The Esophagus From the throat, the bolus passes through a tube called the esophagus into the stomach. You might think that gravity draws food down through the esophagus, but this is not correct. In fact, you can swallow quite well in zero gravity, as astronauts do, or even while standing on your head. The reason is that contractions of smooth muscles, known as peristalsis (pehr uh STAL sis), provide the force that moves food through the esophagus toward the stomach. Peristalsis in the esophagus is shown in Figure 30–12.

After food passes into the stomach, a thick ring of muscle called the cardiac sphincter closes the esophagus. This prevents the contents of the stomach from flowing back. Overeating or drinking excess caffeine can cause a backflow of stomach acid into the esophagus. The result is a burning sensation in the center of the chest known as heartburn. Despite its name, heartburn has nothing to do with the heart. Nonetheless, persistent heartburn can cause serious damage to the esophagus and is a reason to visit a doctor.

FIGURE 30–12 Peristalsis Muscles in the walls of the esophagus contract in waves. Each wave pushes the chewed clump of food, or bolus, in front of it. Eventually, the bolus is pushed into the stomach.

dd

Chemical Digestion in the Stomach The stomach is a large muscular sac that continues the chemical and mechanical digestion of food. The lining of the stomach contains millions of microscopic gastric glands that release many substances into the stomach. Some of these glands produce hydrochloric acid. Other glands release an enzyme called pepsin that is activated in and functions best in acidic conditions. Pepsin breaks proteins into smaller polypeptide fragments.

Another stomach gland produces mucus, a fluid that lubricates and protects the stomach wall. If this protective layer fails, acids may erode the stomach lining and cause a sore called a peptic ulcer. For years, physicians thought that the primary cause of ulcers was too much stomach acid. They prescribed drugs that reduced symptoms but did not cure ulcers. Scientists have since discovered that most peptic ulcers are the result of infection with the bacterium Helicobacter pylori. Most peptic ulcers can now be cured with antibiotics that kill the bacteria.

Mechanical Digestion in the Stomach Alternating contractions of the stomach's three smooth muscle layers thoroughly churn and mix the swallowed food. The churning causes further breakdown of the chunks of swallowed food and allows enzymes greater access to the food. Gradually, a mixture with an oatmeal-like consistency called chyme (KYM) is produced. After an hour or two, the pyloric valve, which is located between the stomach and small intestine, opens, and chyme begins to spurt into the small intestine.

FIGURE 30–13 Helicobacter pylori After many years of blaming lifestyle factors for ulcers, researchers discovered that these bacteria are the cause. H. pylori burrow into the stomach wall and cause inflammation (SEM 6800X).

Table of Contents