During which specific phase of the cardiac cycle is the pressure gradient such that the mitral and tricuspid valves are closed, while the aortic and pulmonary valves are in the process of opening or are fully open, signifying the beginning of ventricular emptying?
Correct: B
Let's analyze the phases:
A. Isovolumetric Contraction: During this phase, ventricular pressure rises above atrial pressure, causing the AV valves (mitral and tricuspid) to close (producing the S1 heart sound). However, the ventricular pressure has not yet exceeded the pressure in the aorta and pulmonary artery, so the semilunar valves remain closed. No blood is ejected, and ventricular volume remains constant.
B. Ventricular Ejection: This phase immediately follows isovolumetric contraction. As ventricular pressure continues to rise, it eventually surpasses the pressure in the aorta and pulmonary artery, forcing the semilunar valves (aortic and pulmonary) open. Blood is then rapidly ejected from the ventricles into the great arteries. During this phase, the AV valves remain closed because ventricular pressure is still higher than atrial pressure. Therefore, this phase signifies the beginning of ventricular emptying with AV valves closed and semilunar valves open.
C. Isovolumetric Relaxation: This phase occurs after ventricular ejection. Ventricular pressure falls below aortic/pulmonary pressure, causing semilunar valves to close (S2 sound). Ventricular pressure is still higher than atrial pressure (initially), so AV valves are also closed. No blood enters or leaves the ventricles.
D. Atrial Systole: During atrial contraction, blood flows from the atria to the ventricles. The AV valves are open, and semilunar valves are closed.