Vets opinion on the spleen

There have been some trainers that say the better long distance horses have large spleens. They claim that the spleen releases a large amount of extra RBC's when horses are put to longer running tests.

Having been a runner for sometime, I never encountered any information of this type for humans. The spleen does hold large amounts of excess RBC's but they are usually released during trauma involving blood loss, not during running. I had never heard about making sure you train long enough so that your spleen releases blood.

So whats the deal with the horses spleen?