The stomach (Fig. 15.4) is a thick-walled, J-shaped organ that lies on the left side of the abdominal cavity deep to the liver and diaphragm. The stomach is continuous with the esophagus above and the duodenum of the small intestine below. The length of the stomach remains at about 25 cm (10 in.) regardless of the amount of food it holds, but the diameter varies, depending on how full it is. As the stomach expands, deep folds in its wall, called rugae, gradually disappear. When full, it can hold about 4 liters (1 gallon). The stomach receives food from the esophagus, stores food, mixes food with its juices (thereby starting the digestion of proteins), and moves food into the small intestine.
Regions of the Stomach
The stomach has four regions. The cardiac region, which is near the heart, surrounds the lower esophageal sphincter where food enters the stomach. The fundic region, which holds food temporarily, is an expanded portion superior to the cardiac region. The body region, which comes next, is the main part. The pyloric region narrows to become the pyloric canal leading to the pyloric sphincter through which food enters the duodenum, the first part of the small intestine.
Digestive Functions of the Stomach
The stomach both physically and chemically acts on food. Its wall contains three muscle layers: One layer is longitudinal, another is circular, and the third is obliquely arranged. This muscular wall not only moves the food along, but it also churns, mixing the food with gastric juice and breaking it down to small pieces. The term gastric always refers to the stomach. The columnar epithelial lining of the stomach has millions of gastric pits, which lead into gastric glands (Fig. 15.4b,c). The gastric glands produce gastric juice, which contains pepsinogen, HCl, and mucus. Chief cells secrete pepsinogen, which becomes the enzyme pepsin when exposed to hydrochloric acid (HCl) released by parietal cells. The HCl causes the stomach to have a high acidity with a pH of about 2, and this is beneficial because it kills most of the bacteria present in food. Although HCl does not digest food, it does break down the connective tissue of meat and activate pepsin. The wall of the stomach is protected by the thick layer of mucus secreted by the mucous cells. If, by chance, HCl penetrates this mucus, the wall can begin to break down, and an ulcer results. An ulcer is an open sore in the wall caused by the gradual disintegration of tissue. It now appears that most ulcers are due to a bacterial infection (Helicobacter pylori) that impairs the ability of mucous cells to produce protective mucus. Alcohol is absorbed in the stomach, but food substances are not. Normally, the stomach empties in about 2-6 hours. When food leaves the stomach, it is a thick, soupy liquid called chyme. Chyme enters the small intestine in squirts by way of the pyloric sphincter, which acts like a valve, repeatedly opening and closing.
Figure 15.4 Anatomy and histology of the stomach. a. The stomach has a thick wall with deep folds that allow it to expand and fill with food. b. The lining of the stomach has gastric glands, which secrete mucus and a gastric juice active in protein digestion.
The abdominal wall and the organs of the abdomen are covered by peritoneum, a serous membrane (Fig. 15.5). The portion of the peritoneum that lines the wall is called the parietal peritoneum. The portion that covers the organs is called the visceral peritoneum. In between the organs, the visceral peritoneum is a double-layered mesentery that supports the visceral organs, including the blood vessels, nerves, and lymphatic vessels Some portions of the mesentery have specific names. The lesser omentum is mesentery that runs between the stomach and the liver. The greater omentum is indeed “greater.” It hangs down in front of the intestines like a large, doublelayered apron. The greater omentum has several functions: It contains fat that cushions and insulates the abdominal cavity; it contains macrophages that can take up and rid the body of pathogens; and it can wall off portions of the alimentary wall that may be infected, keeping the infection from spreading to other parts of the so-called peritoneal cavity.
Figure 15.5 Mesentery formed by two layers of the peritoneal membrane supports the abdominal viscera. Deep folds of the peritoneal membrane, called the greater omentum, cover these organs anteriorly.