Isovolumetric contraction and relaxation are two critical phases in the cardiac cycle that occur during the pumping action of the heart. Let’s break down what happens during these phases.
During isovolumetric contraction, the ventricles are in the early phase of contraction. When the ventricles contract, the pressure inside them rises sharply, but the volume remains constant because all the heart valves (the mitral and tricuspid valves and the aortic and pulmonary valves) are closed. This generates high pressure without any change in volume—hence the term ‘isovolumetric.’ This phase is crucial because it builds enough pressure to eventually open the aortic and pulmonary valves, allowing blood to flow into the arteries.
On the other hand, isovolumetric relaxation occurs after the ventricles have ejected blood into the arteries and the heart is preparing for the next filling phase. During relaxation, the pressure in the ventricles drops but the volume remains constant again because all valves are closed. The pressure decreases as the heart muscle relaxes but, similar to the previous phase, no blood enters or leaves the ventricles.
In summary, both isovolumetric contraction and relaxation involve changes in pressure while the ventricular volume stays constant due to the closure of the heart valves. This mechanism is essential for creating the necessary pressure differentials that facilitate effective blood flow throughout the body.