Gulp: Adventures on the Alimentary Canal

by Mary Roach

Six to eight times a day, unbeknownst to your thinking, feeling self, a peristaltic muscle contraction called a mass movement squeezes the contents of the colon farther along. Eating reliably triggers this, via something called the gastrocolic reflex. The bigger the meal, the more vigorous the push. Any older detritus that had been parked outside the rectum now gets loaded inside. In with the new, out with the old. “It’s a defensive reflex,” explains William Whitehead,† co-director of the Center for Functional Gastrointestinal and Motility Disorders at the University of North Carolina. It prevents the colon from bursting. When a load pushes against the rectum walls with sufficient pressure—as measured by stretch receptors—the defecation reflex is triggered. (You can trigger it prematurely by bearing down; this raises the pressure on the rectum walls to the requisite level.) The defecation reflex causes the rectal wall muscles to contract—that is, squeeze—at the same time the anal sphincter muscles relax. To the conscious mind this registers as urgency—somewhere between “Hello” and “Drop what you’re doing.” The larger or more liquid the load, the more pressing the urge and the tougher it is to hold back. Water will leak out a very small opening. As one gut expert put it, “Not even the sphincter of Hercules can hold back water.” Take this to its end point and you have the simple saline enema—and an urgency that is not easily, if at all, overridden.