There are sprinters and there are distance runners, but the thoroughbreds charging down the tracks this month need to be both. The races stretch a grueling mile and a quarter, so those horses have to be machines that convert air into speed.
1. Slow-Twitch muscle uses oxygen more efficiently, which aids endurance. Thoroughbreds have nearly twice as many slow twitch fibers as sprinty quarter horses, so they can clock speeds up to 40 mph even on the home stretch.
2.The spleen can squirt out 12 bonus liters of blood, jacking up cardiac output. The extra red blood cells provide a Lance Armstrong-style boost to the horse's blood oxygen levels.
3. Big hearts mean fast racers, because the organ can pump more blood (and thus 02) to the muscles. A gene on the X chromosome gives some thoroughbreds a heart that's nearly twice normal size.
4. Latherin, a detergent-like protein unique to equine sweat, foams the hairs apart a bit, allowing more efficient evaporative cooling.
5. The respiratory tract is where the air comes in. No air, no race. Trainers snake a tiny scope into a nostril to monitor prospects' air ways during track exercise sessions.
