🚀 go-pugleaf

> Does anyone know the underlying mechanism that
> "causes" the heart to beat faster during exercise?
> 
> Dennis R. Vogel

	Trust me - you really don't want to know.
	There are many reasons. The easiest to explain is the 
fluid dynamic component.  That is, exercise causes more blood to
flow into the heart, so more has to go out.  And since
	Cardiac Output = Stroke Volume * Heart Rate, 
and that stroke volume can only increase so much, the heart rate
must go up (i.e., the heart beats when it is full).
	There is a lot that can be said about total peripheral
resistance and venous tone at this point.

	The body also senses of lowering of the blood pH (sensed in
the medulla of the brain).  This causes a reflex down the Vagus
nerve as well as the sympathetic chain to stimulate (actually
shorten the refractory period and speed up the Phase IV depolarization)
at the SA node, which is the hearts natural pacemaker.  It also
speeds up conduction at the AV node, which allows beats to pass
much quicker from atrium to ventricle
	The release of Norepinephrine also increases the contractility
of the muscle, which doesn't really affect rate, but allows the heart
to pump more efficiently (albeit while using more energy).

-- 
			      Craig Werner (MD/PhD '91)
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              (1935-14E Eastchester Rd., Bronx NY 10461, 212-931-2517)
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