The semilunar valves close when which of the following occurs?
a. The pressures in the arteries exceed those in the ventricles.
b. The pressures in the ventricles and arteries become equal.
c. The pressures in the atria exceed those in the ventricles.
d. The pressures in the ventricles exceed those in the atria.
ANS: A
Toward the end of systole, as repolarization starts (indicated by the T wave), the ventricles begin to relax. Consequently, ventricular pressures drop rapidly. When arterial pressures exceed those in the relaxing ventricles, the semilunar valves shut.
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