The human body is a weird and amazing machine. It's easy to forget this, alone in the complacency and familiarity of one's own form and rhythms. But then one remembers the existence of runners with the mental and physical stamina to run a hundred miles or more at a stretch and soon the potential of the familiar starts to seem a lot more promising.

Post-race, the blood of a marathoner can paint a picture of diseased organs and biochemical decay. (Getty Images)

But such feats are not without consequence. It's well-known that marathons exert a serious toll on the body as it copes with the demands of running so far at such a pace. Post-race, the blood of a marathoner can paint a picture of diseased organs and biochemical decay thanks to compounds tossed off by the skeletal muscle, heart, and liver, such as troponin, a cardiac enzyme whose elevation signals trauma to the heart. Technically, an ultramarathon is any distance race over the standard 26.2 miles. So what happens to a body that runs even farther?

"The pain signals, or the cumulative fatigue, are nonlinear," according to Jimmy Dean Freeman, an ultrarunner and L.A.-based endurance coach. "So you might feel really poor at mile 30, and then better than you've ever felt that day at mile 60." He's about to experience a lot of nonlinear pain signals, with a planned six 100-mile ultras in the next 13 weeks. This does not bode well for his toenails, common casualties of the sport, which rivals ballerina feet in terms of sheer podiatric carnage.

That said, an ultra is more than just foot torture. Putting a body through such a race is a game of numbers. Freeman lets me in on one of the jokes in ultrarunning, that it's "basically an eating and drinking contest with a little exercise and scenery thrown in." Everything that goes into the body is carefully planned and calibrated; everything that comes out is monitored for signs of peril. Drinking the right amount of water at the correct rate, maintaining the proper electrolyte balance, eating enough of the right kind of calories, and knowing your body well enough to care for that jostling digestive system can each mean the difference between a complete race and a DNF.

Fueling itself is a huge challenge. The body stores glycogen in its liver and muscles; most people's bodies have enough glycogen to about last 20 miles, or a fifth of an ultra. The feeling of running out of sugar is aptly called "bonking"; or hitting a wall. Fat is harder to burn as fuel. What a runner eats during the race can mean the difference between finishing sweaty, or finishing covered in shit. I have a lot of love for runners who poop their pants and keep going. That's some "testament to the human spirit" shit right there. "As many people have failed to finish a hundred mile race based on gastrointestinal misadventures as they have through any sort of mental challenge or physical failure," Freeman said. "The stomach is a demon that slows down, or ends, a lot of days."

But there are worse foes. While pissing blood post-race is somewhat normal due to the hemolysis from repetitive foot-strikes, urine that's brown or black could signal renal failure. When small, exertional tears occur in muscle, the muscle is rebuilt stronger. During such a long race, significant muscle breakdown can occur, dumping myosin into the bloodstream to be filtered out by the kidneys. A dehydrated body, with an increased concentration of myosin in its sludgy blood, is hard enough on the kidneys; add multiple doses of NSAID pain-relievers (like ibuprofen), which shrink the kidneys, and you have a perfect storm for renal failure. There is not a consensus on how, exactly, myoglobin causes renal failure, rather a trio of suspects: obstruction of the delicate tubules of the kidneys, lack of oxygen, or a toxic reaction. Compared to kidneys that stop working, shitting your brains out thanks to hours of sugary drinks and a sloshing gut seems like a quaint right of passage.

It's important to note that renal failure is a rare side-effect, and one that can be guarded against to a certain degree by careful monitoring and planning. Most runners won't land themselves in the hospital for a course of dialysis, but the specter of such injury should not be ignored.

Toenails bruise, turn black, and eventually fall off. Chafing can go from nuisance to Halloween-level gore.

Even when things go well, running 100 miles still brings a host of serious physiological and psychological effects on the body. Freeman tells me that most athletes lose the ability to thermoregulate somewhere after the 10 to 15 hour mark. "In races that have variable temperature, when an athlete fatigues to the point of not being able to run, I've seen people go hypothermic in 80 degrees." An enormous amount of heat is produced by skeletal muscle during running, and the body works hard to get rid of it; muscle temperatures of around 100 degrees Fahrenheit mean that vasodilation and evaporative cooling are a must to keep core temps down. Once a runner reaches the 50 mile point, if they haven't managed themselves well in terms of calories and hydration, if they're not in the proper shape to do the event, and especially if the temperature is below 80 degrees, shivering can occur. "The body's thermostat is off after a long period of running."

One big rule seems to be that minor inconveniences now could spell disaster later. Nipples slightly chafed by a new shirt during the first half of the race could be two bleeding orbs by the end of it. Feet swell, clogging once-well-fit shoes with fluid; toenails bruise, turn black, and eventually fall off. Chafing can go from nuisance to Halloween-level gore. You'd be amazed what a blister could do if you give it 100 miles of rubbing.