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Posted on Sat, Jan 26, 2013 : 5 a.m.

Digestive process is engineering marvel

By Ask Dr. K

DEAR DOCTOR K:

Can you describe how food makes its way through our body, from the mouth to the other end?

DEAR READER:

This question is fun to answer, because the gut is a marvel of nature's engineering. It's both a food processor and a garbage disposal. First, it works as a food processor, breaking down food and liquids into chemical components that the body can absorb as nutrients. Then, what's left -- the garbage -- is expelled by an efficient disposal system.

The food you eat is of no value to you unless it is broken up into microscopically small pieces, and those pieces get absorbed from the gut into the bloodstream. Those tiny pieces become the building blocks for everything your body needs to function.

The gastrointestinal (GI), or digestive, tract is a series of hollow organs. They're linked to form a long tube that runs from mouth to anus. First comes the throat and esophagus, then the stomach, small intestine and colon. The walls of the tube contain muscles that perform different functions. (I've put an illustration of the GI tract on my website, AskDoctorK.com.)

(1) Digestion begins in the mouth, where food is chewed until it's small enough to be swallowed. The teeth chop the food up into small pieces -- but pieces you can see. Chemicals called enzymes in saliva begin to digest these pieces further.

(2) Once food is swallowed, it enters the throat and then the esophagus. Food does not simply drop down the esophagus; it is pushed by contractions of the esophageal muscles. These muscles squeeze in a coordinated way to move food from the top to the bottom of the tube, always pushing it downward.

At the bottom of the esophagus, where it meets the stomach, there is a muscular sphincter. When it's working properly, this sphincter remains shut except when you're swallowing, to prevent stomach acid from irritating the esophagus.

(3) Once the food moves into the stomach, muscles mix it into a soft mush. Saliva, hydrochloric acid and the enzyme pepsin help to further break down the food into a thick liquid called chyme. The stomach delivers chyme to the small intestine.

(4) The main work of digestion takes place in the small intestine. That's where the fats, starches and proteins in your food are turned into the tiniest pieces -- fatty acids, simple sugars and amino acids. These nutrients are absorbed by the intestine's thin lining and then transported in the blood to cells throughout the body.

(5) Finally, what's left of the food arrives in the large intestine, the colon. The walls of this muscular tube soak up most of the remaining water. Bacteria in the colon feast on the little nutrition left in your food. What's left is propelled further down the colon. It settles in the rectum, until you release it during a bowel movement.

The garbage disposal has made room for the next shipment from the food processor!

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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