Originally Posted by Riot

The spleen stores red blood cells. Release is dependent upon catecholamine release and sympathetic nervous system stimulation (fright, flight, exercise, suffocation/asphyxia, excitement, bleeding/hemorrhage, etc).

Horses are better at it than humans (being prey animals), able to increase the number of red blood cells in circulation quite quickly and markedly. This aids oxygenation (along with other physiologic changes happening concurrently).

Appropriate exercise and training regimines cause metabolic adaptations that make such processes more efficient. The metabolic adaptations required by and ideal for sprinting vs long distance exercise vary.

I sent you something in more detail privately.