pressure in the ventricles falls low enough for them to open again, allowing the filling phase to begin.
Timing of the Cardiac Cycle:
The duration of one cardiac cycle (from one heartbeat to the next) is typically around 0.8 seconds in a resting adult.
Diastole lasts about 0.5 seconds.
Systole lasts about 0.3 seconds.
The cardiac cycle repeats continuously, ensuring that blood is constantly circulated through the body.
Key Points
The cardiac cycle is essential for maintaining blood circulation, ensuring that the heart pumps blood efficiently throughout the body.
The cycle is divided into systole (contraction) and diastole (relaxation).
The process is regulated by pressure changes and controlled by electrical impulses originating from the SA node and spreading through the heart's conduction system.
Heart sounds are generated by the closing of the valves during different phases of the cardiac cycle.
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