Each organ in the digestive system plays a specific role in the digestive drama. But the first act occurs in two places that are never listed as part of the digestive tract: your eyes and nose.
When you see appetizing food, you experience a conditioned response. (For the lowdown on how your digestive system can be conditioned to respond to food, see Chapter 14; for information on your food preferences, see Chapter 15). In other words, your thoughts — "Wow! That looks good!" — stimulate your brain to tell your digestive organs to get ready for action.
What happens in your nose is purely physical. The tantalizing aroma of good food is transmitted by molecules that fly from the surface of the food to settle on the membrane lining of your nostrils; these molecules stimulate the receptor cells on the olfactory nerve fibers that stretch from your nose back to your brain. When the receptor cells communicate with your brain — "Listen up, there's good stuff here!" — your brain sends encouraging messages to your mouth and digestive tract.
In both cases — eyes and nose — the results are the same: "Start the saliva flowing," they say. "Warm up the stomach glands. Alert the small intestine." In other words, the sight and scent of food has made your mouth water and your stomach contract in anticipatory hunger pangs.
But wait! Suppose you hate what you see or smell? For some people, even the thought of liver is enough to make them want to barf — or simply leave the room. At that point, your body takes up arms to protect you: You experience a rejection reaction — a reaction similar to that exhibited by babies given something that tastes bitter or sour. Your mouth purses and your nose wrinkles as if to keep the food (and its odor) as far away as possible. Your throat tightens, and your stomach turns — muscles contracting not in anticipatory pangs but in movements preparatory for vomiting up the unwanted food. Not a pleasant moment.
But assume you like what's on your plate. Go ahead. Take a bite.
Lift your fork to your mouth, and your teeth and salivary glands swing into action. Your teeth chew, grinding the food, breaking it into small, manageable pieces. As a result:
At the same time, salivary glands under your tongue and in the back of your mouth secrete the watery liquid called saliva, which performs two important functions:
No protein digestion occurs in your mouth, though saliva does contain very small amounts of lingual lipases, fat-busting enzymes secreted by cells at the base of the tongue; however, the amount is so small that the fat digestion that occurs in the mouth is insignificant.
If you were to lay your digestive tract out on a table, most of it would look like a simple, rather narrow tube. The exception is your stomach, a pouchy part just below your gullet (esophagus).
Salivary enzymes (like amylases) don't lay a finger on proteins and leave fats pretty much alone, but they do begin to digest complex carbohydrates, breaking the long, chainlike molecules of starches into individual units of sugars; this simple experiment enables you to taste firsthand the effects of amylases on carbohydrates.
1. Put a small piece of plain, unsalted cracker on your tongue.
No cheese, no chopped liver — just the cracker, please.
2. Close your mouth and let the cracker sit on your tongue for a few minutes.
Do you taste a sudden, slight sweetness? That's the salivary enzymes breaking a long, complex starch molecule into its component parts (sugars).
3. Okay, you can swallow now.
The rest of the digestion of the starch takes place farther down, in your small intestine.
Like most of the digestive tube, your stomach is circled with strong muscles whose rhythmic contractions — called peristalsis — move food smartly along and turn your stomach into a sort of food processor that mechanically breaks pieces of food into ever smaller particles. While this is going on, glands in the stomach wall are secreting stomach juices — a potent blend of enzymes, hydrochloric acid, and mucus.
One stomach enzyme — gastric alcohol dehydrogenase — digests small amounts of alcohol, an unusual nutrient that can be absorbed directly into your bloodstream even before it's been digested. For more about alcohol digestion, including why men can drink more than women without becoming tipsy, see Chapter 9.
Other enzymes, plus stomach juices, begin the digestion of proteins and fats, separating them into their basic components — amino acids and fatty acids.
Stop! If the words amino acids and fatty acids are completely new to you and if you are suddenly consumed by the desire to know more about them this instant, stick a pencil in the book to hold your place and flip ahead to Chapters 6 and 7, where I discuss them in detail.
