phases of the heart cycle
Biology · Middle School · Thu Feb 04 2021
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The heart cycle, also known as the cardiac cycle, consists of a series of events that occur from the beginning of one heartbeat to the beginning of the next. It can be divided into four main phases:
1. **Ventricular Filling Phase:** This phase occurs during diastole when both the atria and ventricles are relaxed. Blood flows passively from the atria into the ventricles through the open atrioventricular (AV) valves (the tricuspid valve on the right side of the heart and the mitral valve on the left side). Near the end of this phase, the atria contract (atrial systole), pushing additional blood into the ventricles.
2. **Isovolumetric Contraction Phase:** This phase begins with the closure of the AV valves, marking the onset of ventricular systole (contraction). As the ventricles contract, pressure within them rises, but since the semilunar valves (the pulmonary valve on the right and aortic valve on the left) are still closed, no blood is ejected from the heart yet. This results in a brief period when the volume of blood in the ventricles remains constant, hence the name "isovolumetric."
3. **Ventricular Ejection Phase:** As the ventricular pressure exceeds the pressure in the pulmonary artery and aorta, the semilunar valves open, allowing blood to be ejected from the ventricles. Initially, the flow of blood is rapid (rapid ejection), but as the ventricles start to empty, the rate of ejection slows (reduced ejection).
4. **Isovolumetric Relaxation Phase:** This phase occurs at the beginning of diastole when the ventricles start to relax. Since the pressure in the ventricles falls below the pressure in the arteries, the semilunar valves shut to prevent backflow of blood into the heart (this is the "dub" sound of the heartbeat). With all valves closed and ventricles still relaxing, no blood enters the ventricles, so the volume remains unchanged. This phase ends when the pressure in the ventricles falls below the pressure in the atria, causing the AV valves to open, and the cycle starts over again with the ventricular filling phase.