Derby day horse physiology

LiveScience has a story about the qualities of good racing horses. It doesn't talk about breeding too much, but it has a lot of interesting stuff about their physiology:

"Horses have what I call a 'natural blood doper' -- a huge spleen that stores a blood supply very rich in red blood cells," McKeever says.
It's a spleen that any elite athlete would envy. When the horse is just standing around, the percentage of oxygen-carrying red blood cells in its circulatory system runs around 35 to 40 percent. Blood in its huge spleen -- 3 to 4 feet long, 8 inches wide, and 4 inches thick--is a whopping 80 percent red blood cells.
When the horse starts its gallop, the surrounding muscles clamp down on the spleen like a bagpipe and squeeze all that extra blood into the circulation system. The blood ferries extra oxygen to the muscles during the run and for about an hour after exercise.

I sure didn't know that. Also, their lungs have nearly double the oxygen-moving potential of humans, and their stride helps to pump air in and out like a bellows.

Horses really are an interesting combination of fast and large, which brings all these energetic and oxygen restrictions to a head.