Chapter 2
IN THIS CHAPTER
Describing the two ways you process food
Extracting nutrients for your body from what you eat and drink
When you see (or smell) something appetizing, your digestive organs leap into action. Your mouth waters. Your stomach contracts. Intestinal glands begin to secrete the chemicals that turn food into the nutrients that build new tissues and provide the energy you need for work, pleasure, and everyday life. This chapter provides a basic primer on the digestive system from start to finish with a few stops along the way to explain how you metabolize everything from apples to zucchini.
Your digestive system is a collection of organs specifically designed to turn complex substances (food) into basic components (nutrients). These organs form one long, exceedingly well-organized tube that starts at your mouth, continues down through your throat to your stomach, and then goes on to your small and large intestines to end at your anus.
In between, with the help of the liver, pancreas, and gallbladder, the usable (digestible) parts of everything that you eat are converted to simple compounds that your body can easily absorb to burn for energy or to build new tissue. The indigestible residue is bundled off and eliminated as waste.
Figure 2-1 shows the body parts and organs that comprise your digestive system.
The digestive process run by these organs works in two simple ways, one mechanical and one chemical.
The rest of this chapter explains exactly what occurs and where along the digestive tract.
Each organ in the digestive system plays a specific role in the digestive drama. But the first act occurs in three places rarely listed as part of the digestive tract: your brain, your eyes, and your nose.
The next acts take place in your mouth, your stomach, and your small and large intestines.
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, your brain sends encouraging messages to your mouth and digestive tract as the sight and scent of food make your mouth water and your stomach contract in anticipatory hunger pangs.
What if you hate what you see or smell? For some people, even the thought of liver is enough to make them want to leave the room. At that point, your body takes up arms to protect you: You experience a rejection reaction. 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 as muscles contract, not in anticipatory pangs but in movements preparatory for vomiting up the unwanted food. Not a pleasant moment.
But assume that 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 and breaking food 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, and although saliva does contain very small amounts of lingual lipases — fat-busting enzymes secreted by cells at the base of the tongue — the amount is so small that the fat digestion here 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 pouchlike structure just below your esophagus.
Like most of the digestive tube, your stomach is circled with strong muscles whose rhythmic peristaltic contractions 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. Other enzymes, plus stomach juices, begin the digestion of proteins and fats, separating them into their basic components, amino acids and fatty acids.
If the words amino acids and fatty acids are completely new to you and if you’re suddenly consumed by the desire to know more about them this instant, stick a pencil in the book to hold your place and flip to Chapters 6 and 7 for the details.
For the most part, digestion of carbohydrates comes to a temporary halt in the stomach. Stomach acids can break some carb bonds, but overall the liquids here are so acidic that they deactivate amylases, the enzymes that break complex carbohydrates apart into simple sugars. 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.
Open your hand and put it flat against your belly button, with your thumb pointing up to your waist and your little finger pointing down.
Your hand is now covering most of the relatively small space into which your 20-foot-long small intestine is neatly coiled. When the partially digested chyme spills from your stomach into this part of the digestive tube, a whole new set of gastric juices are released:
While these chemicals work, contractions of the small intestine continue to move the food mass down through the tube so your body can absorb sugars, amino acids, fatty acids, vitamins, and minerals into cells in the intestinal wall.
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, as follows:
After you’ve digested your food and absorbed its nutrients through your small intestine, this is what happens:
Inside the cells, nutrients are metabolized — 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; for fats, Chapter 7; for carbohydrates, Chapter 8; for alcohol, Chapter 9; for vitamins, Chapter 10; for minerals, Chapter 11; and for water, Chapter 12.
When 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 the bodies of bacteria, members of the microbiome (see the nearby sidebar “Probiotics: From yogurt to … yogurt”). In fact, about 30 percent of the entire weight of the feces comprises the bodies of these microorganisms, which live in permanent colonies in your colon, where they
When the bacteria have finished and their bodies have been incorporated into the waste, the feces — 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.