Mass peristalsis occurs in ------------- why?
B) small intestine
C) large intestine
Peristalsis, involuntary movements of the longitudinal and circular muscles, primarily in the digestive tract but occasionally in other hollow tubes of the body, that occur in progressive wavelike contractions. Peristaltic waves occur in the esophagus, stomach, and intestines. The waves can be short, local reflexes or long, continuous contractions that travel the whole length of the organ, depending upon their location and what initiates their action.
In the esophagus, peristaltic waves begin at the upper portion of the tube and travel the whole length, pushing food ahead of the wave into the stomach. Particles of food left behind in the esophagus initiate secondary peristaltic waves that remove leftover substances. One wave travels the full length of the tube in about nine seconds. The peristaltic-wave contractions in the esophagus of humans are weak compared with those of most other mammals. In cud-chewing animals, such as cows, reverse peristalsis can occur so that the food is brought back from the stomach to the mouthfor rechewing.
When the stomach is filled, peristaltic waves are diminished. The presence of fat in a meal can completely stop these movements for a short period until it is diluted with gastric juices or removed from the stomach. Peristaltic waves start as weak contractions at the beginning of the stomach and progressively become stronger as they near the distal stomach regions. The waves help to mix the stomach contents and propel food to the small intestine. Usually, two to three waves are present at one time in different regions of the stomach, and about three waves occur each minute.
In the small intestine, local stimulation of the intestinal smooth muscle by the presence of food particles causes contractions that tend to travel from the stimulated point in both directions. Under normal circumstances, the progress of the contractions in an oral direction is quickly inhibited, while the contractions travelling away from the mouth tend to persist. If the intestine is paralyzed by applying such drugs as nicotine or cocaine to the intestinal wall, the contractions initiated by local stimulations travel equally well in both directions. Ordinarily, peristaltic waves appear in the small intestine at irregular intervals and travel for varying distances; some travel only a few inches, others a few feet. They serve to expose food to the intestinal wall for absorption and to move it forward.
In the large intestine (or colon), the peristaltic wave, or mass movement, is continuous and progressive; it advances steadily toward the anal end of the tract, pushing waste material in front of the wave. When these movements are vigorous enough to pass fecal masses into the rectum, they are followed by the desire to defecate. If feces are passed to the rectum and not evacuated from the body, they are returned to the last segment of the colon for longer storage by reverse peristaltic waves. Peristaltic waves are particularly important in helping to remove gas from the large intestine and in controlling bacterial growth by mechanically acting as a cleansing agent that dislodges and removes potential colonies of bacteria.