Unless your weekend hobby is doing improv, you probably feel uneasy discussing your digestive system in polite company. We all love to eat, of course, but we don't care to think about what happens to that chicken chalupa once it disappears down the hatch. We impose a sort of "don't ask, don't tell" policy on our own digestive systems. This strategy works well enough, until your gallbladder refuses to stay mum or your gut decides to show a little insubordination. Then, suddenly, you want to know what's going on down there in the dark.
Disgestive System Anatomy
Digestion is the physiological process of turning food into energy. The digestive system is also called the gastrointestinal (GI) system (gastro- comes from the Greek word for stomach). It consists of the digestive tract and its accessory organs:
Digestive Tract
The digestive tract, or GI tract, is the path through which food travels as it's transformed into fuel for the body. It begins at the lips, includes the oral cavity, throat, esophagus, stomach, small intestine, and large intestine, and ends at the anal canal.
Accessory Organs
Accessory organs are not part of the digestive tract itself, but they facilitate the process of digestion. These organs include the tongue, teeth, salivary glands, liver, gallbladder, and pancreas. The appendix is not exactly an accessory organ, but it likely played a role in digestion of food sometime in the past. It is now vestigial, meaning it has lost that original function.
How It Works
A diagram of the GI tract resembles a Rube Goldberg–like mouse trap: a complicated apparatus that performs a deceptively simple function. Indeed, the digestive system is a finely tuned network that interacts in complex ways with other body systems to perform the straightforward task of turning food into fuel.
That chicken chalupa, for instance, can't move directly from your stomach into the body's cells. The food particles must first be broken down to extract their proteins, fats, vitamins, and other nutrients — the fuel your cells need to carry out their normal functions.
The GI tract carries out these essential duties related to digestion:
Motility
How is a meal conveyed from your mouth to your stomach and then through your intestinal tract? It must first descend through the esophagus, the hollow muscular tube that begins at the back of the throat and terminates at the top of the stomach. It does so with the help of gravity and voluntary muscles contractions controlled by swallowing. In the lower half of the esophagus and in the stomach and intestinal tract, food is propelled by slow, involuntary smooth muscle contractions called peristalsis.
Alternating contractions of rings of longitudinal muscle, called segmental contractions, knead and mash the food and churn it with digestive juices. Together, peristalsis and segmental contractions make up the mechanism known as motility (from the Latin word motus, meaning motion). Two special bands of muscles along the way, called the upper esophageal sphincter (just below the junction of the throat and the esophagus) and the lower esophageal sphincter (just above the junction of the esophagus and stomach), contract when the esophagus isn't in use so that food and stomach acid do not flow back up into the mouth.
Secretion
The digestive system must call on accessory organs such as the liver to break down the food as it moves through the GI tract. These organs secrete enzymes, acids, and other substances that modify the chemical composition of food so that nutrients can be absorbed. The salivary glands jump-start the process by secreting fluid and mucus to lubricate and bind food as you chew. They also secrete an enzyme that begins to dissolve starches. Then other accessory organs report for duty. The liver and gallbladder, for example, work in tandem to secrete, store, and release bile, which helps the body absorb fats. The pancreas secretes several essential digestive enzymes and bicarbonate, a substance that neutralizes acidic stomach juices.
Absorption
Food is reduced primarily to glucose molecules. The end products of digestion also include amino acids (proteins) and fats. Absorption is the passage of these molecules across the mucous membranes of the cells that line the GI tract. Most absorption occurs within the compactly arranged coil of the small intestine. Its lining looks something like a corrugated cardboard tube, with ridges that increase the surface area for absorption. These ridges are lined with furrowed cells specially designed for the task. The glucose molecules then pass into the circulatory system, which transports and distributes them throughout the body.
Elimination
Well, now we've reached the messy part. Like any other bodily process, digestion produces waste, such as insoluble fiber, sloughed-off cells, and digestive secretions. These waste products form feces, or stool. It consolidates in the colon (part of the large intestine), where water is removed so that the mass of stool that can be easily expelled from the body through the anus. Too little water causes constipation, and too much means diarrhea. The stool is stored in the rectum until voluntary defecation (fecal evacuation) occurs. This mechanism is controlled by a set of drawstring-like sphincter muscles regulated by both voluntary and involuntary muscle contractions.
