Blood volume and heart rate

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From: Amy Tetervin 
Email: tetervin@bms.com

Been following the discussion of heart rate and gait and now I wonder if some of the vets or physiologists out there can tell us at what point the spleen dumps it's hoard of blood into the circulation and how this might affect heart rate or recovery. My impression is that a horse has to be running at top speed before the spleen lets go of this reserve of blood so perhaps finishing a race at a fast gallop could be a good strategy? Any differences among breeds when it comes to the amount held in reserve? Thanks- Amy