Your Horse's Heart

ventricle is filling blood flows around the body from the left ventricle to the right ventricle. In the middle of the circulation between those two ends lies the microcirculation which supplies functioning tissues with the oxygen and fuel they need and removes waste products to transport them to where they can be disposed of.
With the exception of extreme circumstances such as severe shock the tissue or peripheral circulation functions entirely autonomously without any reference whatsoever to what is going on centrally in the heart. Small arteries called arterioles lying at the beginning of the microcirculation are controlled almost exclusively by local factors such as potassium levels oxygen tension and acidity and these are determined by local metabolic activity. So long as there is adequate pressure to drive flow local arterioles will ensure tissues get what they need. The role of the heart is simply to maintain the pressure.
If maintaining pressure and thus flow is the primary role of the heart how might pressure change and how is that change monitored? Momenttomoment control of blood pressure is determined by special pressure sensitive tissues distributed throughout the vascular system. If pressure falls these sensors send a message to the brainstem that results in an increase in heart rate and thus output. In the horse this is initially achieved by reducing activity in a branch of the autonomic nervous system the parasympathetic system which reduces inhibition of the heart. This works because in the horse at rest the heart rate is actually being held down by the parasympathetic activity.
At the same time adjustments in return of blood from peripheral tissues in pressures in the central venous reservoir the large veins leading to the heart and in contractility or responsiveness of the heart itself are taking place. Such mechanisms conspire to promote an increase in filling of the heart so filling will be efficient at the higher heart rate to support the increased output.
Exercise
Progressive increase in the amount of muscle tissue functioning vigorously as exercise intensity increases will be associated with progressive dilation of all the arterioles in that muscle and with filling of the muscle capillary bed all of the thinwalled microscopic vessels fed by the arteriole and that supply nutrients directly to the tissue. This shift in blood volume from the largest veins to the capillaries and the fall in blood flow resistance that results from dilating arterioles will result in a