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In article <768@mtx5w.UUCP>, drv@mtx5w.UUCP writes:
> Does anyone know the underlying mechanism that
> "causes" the heart to beat faster during exercise?

	The heart is a pump; the volumetric pumping rate of the heart is
referred to as "cardiac output", which is about 5.6 liters per minute for
an average young adult male.  Since the heart is a cyclic, volume-displacement 
pump, there are two parameters which determine cardiac output: (1) its cyclic
rate (i.e., heart rate), and (2) its stroke volume.  Of these two parameters,
the heart rate is responsible for the greatest variation in cardiac output.
	Cardiac output is essentially proportional to the overall metabolism
of the body.  Exercise results in an obvious increase in body metabolism.
Exercise can result in body tissues requiring as much as TWENTY times the
normal amounts of oxygen and other blood-transported nutrients.  In order to
supply this increased demand, the cardiac output may be required to increase
as much as SIX times, to over 30 L/min.
	In the particular case of exercise, the following are some of the
mechanisms which increase cardiac output:

1.	The autonomic nervous system increases heart rate and stroke volume
	(by increasing strength of ventricular contractions) just by THINKING
	about exercise.  Really!

2.	Heart rate and stroke volume are further increased by the sympathetic
	nervous system; this results from the motor cortex activity actually
	associated with exercise. 

3.	Increased metabolism resulting from muscular activity results in the
	formation of chemicals which exhibit a vasodilator effect, thereby
	reducing "peripheral resistance" (i.e., blood vessels expand to
	allow a greater blood flow).  The resultant increase in blood flow
	results in greater venous return to the right atrium, which causes
	an increase in right atrial pressure.  This increase in right atrial
	pressure increases the volume of the right atrium, which stretches a
	specialied portion of heart muscle called the "sino-atrial node",
	resulting in increased heart rate.  There is yet a second mechanism
	which comes into play when the right atrium increases volume; it is
	called the "Bainbridge Reflex". Stretch receptors in the right atrium
	transmit aferrent signals to the medulla of the brain via the vagus
	nerves.  Reception by the medulla results in transmission of eferrent
	signals by both the vagus and sympathetic nerves back to the heart to
	further increase heart rate and stroke volume (by stronger ventricular
	contractions).

	Of the above, the vasodilation effects of exercise reult in the
greatest overall increase in heart rate through the mechanisms as described
in (3) above.

==>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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