1. The cardiac cycle is divided into systole and diastole, during which the ventricles contract and relax, respectively. During systole, blood is ejected from the ventricles and is pumped into either the aorta or pulmonary artery. The pressure in the ventricles rapidly increases; however, the volume of blood in the ventricles initially remains unchanged. This is referred to as isovolumetric contraction. Which of the following is the most likely reason for isovolumetric contraction?

A) Decreased ventricular contractility due to overstretching of cardiac sarcomeres.
B) The pressure in the aorta is initially greater than that of the left ventricle.
C) The pressure within the pulmonary vein is initially lower than that of the right ventricle.
D) The action potential that propagates through the bundle of His and Purkinje fibers has not yet depolarized all of the ventricles.

Isovolumetric contraction in the cardiac cycle occurs when the ventricles contract, increasing pressure before blood ejection, without a change in volume.

Explanation:

The most likely reason for isovolumetric contraction in the cardiac cycle is that initially, as the ventricular muscles contract, the pressure in the ventricles rises but is not yet high enough to open the semilunar valves for blood ejection.

During isovolumetric contraction, the volume of blood in the ventricles remains constant as the pressure increases before the ejection phase.

This phase allows for the ventricles to build up enough pressure for efficient blood ejection into either the aorta or pulmonary artery.

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