URGENT: How does intraventricular pressure rise during the isovolumic ventricular contraction stage?
According to a website, the intraventricular pressure of the heart increases rapidly during the isovolumic ventricular contraction stage of the cardiac cycle. There isn't a change in the ventricular volume, so how does the pressure change? Is it due to the force that the contracting ventricular cardiomyocytes exert on the blood or the decrease in the area of the interior of the ventricles? Also, does the "ventricular volume" refer to the volume of blood in the ventricles?
Update:Caitie, just to clarify, what causes atrioventricular valve closure and what is the difference between tensing and contraction? Do these two actions cause intraventricular pressure to increase?
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First, yes, "ventricular volume" refers to the volume of blood in the ventricles.
Second, isovolumic ventricular contraction refers to the stage of very early systole, when the atria have pumped blood into the ventricles and the ventricles have begun to depolarize, preparing for contraction, but contraction has not happened yet. The ventricular muscles have not yet begun to squeeze blood out of the heart. The pressure increases in the ventricles because, at this state, diastole has just finished and the valves to the atria have closed, preventing backwash of blood into the atria when the ventricles do contract. Since the ventricular muscles are tensing for contraction and the blood has nowhere to go yet, this is why the pressure inside the ventricles increases for this short time. When this happens, the QRS spike on an ecg reading appears because the ventricles are depolarizing and this produces the electrical discharge we read as the QRS complex.
It is diastolic relaxation (which is an active, energy dependant process). This phase has also been referred to as protodiastole, i.e. the brief period of time when the aortic valve has closed but the mitral valve has not opened yet.