Stop again!! For the most part, digestion of carbohydrates comes to a screeching — though temporary — halt in the stomach because the stomach juices are so acidic that they deactivate amylases, the enzymes that break complex carbohydrates apart into simple sugars. However, stomach acid can break some carbohydrate bonds, so a bit of carb digestion does take place.
Back to the action. Eventually, your churning stomach blends its contents into a thick soupy mass called chyme (from cheymos, the Greek word for juice). When a small amount of chyme spills past the stomach into the small intestine, the digestion of carbohydrates resumes in earnest, and your body begins to extract nutrients from food.
The small intestine
Open your hand and put it flat against your belly button, with your thumb pointing up to your waist and your pinkie pointing down.
Your hand is now covering most of the relatively small space into which your 20-foot-long small (20 feet? small?) intestine is neatly coiled. When the soupy, partially-digested chyme spills from your stomach into this part of the digestive tube, a whole new set of gastric juices are released. These include:
I Pancreatic and intestinal enzymes that finish the digestion of proteins into amino acids
I Bile, a greenish liquid (made in the liver and stored in the gallbladder) that enables fats to mix with water
I Alkaline pancreatic juices that make the chyme less acidic so that amy-lases (the enzymes that break down carbohydrates) can go back to work separating complex carbohydrates into simple sugars
I Intestinal alcohol dehydrogenase, which digests alcohol not previously absorbed into your bloodstream
While these chemicals are working, contractions of the small intestine continue to move the food mass down through the tube so that your body can absorb sugars, amino acids, fatty acids, vitamins, and minerals into cells in the intestinal wall.
^^-Sify^ The lining of the small intestine is a series of folds covered with projections that have been described as "finger-like" or "small nipples." The technical name for these small fingers/nipples is villi. Each villus is covered with smaller projections called microvilli, and every villus and microvillus is programmed to accept a specific nutrient — and no other.
Nutrients are absorbed not in their order of arrival in the intestine but according to how fast they're broken down into their basic parts:
1 Carbohydrates — which separate quickly into single sugar units — are absorbed first.
1 Proteins (as amino acids) go next.
i Fats — which take longest to break apart into their constituent fatty acids — are last. That's why a high-fat meal keeps you feeling fuller longer than a meal such as chow mein or plain tossed salad, which are mostly low-fat carbohydrates.
i Vitamins that dissolve in water are absorbed earlier than vitamins that dissolve in fat.
Peephole: The first man to watch a living human gut at work
William Beaumont, M.D., was a surgeon in the United States Army in the early 19th century. His name survives in the annals of medicine because of an excellent adventure that began on June 6, 1822. Alexis St. Martin, an 18-year-old French Canadian fur trader, was wounded by a musket ball that discharged accidentally, tearing through his back and out his stomach, leaving a wound that healed but didn't close.
St. Martin's injury seems not to have affected what must have been a truly sunny disposition: Two years later, when all efforts to close the hole in his gut had failed, he granted Beaumont permission to use the wound as the world's first window on a working human digestive system. (To keep food and liquid from spilling out of the small opening, Beaumont kept it covered with a cotton bandage.)
Beaumont's method was simplicity itself. At noon on August 1, 1825, he tied small pieces of food (cooked meat, raw meat, cabbage, bread) to a silk string, removed the bandage, and inserted the food into the hole in St. Martin's stomach.
An hour later, he pulled the food out. The cabbage and bread were half digested; the meat, untouched. After another hour, he pulled the string out again. This time, only the raw meat remained untouched, and St. Martin, who now had a headache and a queasy stomach, called it quits for the day. But in more than 230 later trials, Beaumont — with the help of his remarkably compliant patient — discovered that although carbohydrates (cabbage and bread) were digested rather quickly, it took up to eight hours for the stomach juices to break down proteins and fats (the beef). Beaumont attributed this to the fact that the cabbage had been cut into small pieces and the bread was porous. Modern nutritionists know that carbohydrates are simply digested faster than proteins and that digesting fats (including those in beef) takes longest of all.
By withdrawing gastric fluid from St. Martin's stomach, keeping it at 100° F (the temperature recorded on a thermometer stuck into the stomach), and adding a piece of meat, Beaumont was able to clock exactly how long the meat took to fall apart: 10 hours.
