6: Digestive Tract
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I think that I shall never see
A tract more alimentary.
A tube whose velvet villi sway
Absorbing food along the way.
Whose surface folded and striate
Does rapidly regenerate.
A magic carpet whose fuzzy nap
Minuscule molecules entrap.
Then, microvilli with enzymes replete
The last hydrolyses complete.
Small intestine described in The Superbowel, by George Fruhman, anatomy professor
In the simplest sense, the digestive tract is a long tube down the center of the body (Figure 6.1). A child swallowing a marble demonstrates that this tube is continuous with the outside of the body—the marble goes in one end and out the other
A more dramatic example is the use of the digestive tract to smuggle drugs packed in latex condoms or plastic bags. Diarrhea en route is a problem! One smuggler with really bad luck was on the Avianca flight from Colombia that crashed in New York in 1990. His contraband was found during abdominal surgery to treat his injuries. Many such smugglers have died of drug overdose when the condoms or bags broke or leaked—or when they made the mistake of using “natural” condoms made from digestible animal tissue.
Once something is swallowed, involuntary muscles move it downward. From the esophagus to the anus, circular muscles squeeze, and longitudinal muscles contract lengthwise. Coordinated muscle actions create “waves” of motion (peristalsis) that move the contents downward. Squeezing motions mix the food with digestive enzymes, and aid absorption by increasing contact with the absorptive lining of the digestive tract.
Digestion changes nutrients into a form that can be absorbed. Vitamins, minerals, cholesterol, and alcohol don’t need digestion— they’re absorbed “as is.” Starch, double sugars, protein, and triglycerides must first be digested (broken down) by digestive enzymes into simpler components:
- Starch and double sugars are broken into single sugars.
- Proteins are broken down to amino acids.
- Triglycerides have at least two of their fatty acids removed.
Like other enzymes, each digestive enzyme works best within a narrow range of temperature and acidity, and each is usually named after the molecule it acts on, ending in -ase: lactase acts on lactose, sucrase on sucrose, etc. Digestive actions (chewing, liquefying, mixing) increase the contact between digestive enzymes and the molecules they break apart.
