Mixing and Emptying Actions

Food entering the stomach stretches the smooth muscles in its wall. The stomach may enlarge, but its muscles maintain their tone, and internal pressure of the stomach normally is unchanged. A person may eat more than the stomach can comfortably hold, and when this happens, the internal pressure may rise enough to stimulate pain receptors. The result is a stomachache. Clinical Application 17.2 discusses this common problem along with its associated indigestion.

Following a meal, the mixing movements of the stomach wall aid in producing a semifluid paste of food particles and gastric juice called chyme (kim). Peristaltic waves push the chyme toward the pyloric region of the stomach, and as chyme accumulates near the pyloric sphincter, this muscle begins to relax. Stomach contractions push chyme a little (5-15 milliliters) at a time into the small intestine. These contraction waves push most of the chyme backward into the stomach, mixing it further. The lower esophageal sphincter prevents reflux of stomach contents into the esophagus. Figure 17.22 illustrates this process.

(a) Stomach

(a) Stomach

Movements Stomach

Stomach movements. (a) As the stomach fills, its muscular wall stretches, but the pyloric sphincter remains closed. (b) Mixing movements combine food and gastric juice, creating chyme. (c) Peristaltic waves move the chyme toward the pyloric sphincter, which relaxes and admits some chyme into the duodenum.

Figure

Stomach movements. (a) As the stomach fills, its muscular wall stretches, but the pyloric sphincter remains closed. (b) Mixing movements combine food and gastric juice, creating chyme. (c) Peristaltic waves move the chyme toward the pyloric sphincter, which relaxes and admits some chyme into the duodenum.

The rate at which the stomach empties depends on the fluidity of the chyme and the type of food. Liquids usually pass through the stomach quite rapidly, but solids remain until they are well mixed with gastric juice. Fatty foods may remain in the stomach three to six hours; foods high in proteins move through more quickly; carbohydrates usually pass through more rapidly than either fats or proteins.

As chyme fills the duodenum, its internal pressure increases, and the intestinal wall stretches. These actions stimulate sensory receptors in the wall, triggering an en-terogastric reflex. The name of this reflex, like those of other digestive reflexes, describes the origin and termination of reflex impulses. Thus, the enterogastric reflex begins in the small intestine (entero) and ends in the stomach (gastro).

As a result of the enterogastric reflex, fewer parasympathetic impulses arrive at the stomach, inhibiting peristalsis, and the intestine fills less rapidly (fig. 17.23). Also, if chyme entering the intestine is fatty, the intestinal wall releases the hormone cholecystokinin, which further inhibits peristalsis.

As chyme enters the duodenum (the first portion of the small intestine), accessory organs add secretions. These organs include the pancreas, the liver, and the gallbladder.

Vomiting results from a complex reflex that empties the stomach another way. Irritation or distension in the

Vomition Reflex

stimulated

Figure

The enterogastric reflex partially regulates the rate at which chyme leaves the stomach.

stimulated

Figure

The enterogastric reflex partially regulates the rate at which chyme leaves the stomach.

stomach or intestines can trigger vomiting. Sensory impulses travel from the site of stimulation to the vomiting center in the medulla oblongata, and motor responses follow. These include taking a deep breath, raising the soft palate and thus closing the nasal cavity, closing the opening to the trachea (glottis), relaxing the circular muscle fibers at the base of the esophagus, contracting the diaphragm so it presses downward over the stomach, and contracting the abdominal wall muscles to increase pressure inside the abdominal cavity. As a result, the stomach is squeezed from all sides, forcing its contents upward and out through the esophagus, pharynx, and mouth.

Activity in the vomiting center can be stimulated by drugs (emetics), by toxins in contaminated foods, and sometimes by rapid changes in body motion. In this last situation, sensory impulses from the labyrinths of the inner ears reach the vomiting center and can produce motion sickness. The vomiting center can also be activated by stimulation of higher brain centers through sights, sounds, odors, tastes, emotions, or mechanical stimulation of the back of the pharynx.

Nausea emanates from activity in the vomiting center or in nerve centers near it. During nausea, stomach movements usually are diminished or absent, and duodenal contents may move back into the stomach.

99 How is chyme produced?

^9 What factors influence how quickly chyme leaves the stomach?

^9 Describe the enterogastric reflex. □ Describe the vomiting reflex. Q What factors may stimulate the vomiting reflex?

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Responses

  • ryan
    What factors may atimulate the vomiting reflex?
    6 years ago
  • Elisabeth
    What factors may stimulate the vomiting reflex?
    6 years ago
  • daniel
    What factors influence how quickly chyme leaves the stomach?
    6 years ago
  • EDITH
    What influences the rate of chyme moving through the stomach?
    6 years ago

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