Re: RC: Re: Tieing up

[Tivers@aol.com]

Vet Clin North Am Equine Pract 1985 Dec;1(3):513-31 

Cardiovascular adaptations to exercise and training.

Evans DL

The cardiovascular system provides the link between pulmonary ventilation
and oxygen usage at the
cellular level. During exercise, efficient delivery of oxygen to working
skeletal and cardiac muscles is vital
for maintenance of ATP production by aerobic mechanisms. The equine
cardiovascular response to
increased demand for oxygen delivery during exercise contributes largely to
the over 35-fold increases in
oxygen uptake that occur during submaximal exercise. Cardiac output during
exercise increases greatly
owing to the relatively high heart rates that are achieved during exercise.
Heart rate increases
proportionately with workload until heart rates close to maximal are
attained. It is remarkable that exercise
heart rates six to seven times resting values are not associated with a
fall in stroke volume, which is
maintained by splenic contraction, increased venous return, and increased
myocardial contractibility.
Despite the great changes in cardiac output, increases in blood pressure
during exercise are maintained
within relatively smaller limits, as both pulmonary and systemic vascular
resistance to blood flow is
reduced. Redistribution of blood flow to the working muscles during
exercise also contributes greatly to
the efficient delivery of oxygen to sites of greatest need. Higher work
rates and oxygen uptake at
submaximal heart rates after training imply an adaptation due to training
that enables more efficient
oxygen delivery to working muscle. Such an adaptation could be in either
blood flow or arteriovenous
oxygen content difference. Cardiac output during submaximal exercise does
not increase after training, but
studies using high-speed treadmills and measurement of cardiac output at
maximal heart rates may reveal
improvements in maximal oxygen uptake due to increased stroke volumes, as
occurs in humans.
Improvements in hemoglobin concentrations in blood during exercise after
training are recognized, but at
maximal exercise, hypoxemia may reduce arterial oxygen content. More
effective redistribution of cardiac
output to muscles by increased capillarization and more efficient oxygen
diffusion to cells may also be an
important means of increasing oxygen uptake after training. >

Many thanks to a friend for doing the work for me. Now we're ready for your 
cite, Heidi.

ti