By Alyse Knorr - May 3, the day of the 134th Kentucky Derby, marked one of horse racing's most anticipated days of the year.

Washington, D.C. - Scripps Howard Foundation Wire - infoZine - But by the end of the race, it would be remembered as one of the sport's most tragic.

Just minutes after passing the winning post at the Churchill Downs track, Eight Belles, the filly who finished second in the famous race, collapsed with two broken front ankles.

The acute nature of her injuries forced an emergency veterinarian to euthanize her in the middle of the track, launching the country into a heated debate over the ethics of the horseracing industry.

But while animal rights activists and shocked fans have been pointing to the industry's use of drugs and steroids, thoroughbreds face a number of health risks completely unrelated to chemical factors.

Thoroughbreds' bodies have some naturally weak points that make them particularly vulnerable to certain kinds of racing injuries. These weak points are unrelated to drugs and exist simply because of the breed's evolution.

"The horse by itself is already the most athletic species of all domestic mammals," said Dr. Laurent Couetil, from the Purdue University School of Veterinary Medicine.

Their athleticism is partly a result of a high-powered cardiovascular system that allows horses to take in more oxygen and supply their muscles with more blood for faster contraction.

A bull of similar size to a thoroughbred, for example, has only half the lung volume, and its heart is only half as large.

"Everything has been done to have an apparatus very well-adapted to fast racing," Couetil said.

But that large oxygen intake carries with it the risk of inhaling more irritable particles that can cause allergies, asthma and chronic bronchitis.

And the large amount of blood pumped by the heart during a race puts pressure on the lungs that can cause bleeding, a condition known as exercise-induced pulmonary hemorrhage, or EIPH.

Knee and joint problems are also common in racehorses, Couetil said, because of repeated trauma to joint cartilage. This trauma can lead to painful fluid buildup and arthritis.

The high speeds of racing also put a large amount of stress on a horse's musculoskeletal system, which causes microdamage that, over time, can cause a more serious fracture.

In 2006, Kentucky Derby winner Barbaro shattered his hind leg racing in the Preakness Stakes. Like Eight Belles, he had to be euthanized, after doctors tried several treatments over eight months.

The deaths of both Barbaro and Eight Belles have sparked bitter discussions and intense investigations into steroids, selective breeding and overworking.

On June 19, the House of Representatives held a hearing on the topic and questioned veterinarians, farm owners and other industry members to explore the idea of government intervention.