The Incredible Human Machine: Digestion Transcript
Speaker:Narrator
(Music)
NARRATOR: Every timewe swallow a morsel of food, we set it on a journey designed to suck everything useful out of it. Sit down to lunch, and in the 30 or so hours and 20+ feet it takes to digest it, we convert plants or animals into energy and absorb their chemical building blocks into our own flesh and blood. Carbohydrates, proteins, fats, vitamins, minerals—all of our nutrients come from what we eat. Digestion actually starts before food crosses our lips. Just the idea of food can get our mouths to water. We salivate roughly a pint every day, and in that saliva are enzymes that, along with our teeth, begin to break the food we eat down. But not before we get to savor it first. Some 10,000 taste buds line our tongues, each one home to about 50 taste receptor cells that tell the brain what we’re eating. And in case you scorch some of those precious receptors off, not to worry. Wait a week to 10 days, and you’ll grow back a brand new set. Once the muscular tongue maneuvers food into our esophagus, we’re on autopilot. If it looks familiar, we’ve been here before. There’s the voice box again. When we eat, a flap of skin called the epiglottis seals off our windpipes, except when it doesn’t.
(Man coughing)
Then, it’s back to the beginning.
A typical journey down the esophagus takes all of about 5 seconds. From here on out, it’s squeezed like toothpaste. Once it passes into the stomach, as it’s doing here, it’s time to slow down a bit and digest. Normally this stretchy j-shaped bag isn’t much bigger than a fist. But after a big meal, it can expand to more than 20 times that size. For the next several hours, highly acidic gastric juices spew from the stomach’s walls, breaking down proteins in our food, while muscle contractions knead and churn it into pulp. The acid here is so strong, the stomach must continually secrete a layer of protective mucus so that it doesn’t digest itself. It absorbs very few nutrients, though. For that we pass into the 20-foot disassembly line of small intestine with the help of a specialized camera pill. For some 5 hours, foods’ building blocks are pushed, prodded, sprayed with digestive juices, and wrung like a rag here, until its vital elements are forced through the intestinal wall and into the bloodstream.
From here, just about all of our meal’s nutrients will flow directly into the liver, the body’s largest internal organ. The liver breaks down, repackages, and delivers nutrients to our cells for growth and power. Our bodies ultimately try to balance energy intake, but sometimes more goes in than out. The result: fat. Here’s how fat looks on the inside and on the outside. It doesn’t take a lot of excess calories to make you gain weight. Just 15 more a day than you need, about the amount in four pistachio nuts, will add a pound and a half of fat over a year. Once the small intestine has taken in everything worth ingesting, the rest is pushed along to the large intestine. For another 20 hours or so, the last of the water is absorbed and trillions of bacteria work to break down the remaining contents. In the end, anything we can’t digest gets flushed out of our bodies. Food’s complicated journey has a larger purpose. Once we’ve extracted what we need to feed the incredible machine and gotten our cellular engines humming, it’s nothing short of astounding what we can do with them.
[End of Audio]
From “The Incredible Human Machine: Digestion,”2012. Adapted with permission from Wiley.