The odds have been set and poll positions assigned for Saturday's Kentucky Derby, often called the "most exciting two minutes in sports."
The big question: Does favorite Brother Derek (3-1) have a big enough spleen to win?
Or will undefeated and second-ranked Barbaro (4-1) suck in enough oxygen to earn a victory lap? Maybe Flashy Bull, a 50-1 long shot, has already made up his mind to wear the rose blanket awarded in the winner's circle.
These are the questions Kenneth McKeever ponders when considering the science that separates the winners from the also-rans.
Spleen of a champion
McKeever works at the Equine Science Center at Rutgers University. He knows that horses have evolved several physiological features that make them ideal for running. For example, horses can breathe only through their nostrils, which are situated on the sides of their snouts. This is likely an adaptation for preventing kicked-up dust and dirt from entering their lungs when running with their mouth open.
Derby contenders keep one of their best tricks hidden close to their hindquarters.
"Horses have what I call a 'natural blood doper'—a huge spleen that stores a blood supply very rich in red blood cells," McKeever says.
It's a spleen that any elite athlete would envy. When the horse is just standing around, the percentage of oxygen-carrying red blood cells in its circulatory system runs around 35 to 40 percent. Blood in its huge spleen—3 to 4 feet long, 8 inches wide, and 4 inches thick—is a whopping 80 percent red blood cells.
When the horse starts its gallop, the surrounding muscles clamp down on the spleen like a bagpipe and squeeze all that extra blood into the circulation system. The blood ferries extra oxygen to the muscles during the run and for about an hour after exercise.
The burst of cells increases the blood's thickness substantially, a condition that might overwhelm some animals, but horses are well prepared. A horse heart is larger relative to body mass than most animals, and it pumps extra strong.
"They have a tremendous ability to utilize oxygen," McKeever told LiveScience. "Their lungs move 140 milliliters per kilogram per minute. The most elite human athletes can only do about 80. Still, if we could build a better horse, it would have even bigger lungs."
But inhaling all that oxygen is no simple task. When galloping, horses can only inhale while striding their front legs forward, explains Eric Birks of the University of Pennsylvania School of Veterinary Medicine.
"Because they have such a big mass of abdominal contents, if they tried to inhale when landing on their front feet, this mass and any food in their stomach and intestines would shift forward and limit the expansion of their lungs," Birks explained.
Since horse lungs work like a giant bellows, the beast with the longest stride should be able to inhale the most oxygen and race the fastest for the longest time.
"It just happens that the horse with the longest measured stride was Secretariat, and he was obviously a winner," Birks said.
Secretariat is widely considered the greatest racing thoroughbred of all time, having won the Triple Crown of his sport in 1973 and covered the one-and-a-quarter-mile dirt track at Churchill Downs in a record 1:59.40.
Most horses have about the same average stride length, and previous studies have suggested that being average is the best recipe for success.
"However, winning horses in recent years, such as Smarty Jones, have been shorter and had shorter strides," Birks said. "This says there's a lot more that goes into winning a horse race than length of stride."
All bets are off
One might think that armed with physiologic knowledge of what makes a good racing horse, scientists might enjoy a lucrative gambling side career.
"Over the years, the more we learn about horse physiology, the less likely we are to bet," Birks said. "Based on most of our evaluations, the horse that wins on Saturday is only 5 percent better than the horse that loses."
Horse races are often won just by the tip of a nose, or at most a body length or two, and factors such as the ability of the jockey, the track surface, nutrition and training all play into the outcome.
But the mentality of a horse might be most important of all.
"When it comes down to it, a lot of times what's going on in that horse's head is what makes him a good runner," McKeever said. "In the lab when we have a horse on the treadmill, he'll often paw at the tread, as if he's saying 'hey, I want to go. I want to have some fun.' So a lot of what makes them a good racer is up in the head."