4. Which of the following statements are accurate?
A. The heartbeat in a human heart is initiated by the atrioventricular mode.
B. During systole, the heart's ventricle(s) is/are relaxing and filling with blood.
C. During systole, blood will be pumped from the heart into the veins of the body
D. The pulse is used to determine one's heart rate.
E. The nervous system and certain hormones can alter pacemaker activity.

options D and E

Explanation:

The heartbeat in a human is mostly initiated by the sinoatrial node (SA node also known as the heart's natural pacemaker. Made up of a specialized bundle of cell that receives an impulse causing contraction of the atria wall allowing blood to flow into the ventricles. During systole, the ventricles are not relaxed but contracted to allow for bloodflow out of the ventricles to the aorta which is the largest artery in the human body.

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