What is the force that acts upon the sinoatrial node in the heart that
makes the sinoatrial node send out the electrical charge that causes the heart to beat?
Wow, that's a complex one! Basically, no outside force acts on the SA node.
That's why it's called the "pacemaker" of the heart. In simple terms, the membranes of
the cells of the SA node have special proteins which periodically allow a small number
of positively charged ions to enter the cell. The periodicity with which this happens
is run by a clock that is right in these cells. The electrical signal generated by the
SA node that tells the heart to beat is triggered whenever the SA node cells become
slightly more positively charged inside, which is just what happens whenever these
proteins let in a few positive ions. So there it is in basic terms. For the real
story, you need to understand quite a bit about electricity and ions and the energy
that determines diffusion, but if your in high school or beyond, you're probably
ready to give it a try if you are really interested. I recommend a college biology
book, such as the one by Campbell, or, for more advanced stuff, a real physiology
book such as "Animal Physiology" by Augustine, Eckert, & Randall.
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Update: June 2012