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The Cardiac Cycle – Overview

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The Cardiac Cycle – Overview

The cardiac cycle includes the stages of contraction and relaxation between the atria and ventricles that occur in one heartbeat (see Figure 1.2). There are five stages in all: 

  1. The first stage is early diastole in which the whole heart is relaxed, the semilunar valves are closed, and the atrioventricular valves (AV). Blood flows in from the body via the right atrium and from the lungs via the left atrium. As the pressure in the atria begins to increase, the AV valves open, and the relaxed ventricles begin to fill.
  2. The second stage of the cardiac cycle is atrial systole. During this stage, the still-relaxed ventricles fill and the atria empty. This is the state of ventricular diastole. 
  3. In the third stage, the ventricles begin to contract, and both the AV valves and semilunar valves are closed. Ventricular volume remains unchanged, but there is a sharp rise in ventricular pressure. This phenomenon is known as isovolumetric contraction.  
  4. The fourth stage is the emptying of blood in the ventricles through ventricular systole. The semilunar valves are open, and the AV valves are closed. Approximately 25% to 40% of the ventricular blood volume is retained at the end of this stage.
  5. The fifth stage is when the ventricles begin to relax. This is the stage of isovolumetric relaxation. The pressure declines, no change in ventricular blood volume is noted, and the semilunar valves close because of pressure in the aorta.

 Figure 1.2. The Wiggers Diagram

Wiggers diagram.

Wiggers Diagram