Digestion in Human Beings

 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.


The human digestive system is a complex series of organs and glands that processes food. In order to use the food we eat, our body has to break the food down into smaller molecules that it can process; it also has to excrete waste.
Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food as it makes its way through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals.
The Digestive Process: The start of the process - the mouth: The digestive process begins in the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smaller molecules).
On the way to the stomach: the esophagus - After being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to the stomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach. This muscle movement gives us the ability to eat or drink even when we're upside-down.
In the stomach - The stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested and mixed with stomach acids is called chyme.
In the small intestine - After being in the stomach, food enters the duodenum, the first part of the small intestine. It then enters the jejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder), pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food.
In the large intestine - After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food. Many microbes (bacteria like Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process. The first part of the large intestine is called the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon.
The end of the process - Solid waste is then stored in the rectum until it is excreted via the anus.
Digestive System Glossary: abdomen - the part of the body that contains the digestive organs. In human beings, this is between the diaphragm and the pelvis alimentary canal - the passage through which food passes, including the mouth, esophagus, stomach, intestines, and anus. anus - the opening at the end of the digestive system from which feces (waste) exits the body. appendix - a small sac located on the cecum. ascending colon - the part of the large intestine that run upwards; it is located after the cecum. bile - a digestive chemical that is produced in the liver, stored in the gall bladder, and secreted into the small intestine. cecum - the first part of the large intestine; the appendix is connected to the cecum. chyme - food in the stomach that is partly digested and mixed with stomach acids. Chyme goes on to the small intestine for further digestion. descending colon - the part of the large intestine that run downwards after the transverse colon and before the sigmoid colon. digestive system - (also called the gastrointestinal tract or GI tract) the system of the body that processes food and gets rid of waste. duodenum - the first part of the small intestine; it is C-shaped and runs from the stomach to the jejunum. epiglottis - the flap at the back of the tongue that keeps chewed food from going down the windpipe to the lungs. When you swallow, the epiglottis automatically closes. When you breathe, the epiglottis opens so that air can go in and out of the windpipe. esophagus - the long tube between the mouth and the stomach. It uses rhythmic muscle movements (called peristalsis) to force food from the throat into the stomach. gall bladder - a small, sac-like organ located by the duodenum. It stores and releases bile (a digestive chemical which is produced in the liver) into the small intestine. gastrointestinal tract - (also called the GI tract or digestive system) the system of the body that processes food and gets rid of waste. ileum - the last part of the small intestine before the large intestine begins. intestines - the part of the alimentary canal located between the stomach and the anus. jejunum - the long, coiled mid-section of the small intestine; it is between the duodenum and the ileum. liver - a large organ located above and in front of the stomach. It filters toxins from the blood, and makes bile (which breaks down fats) and some blood proteins. mouth - the first part of the digestive system, where food enters the body. Chewing and salivary enzymes in the mouth are the beginning of the digestive process (breaking down the food). pancreas - an enzyme-producing gland located below the stomach and above the intestines. Enzymes from the pancreas help in the digestion of carbohydrates, fats and proteins in the small intestine. peristalsis - rhythmic muscle movements that force food in the esophagus from the throat into the stomach. Peristalsis is involuntary - you cannot control it. It is also what allows you to eat and drink while upside-down. rectum - the lower part of the large intestine, where feces are stored before they are excreted. salivary glands - glands located in the mouth that produce saliva. Saliva contains enzymes that break down carbohydrates (starch) into smaller molecules. sigmoid colon - the part of the large intestine between the descending colon and the rectum. stomach - a sack-like, muscular organ that is attached to the esophagus. Both chemical and mechanical digestion takes place in the stomach. When food enters the stomach, it is churned in a bath of acids and enzymes. transverse colon - the part of the large intestine that runs horizontally across the abdomen.