Scientists at the University of California-Davis have uncovered new information that might explain why horses bleed internally when they exercise. According to a recent edition of The Horse Report, which is published by the school's Center for Equine Health, a horse's heart is unable to relax quickly enough between beats during extreme exercise. This situation leads to unusually high blood pressure in the heart, which in turn increases pressure throughout the lungs.

The heart studies were performed by Dr. Jim Jones and Dr. John Pascoe, who worked closely with the Japan Racing Association. The researchers used surgically-implanted catheters in conjunction with ultrasound crystals that were attached surgically to the heart to measure the organ's mechanics during exercise.