Beaumont and St. Martin separated in 1833 when the patient, now a sergeant in the United States Army, was posted elsewhere, leaving the doctor to write "Experiments and Observations on the Gastric Juice and the Physiology of Digestion." The treatise is now considered a landmark in the understanding of the human digestive system.
After you've digested your food and absorbed its nutrients through your small intestine:
1 Amino acids, sugars, vitamin C, the B vitamins, iron, calcium, and magnesium are carried through the bloodstream to your liver, where they are processed and sent out to the rest of the body.
1 Fatty acids, cholesterol, and vitamins A, D, E, and K go into the lymphatic system and then into the blood. They, too, end up in the liver, are processed, and are shipped out to other body cells.
Inside the cells, nutrients are metabolized, or burned for heat and energy or used to build new tissues. The metabolic process that gives you energy is called catabolism (from katabole, the Greek word for casting down). The metabolic process that uses nutrients to build new tissues is called anabolism (from anabole, the Greek word for raising up).
How the body uses nutrients for energy and new tissues is, alas, a subject for another chapter. In fact, this subject is enough to fill seven different chapters, each devoted to a specific kind of nutrient. For information about metabolizing proteins, turn to Chapter 6. I discuss fats in Chapter 7, carbohydrates in Chapter 8, alcohol in Chapter 9, vitamins in Chapter 10, minerals in Chapter 11, and water in Chapter 13.
After every useful, digestible ingredient other than water has been wrung out of your food, the rest — indigestible waste such as fiber — moves into the top of your large intestine, the area known as your colon. The colon's primary job is to absorb water from this mixture and then to squeeze the remaining matter into the compact bundle known as feces.
Feces (whose brown color comes from leftover bile pigments) are made of indigestible material from food, plus cells that have sloughed off the intestinal lining and bacteria — quite a lot of bacteria. In fact, about 30 percent of the entire weight of the feces is bacteria. No, these bacteria aren't a sign you're sick. On the contrary, they prove that you're healthy and well. These bacteria are good guys, microorganisms that live in permanent colonies in your colon, where they:
1 Manufacture vitamin B12, which is absorbed through the colon wall
1 Produce vitamin K, also absorbed through the colon wall i Break down amino acids and produce nitrogen (which gives feces a characteristic odor)
i Feast on indigestible complex carbohydrates (fiber), excreting the gas that sometimes makes you physically uncomfortable — or a social pariah
Chew! Chew! All aboard the Nutrient Express!
Think of your small intestine as a busy train station whose apparent chaos of arrivals and departures is actually an efficient, well-ordered system. (Please forgive the terrible pun in the title of this sidebar. My husband — who inherited a gift for this sort of thing from his mother — made me do it.)
As I was saying, the small intestine resembles a three-level, miniature Grand Central Terminal:
I Level 1 is the duodenum (at the top, right after your stomach).
I Level 2 is the jejunum (in the middle).
I Level 3 is the ileum (the last part before the colon).
This three-station tube hums away as nutrients arrive and depart, with millions of "trains" (the nutrients) running on millions of "tracks" (the microvilli) designed to accommodate only one kind of train — and no other.
The system absorbs and ships out nutrients accounting for more than 90 percent of all the protein, fat, and carbohydrates that you consume, plus smaller percentages of vitamins and minerals. The train schedule looks something like this:
Level 1 Duodenum Iron, calcium, magnesium
Level 2 Jejunum Simple sugars (the end products of carbohydrate digestion) and water-soluble vitamins (vitamin C and the B vitamins, other than vitamin B12)
Level 3 Ileum Amino acids (the end product of protein digestion), fat-soluble vitamins
(vitamins A, D, E, and K), fatty acids (the end products of fat digestion), cholesterol, vitamin B12, sodium, potassium, and alcohol
When the bacteria have finished, the feces — perhaps the small remains of yesterday's copious feast — pass down through your rectum and out through your anus. But not necessarily right away: Digestion of any one meal may take longer than a day to complete.
After that, digestion's done!
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