What is true about heart valves?
a.they enforce a one-way blood flow through the heart.
b.they operate passively (no active contraction required).
c.they separate atria from ventricles, and ventricles from the large arteries that leave them.
d.they enforce a one-way blood flow through the heart, operate passively (no active contraction required), and separate atria from ventricles, and ventricles from the large arteries that leave them?