What causes the opening and closing of the heart valves?
A. The action of the papillary muscles and chordae tendineae
B. A pressure difference on the two sides of the valve
C. Turbulent flow in the atria and ventricles
D. Na+ and K+ fluxes during ventricular depolarization
E. Stimulation by the sympathetic and parasympathetic nerves
Answer: B
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