I AM THE UGLY duckling of your anatomy. Other organs behave with quiet modesty. Not me. Constantly I remind you of my existence: with embarrassing rumbles, crampy pain, over activity at one time, under activity at another. I am your 26-foot-long intestinal tract.
You think of me, vaguely, as a coiled tube looping through your body. I am far more than that. I expect I could be best described as an elaborate food-processing plant. You assume you feed me. Actually, I feed you. Most of the food you eat would be as deadly as rattlesnake venom if it got into your bloodstream. I make it acceptable, changing it into normal components of your bloodstream - food for your trillions of cells, energy for your muscles. I convert the crisp fat in your breakfast bacon into fatty acids and glycerol. I turn the protein in your dinner lamb chop into amino acids. I change the carbohydrate in your mashed potatoes into sugary glucose. Without my chemical wizardry, even though you gorged your self, you would starve to death.
Except for cellulose - nut husks, celery strings and such - I digest virtually everything you eat and then pass it into your blood or lymph system. My final waste is composed half of countless millions of dead bacteria and half of the lubricating mucus I have secreted along the way, together with odds and ends I could not absorb.
My architecture is uniquely suited for the tasks of digestion. First comes my small intestine, which consists of a ten-inch duodenum, adjacent to the stomach; then eight feet of jejunum, about 1.5 inches in diameter; then 12 feet of slightly smaller ileum. Next comes my big gut - five feet of large intestine. To a great degree my upper portion is free of microbes - strong stomach acids kill most of them off. But my lower portion contains a veritable microbe zoo - upward of 50 varieties with a total population in the trillions.
Digestion, of course, starts in your mouth and stomach. The mouth grinds, the stomach churns; eventually, food that is about the consistency of cream soup is squirted into me through a gatekeeper valve. I may get a glass of water ten minutes after it is drunk, but a pork chop may not come along for four hours. The food the stomach delivers to me is highly acid. If I got too much at a time the acid would damage my lining and stop activity of my all-important digestive enzymes.
I take care of the acid rather neatly. My duodenum produces a substance called secretin, which empties into your blood stream. This prods your pancreas into instant secretion of its alkaline digestive juice. This juice - about a quart a day - pours into my duodenum, neutralizing acids. Let this process fail, and your apt to get what is commonly called a "stomach" ulcer. (Actually, nearly 75 percent of ulcers of this type occur in my duodenum.) The pancreatic juice also contains three main enzymes that tear proteins, fats and carbohydrates apart into basic building blocks.
Other fluids constantly pour into me from a number of sources: two daily quarts of saliva (which moistens the food, aids in swallowing and begins the digestion of starches), three quarts of gastric juice from the stomach (which destroys bacteria in the food, clots milk and splits proteins), bile from the liver (which breaks big fat globules into minute ones the pancreatic enzymes can process) and more than two quarts of intestinal juice from innumerable glands. That's roughly two gallons of fluid!
To the naked eye, the interior of my three small-bore sections has a velvety look. A microscope, however, reveals intricate cavities and projections. In fact, if my interior were smooth it would present only about six square feet of absorptive surface. Instead, it presents about 90 square feet. Perhaps my most important components are my millions of villi - microscopic finger like projections on my walls. Their job is to take processed food from my contents and put it into circulation throughout your body - proteins and carbohydrates via your bloodstream, fats via your lymphatic system.