what is the 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 and the movement of blood through the heart and circulatory system. Here are the main points of the cardiac cycle:

  1. Diastole: The cardiac cycle begins with diastole, which is the relaxation phase of the heart. During diastole, the heart chambers fill with blood as the atria and ventricles relax.
  2. Atrial Contraction: The atria contract, pushing the remaining blood into the ventricles. This phase is called atrial systole.
  3. Ventricular Contraction: The ventricles contract, causing the atrioventricular valves (tricuspid and mitral valves) to close, preventing the backflow of blood into the atria. This phase is called ventricular systole.
  4. Isovolumetric Contraction: Initially, when the ventricles contract, the pressure inside them rises rapidly, but the ventricular volume remains constant. This phase is called isovolumetric contraction because no blood is ejected yet.
  5. Ventricular Ejection: As the ventricular pressure exceeds the pressure in the pulmonary artery and aorta, the semilunar valves (pulmonary and aortic valves) open, allowing blood to be ejected from the ventricles into the respective arteries. This phase is called ventricular ejection.
  6. Isovolumetric Relaxation: After ventricular ejection, the ventricles relax, and the semilunar valves close to prevent blood from flowing back into the ventricles. This phase is called isovolumetric relaxation because no blood enters or leaves the ventricles.
  7. Ventricular Filling: As the ventricles relax further, the pressure inside them decreases. Blood flows passively from the atria into the ventricles, filling them. This phase is called early ventricular filling.
  8. Atrial Contraction: The atria contract again to push the remaining blood into the ventricles, completing the ventricular filling. This phase is called late ventricular filling or atrial systole.
  9. Repeat: The cardiac cycle repeats itself with each heartbeat, ensuring a continuous flow of blood throughout the body.

It’s important to note that the cardiac cycle is coordinated by electrical signals within the heart, primarily initiated by the sinoatrial (SA) node, which acts as the heart’s natural pacemaker. These electrical signals regulate the timing and sequence of events during each cardiac cycle.

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