18. The Heart

Which of the following is longer?

(a) The refractory period of a cardiac contractile cell.

(b) The refractory period of skeletal muscle fiber.

An experimental toxin makes the refractory period of cardiac muscle cells equal in length to that of skeletal muscle fibers. Predict the consequences of this toxin.

Mark the following statements as true or false. If a statement is false, correct it to make a true statement.

The rapid depolarization phase of the contractile cell action potential is due to the opening of voltage-gated potassium ion channels.

What is the cardiac cycle? What phases and events are necessary to complete a cardiac cycle?

The cardiac skeleton of the heart has which two of the following functions?

(a) It physically isolates the muscle fibers of the atria from those of the ventricles.

(b) It maintains the normal shape of the heart.

(c) It helps distribute the forces of cardiac contraction.

(d) It allows more rapid contraction of the ventricles.

(e) It strengthens and helps prevent overexpansion of the heart.

What are the sources and significance of the four heart sounds?