Complex sentence: Thoroughbred racehorses perform with maximum exertion, which has resulted in high accident rates and health problems such as bleeding from the lungs, low fertility, abnormally small hearts and a small hoof to body mass ratio.
Split into simpler sentences: Thoroughbred racehorses perform with maximum exertion, which has resulted in high accident rates and health problems such as bleeding from the lungs. Other health concerns include low fertility, abnormally small hearts and a small hoof to body mass ratio.