Name: Rob
Status: other
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001
Question:
From a previous question, I understand the Sinoatrial node acts on it's
own to send the electrical pulses to the heart which in turn, regulates
the heart beat. I am having trouble understanding how this is an
attonomous act. In the case of an adrenaline release, the heart speeds
up. Doesn't the brain control the release of the adrenal gland? In this
case, is it true that the brain is indirectly controlling the heart
rate? What about during exercise? How does the Sinoatrial node "know"
that the muscles need more oxygen and to increase heart rate? What is in
indirect control of the heartbeat in this situation?
Replies:
There are many different mechanisms which control the strength and number of
beats of the heart. The SA node controls the normal heartbeat. It is a
small area of cells in the upper right atrium which is more excitable and
depolarizes at a faster rate than the other cardiac cells. It spreads over
the surface of the upper part of the heart and so on. The cardiac cells are
also sensitive to stretching, kind of like a rubber band. Since blood flow
is a cycle, when more blood returns to the heart as a result of exercise, the
heart muscle has to stretch to accommodate it. It then "snaps" back harder to
force the extra blood out, which puts more pressure on the arteries. There
are pressure receptors in the upper aorta that also sense the pressure of
blood in the system. There are also receptors in the aorta that sense the
amount of carbon dioxide (not oxygen). The SA node is also sensitive to
adrenaline which causes it to depolarize faster and to acetylcholine which
causes it to depolarize slower. The body always has many checks and balances
to keep everything in a state of homeostasis.
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