10.7: Stomach

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The stomach is like a large sac (Figure 10.1) whose walls can stretch to store large amounts of food. Food is normally prevented from moving back into the esophagus by proper functioning of the esophagus and proper pressures in the thoracic cavity. Contraction of a ring of muscle (the pyloric sphincter) at the lower end of the stomach temporarily prevents food from moving into the small intestine.

Secretion and Absorption

The inner lining of the stomach is a thick layer containing many secreting cells. Some of these cells secrete hydrochloric acid (HCl), and others secrete pepsin. When HCl and pepsin combine, they cause the rapid breakdown of large protein molecules, which are usually split into short chains of amino acids. HCl also kills bacteria and other microorganisms that have been swallowed. A third secretion from the stomach lining consists of intrinsic factor. Upon reaching the small intestine, intrinsic factor promotes the absorption of vitamin B12. This vitamin is important in the production of red blood cells. Finally, the stomach secretes a hormone that helps control hunger.

The lining of the stomach absorbs water and small molecules that have become dissolved (e.g., simple sugars, salts, alcohol, certain medications). These materials enter the blood.

Movements

The middle layer of the stomach wall contains sheets of smooth muscle. Rhythmic contractions of this muscle churn the food and stomach secretions. The churning thoroughly mixes all materials and aids absorption by bringing dissolved materials into contact with the stomach lining.

Once the stomach contents have been adequately liquefied, the muscular contractions of the stomach become strong peristaltic waves. At the same time the pyloric sphincter relaxes somewhat so that a portion of the stomach contents is pushed into the small intestine. The pyloric sphincter then closes, and stomach churning and absorption continue until the small intestine is ready to receive more material. The functioning of the smooth muscle and the pyloric sphincter is controlled by the autonomic nervous system and Auerbach's plexus.

Age Changes

Aging causes small changes in the structure and functioning of the stomach, including a slight thinning of the stomach lining, a small decrease in HCl production, a possible decline in pepsin and intrinsic factor secretion, and a minimal slowing of stomach emptying. These changes are usually so slight that they do not prevent the stomach from performing its routine functions. However, they can alter the absorption of some medications and the functioning of the small intestine.

Abnormal Changes

Though the normal stomach functions well regardless of age, several abnormal and disease conditions become more frequent and severe with age.

Atrophic Gastritis

Atrophic gastritis results in an excessive thinning of the stomach lining. The causes of many cases of this abnormality are unknown, but many other cases result from the immune system attacking the stomach.

Atrophic gastritis results in inadequate production of HCl and intrinsic factor. The consequences include poor protein digestion, alterations in the number and types of bacteria in the GI tract, and poor absorption of vitamin B12. Finally, atrophic gastritis is a risk factor for stomach cancer.

Poor protein digestion can lead to indigestion and malnutrition. The alterations in bacteria can also adversely affect nutrition. The reduction in vitamin B12 absorption leads to a significant reduction in red blood cell (RBC) production. The number of RBCs in the blood eventually becomes too low, and the person becomes anemic. Anemia caused by inadequate production of intrinsic factor is called pernicious anemia. The effects of pernicious anemia include sleepiness and persistent fatigue. These symptoms are not part of aging.

Atrophic gastritis can be treated with medications to relieve gastric discomfort and vitamin B12 supplements to prevent anemia.

Acute Gastritis

A second age-related stomach disorder is short-term stomach inflammation (acute gastritis). Reasons for the increased incidence of acute gastritis include reductions in the resistance of the stomach to environmental insults; increases in stomach infections due to lowered stomach acid production; and increases in the use of medications such as analgesics to relieve pain. Many analgesics (e.g., aspirin, steroids, nonsteroidal anti-inflammatory drugs) irritate the stomach.

The main problem arising from acute gastritis is the discomfort it causes. When attacks are frequent or severe, affected individuals may eat less, lose weight, and develop malnutrition.

Most cases of acute gastritis can be prevented by avoiding specific foods and medications. Taking antacids can relieve the symptoms in some situations.

Peptic Ulcer

In a peptic ulcer, stomach acid and enzymes cause cells lining the GI tract to die and peel away, leaving a pit in the wall of the tract (Figure 10.3).

The occurrence of peptic ulcers in the esophagus has been mentioned in connection with gastric refluxing. These ulcers also occur in the stomach (gastric peptic ulcers) and the beginning of the small intestine (duodenal peptic ulcers). Although duodenal peptic ulcers are more common than gastric ones among younger adults, gastric peptic ulcers become higher in frequency among the elderly.

Gastric peptic ulcers often result from weakening of the stomach lining. Common causes include bacteria (H.pylori) or having unusually high levels of anti-inflammatory steroids in the blood. The elderly are more likely to have such elevated steroid levels since many medications used to relieve pain and inflammation contain them. Even nonsteroid pain relievers such as aspirin can increase the risk of gastric peptic ulcers.

A gastric peptic ulcer causes considerable pain. Usually the pain becomes worse shortly after eating because more stomach acid is produced then. Although the pain is generally of lower intensity at advanced age, other complications are usually more serious. If scar tissue forms, it can shrink and narrow the stomach, leading to partial obstruction. Peptic ulcers that bleed slowly lead to anemia, while those that bleed profusely can cause sudden death. When an ulcer becomes very deep, the stomach may perforate, allowing its contents to leak into the abdominal cavity. The consequences may include severe pain, bleeding, digestion of neighboring organs, a severe drop in blood pressure, and death.

The incidence and severity of gastric peptic ulcers can be reduced by avoiding risk factors such as ulcer-promoting medications. Treatments include antibiotics or avoiding foods and medications that promote stomach irritation and ulcer formation. Conversely, medications such as antacids can promote healing or retard worsening of the ulcer. Some ulcers require surgery.