The Human Digestion Process (or, What Happens after You Eat Food)

Digestion is the process of changing food into a form that the body can absorb and use as energy or as the raw materials to repair and build new tissue. Digesting food is a two-part process that's half mechanical, half chemical.

• Mechanical digestion begins in your mouth as your teeth tear and grind food into small bits and pieces you can swallow without choking. The muscular walls of your esophagus, stomach, and intestines continue mechanical digestion, pushing the food along, churning and breaking it into smaller particles.

• Chemical digestion occurs at every point in the digestive system, beginning when you see or smell food. These sensory events set off nerve impulses from your eyes and nose that trigger the release of enzymes and other substances that will eventually break down food to release the nutrients inside. The body then burns these nutrients for energy or uses them to build new tissues and body parts.

How sight and smell relate to digestion

At first glance — or sniff — the digestive link between your eyes, nose, and stomach sounds a tad weird. But think about it: How many times has the sight or scent of something yummy like a simmering stew or baking bread set your tummy rumbling?

The sight of an appetizing dish or the aroma (actually scent molecules bouncing against the nasal tissues) sends signals to your brain: "Good stuff on the way." As a result, your brain — the quintessential message center — shoots out impulses that

• Make your mouth water.

• Make your stomach contract (hunger pangs).

• Make intestinal glands start leaking digestive chemicals.

All that from a little look and sniff. Imagine what happens when you actually take a bite!

Tasting and chewing in the digestion process

You know that small bag of potato chips you have stashed way at the back of your desk drawer? Well, dig it out and take a chip.

As the chip hits your tongue, your mouth acts as though someone had thrown the "on" switch in a fun house.

• Your teeth chew, breaking the chip into small manageable pieces.

• Your salivary glands release a watery liquid (saliva) to compact the chip into a mushy bundle (a bolus in digestive-geek speak) that can slide easily down your throat on a stream of saliva.

• Enzymes (which you can think of as digestive catalysts in this case) in the saliva begin to digest carbohydrates in the chip.

• Your tongue lifts to push the whole ball of wax . . . no, bolus, back toward the pharynx, the opening from your mouth to your esophagus, and then through a muscular valve called the upper esophageal sphincter, which opens to allow the food through. In other words, you're about to swallow.

Swallowing food: The slide from esophagus to stomach

If you think about it, the human digestive system is a wonder. As food enters the esophagus, your salivary glands release a rush of saliva to help food slide more easily down the tube. Then your esophageal muscles swing into action.

Like the rest of your digestive tract, your esophagus is ringed with muscles that contract to produce wavelike motions — which you can refer to as peristalsis or (no surprise here) peristaltic contractions, if you're so inclined — pushing food down toward your stomach.

At the bottom of the esophagus — an area known as the gastroesophageal junction — a muscular valve called the lower esophageal sphincter (LES) opens to allow food through. Then the LES closes to prevent reflux, the flow of stomach contents back into the esophagus. A malfunctioning LES is public enemy No. 1 in the reflux world.

Digesting food in the stomach

Point to your stomach. Go ahead. Don't be shy. Odds are your finger is aimed somewhere around your belly button, an interesting site to be sure, but definitely not your stomach. Your stomach, a wide, pouchy part of the digestive tube, is located on the left side of your body above your waist and behind your ribs.

Like the walls of your esophagus, the walls of your stomach are strong and muscular. They contract with enough force to break food into ever smaller pieces as glands in the stomach walls release stomach juices — a highly technical term for a highly acidic blend of enzymes, hydrochloric acid (HCl), and mucus. The stomach juices begin the digestion of proteins and fats into their respective bodily building blocks — amino acids and fatty acids.

Churned by the stomach walls and degraded by the stomach juices, what started as food — apples, pears, potato chips, steak, cake, you name it — is now a thick, soupy mass called chyme (from chymos, the Greek word for juice). The stomach's wavelike contractions push this messy but still intact substance along to the small intestine where your body begins to pull out the nutrients it needs.

Pulling out nutrients in the small intestine

Here's an easy anatomy lesson to find your small intestine: Open your hand and put it flat slightly below your belly button, with your thumb pointing up and your pinky pointing down. Your hand is now covering most of the relatively small space into which your 20-foot-long small intestine is neatly coiled. Just like your esophagus and stomach, contracting muscles line your small intestines to push food along.

But your small intestine is nobody's copycat. This part of your digestive system has its own set of digestive juices including

• Alkaline goop from the pancreas that powers special enzymes (called amylases) to digest carbs

• Bile from the liver and gallbladder that acts as an emulsifier (a compound that enables fats to mix with water)

• Pancreatic and intestinal enzymes that complete the separation of proteins into amino acids

More contractions shove the chyme along the intestines while specialized cells in the intestinal walls grab onto sugars, amino acids, fatty acids, vitamins, and minerals, which are then sent off into your body for energy or as building blocks for new tissue.

Then, after your small intestine has squeezed every last little bit of useful material (other than water) out of the food, the indigestible remainder (think dietary fiber) moves toward its inevitable end in your large intestine.

The end of the digestive line: Poo-poo

Your large intestine is also sometimes called the colon. Think of this area as a giant sponge and press whose only jobs are to absorb water from the mass you deliver to it and then squeeze the dry leftovers into compact bundles of waste — which you may know as feces and any 2-year-old as poo-poo (or poop, caca, whatever).

After resident colonies of friendly bacteria digest any amino acids remaining in the waste and excrete smelly nitrogen — in a process scientists call passing gas — muscular contractions in the rectum push the feces out of your body, and digestion is finally done.