Digestion, begun in the mouth, continues in the stomach. The stomach is one of the main sites of protein and lipid breakdown. The acidic environment of the stomach facilitates digestion. In addition, several enzymes have evolved to act at a very low pH of 1–3, that found in the stomach. Most enzymes, by contrast, are denatured (that is, unfolded and thus made inactive) in acidic environments.

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The stomach is maintained at a low pH through the secretion of hydrochloric acid (HCl) by specific cells lining the stomach. A low pH could damage the stomach when no food is present, so the amount of HCl is carefully regulated. The sight, smell, and taste of food send signals to the brain that stimulate a sense of appetite, and to the stomach that stimulate the secretion of HCl and digestive enzymes that break down proteins and lipids.

When food arrives in the stomach and digestion there begins, cells lining the stomach secrete a peptide hormone called gastrin. Gastrin stimulates the cells lining the stomach to increase their production of HCl still further. If the pH of the stomach becomes too low, gastrin secretion is inhibited, an example of a negative feedback loop (Chapter 38).

The stomach has several mechanisms to protect itself from the acidic environment and presence of digestive enzymes. For example, glands in the lining of the stomach secrete mucus to protect the stomach wall. In addition, cells in the stomach secrete some digestive enzymes in an inactive form; otherwise, the cells themselves would be digested. After secretion, the inactive enzymes are activated by a change in their chemical structure. The primary digestive enzyme produced in the stomach is pepsin, an enzyme that breaks down proteins into amino acids (Table 40.3). Cells lining the stomach release pepsin in an inactive form called pepsinogen that is activated by the low pH of the stomach. The stomach also secretes lipases that break down lipids (Table 40.3). Like pepsin, gastric lipase works best in an acidic environment.

Animals often eat large amounts of food in a short time, but it takes much longer to digest that food. Therefore, the stomach serves as a storage compartment as well as a digestion compartment. In humans, it typically takes about four hours for the stomach to empty, allowing digestion and absorption of nutrients to occur between meals. In other animals, stomach emptying can take much longer. Most carnivorous animals consume large and infrequent meals that are followed by long periods of digestion and nutrient absorption compared with humans and other animals that eat more regularly. Snakes such as pythons that engulf large prey whole (Fig. 40.14) elevate their metabolic rate to high levels during digestion and spend several days digesting and absorbing the nutrients from their meal. To digest their single large meal, their gut undergoes extensive remodeling that produces new secretory and absorptive cell surfaces. These extra cells are not retained between meals because of the high energy cost of maintaining them.

The stomach walls contract to mix the contents of the stomach, aiding their digestion. Waves of muscular contraction also move the food toward the base of the stomach. There, the pyloric sphincter, a band of muscle, opens and allows small amounts of digested food to enter the small intestine. Opening and closing of the pyloric sphincter regulates the rate at which the stomach empties, allowing time for the food products released into the small intestine to be further digested and then absorbed.