In the mammalian heartbeat
A) contractions are initiated in the wall of the left atrium.
B) ventricles and atria contract at the same time.
C) the left ventricle and the left atrium contract at the same time.
D) the cycle is initiated in the sinus venosus.
E) the two ventricles contract at the same time.

Question

Quizplus · Accepted Answer

In the mammalian heart, the heartbeat cycle is initiated by the sinoatrial node (not the sinus venosus, which is a term more relevant to the hearts of fish and embryonic hearts), located in the right atrium, not the left atrium or the wall of the left atrium. The atria contract first, followed by the ventricles, meaning they do not contract at the same time. However, the two ventricles do contract simultaneously to efficiently pump blood to the lungs and the rest of the body.

In the Mammalian Heartbeat