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37. How the pacemaker controls the heartbeat:
a) Separate nerves from the brain to the pacemaker help speed up or slow down the pace of the pacemaker.
b) Adrenaline carried to the heart by the blood acts on the pacemaker by slowing it down.
c) Once stimulated, the pacemaker sends an electrical stimulation to the rest of the heart.
d) This stimuli first causes the atria to contract, forcing blood into the chambers of the venticles.
e) When full, the ventricles in turn are stimulated a split second later to contract. The atrio-ventricular valves close (preventing back flow to the atria), and blood is forced from the ventricles into either the pulmonary artery or aorta. In the meantime, the atria refill.
f) At the end of ventricular contraction, the semi-lunar valves close, preventing back flow of blood from the main arteries. When pressure in the ventricle subsides sufficiently, the atrio-ventricular valves will agian reopen to refill the ventricles as the next contraction begins.
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