Horses use their spleens for natural blood doping

A Thoroughbred standing at the starting gate carries a hidden reserve of energy tucked away in its abdomen. While a human spleen is roughly the size of a fist and mostly manages old blood cells, a horse's spleen is a massive, muscular organ that holds up to twelve liters of highly concentrated red blood cells. When the horse feels the sudden rush of adrenaline at the start of a race, its nervous system signals the spleen to squeeze like a sponge. In less than sixty seconds, this contraction injects a massive dose of oxygen-carrying cells into the bloodstream, effectively supercharging the animal from the inside out.