In May, only one word is on the minds of people from Kentucky—Derby! Always held on the first Saturday in May, this year was the 145th running of the Kentucky Derby horse race. This year’s outcome was a shocker: For the first time in race history, the horse that crossed the finish line first was disqualified and the second horse became the official winner. The race that is known as “the most exciting two minutes in sports” certainly earned that nickname this year!
At the Derby, three-year-old Thoroughbred horses race 1 1/4 miles around a dirt racetrack, vying for a $2 million purse and all the glory. Regardless of the result, this amazing feat in such a short amount of time makes me wonder how horses power themselves around the track so fast.
Horses are elite athletes when compared to other mammals. The horse has a unique respiratory anatomy, which allows them to move extremely large volumes of air—up to 80 liters per second (l/s)—in and out of the lungs. In comparison, the human respiratory tract—which is highly specialized for speech rather than exercise—has a peak maximum air flow around four l/s. A galloping horse’s breathing is tied to its stride frequency, which means it inhales only when the front hooves are striding outward and exhales only when all four legs come together. This also means that horses with longer strides have the advantage of longer breathing time.
In people, the cardiovascular and muscular systems—not lung capacity—limit performance. When elite human athletes work out, they can use six or eight times more oxygen than when they’re resting, whereas a horse can consume about 40 times more oxygen than at rest. This is helped in part by horses’ respiratory dynamics but also by their massive spleens, which store a very large number of red blood cells. Before exercising, 35 percent of a person’s—or horse’s— total blood volume is made up of red blood cells. During exercise, this percentage stays the same in people, but horses are able to increase their red blood cell numbers to more than 65 percent. This gives the horse an added ability to transport more oxygen within the blood during exercise.
A horse with a perfect combination of physiological traits and a solid mental temperament has a distinct racing advantage and could be a Derby contender. So as we watch horses race, we can simply marvel at the beauty of one of the most elite physiological athletes on earth.
Amanda Jo LeBlanc, PhD, is a cardiovascular physiologist and an associate professor at the University of Louisville School of Medicine. She studies microvascular function in aging.