The J-shaped stomach is a pouchlike portion of the alimentary canal. It lies just inferior to the diaphragm in the left upper quadrant of the abdominopelvic cavity. The basic functions of the stomach are temporary storage of food, mixing food with gastric juice, and starting the chemical digestion of proteins.
The stomach may be subdivided into four regions: the cardia, fundus, body, and pyloric part. The cardia (closest to the heart) is a relatively small area that receives food from the esophagus. The fundus expands superior to the level of the cardia and serves as a temporary storage area. The body is the largest region of the stomach, and it is located between the fundus and pyloric part. The pyloric part is the narrow portion located near the junction with the duodenum.
The pyloric sphincter is a thickened ring of circular muscle cells that is located at the junction of the stomach and duodenum. This muscle usually is contracted, closing the stomach outlet, but it relaxes to let stomach contents pass into the small intestine.
The stomach possesses some specializations to accommodate its functions. The muscular layer contains a third layer of oblique muscle cells, which allows the stomach to better mix food with gastric secretions. The mucosa is quite thick, when compared to other organs of the alimentary canal. In an empty stomach, the mucosa and submucosa are organized into numerous folds called gastric rugae (ru-je). These folds allow the lining to stretch as the stomach fills with food. The mucosa is dotted with numerous pores called gastric pits. Gastric pits receive secretions from gastric glands that extend deep into the mucosa.
The secretion of the gastric glands is known as gastric juice. Mucous neck cells, located near the opening to the gastric pit, secrete mucus to coat and protect the mucosa from the action of digestive secretions. Chief cells, located in the deepest portions of the gastric glands, secrete the digestive enzymes pepsinogen (inactive form of pepsin), gastric lipase, and rennin. Parietal cells, located in the midportion of the gastric glands, secrete hydrochloric acid (HCl) and intrinsic factor.
As food is mixed with gastric juice and as chemical digestion occurs, it is converted into an acidic, semiliquid substance called chyme (k–m). Small amounts of chyme are released intermittently into the duodenum by the relaxing of the pyloric sphincter.
The rate of gastric secretion is controlled by both neural and hormonal means and is a good example of a positive- feedback mechanism. Gastric juice is produced continuously, but its secretion is greatly increased whenever food is on the way to, or already in, the stomach. The sight, smell, or thought of appetizing food, food in the mouth, or food in the stomach stimulates the transmission of parasympathetic nerve impulses that increase the secretion of gastric juice. These nerve impulses also, along with food in the stomach and stomach stretching, stimulate certain stomach cells to secrete a hormone called gastrin. Gastrin is absorbed into the blood and is carried to gastric glands, increasing their secretions.
As stomach contents are gradually emptied into the small intestine, there is a decrease in the frequency of parasympathetic nerve impulses and an increase in the frequency of sympathetic nerve impulses to the stomach, which reduces the secretion of gastric juice. When chyme passes from the stomach into the small intestine, it stimulates the intestinal mucosa to release two hormones: cholecystokinin (ko-le-sis-to-kin’-in) (CCK) and secretin (se ‘-kre ‘-tin), which reduce both the motility of the stomach and the secretion of gastric juice.
Food entering the stomach is thoroughly mixed with gastric juice by ripplelike, mixing contractions of the stomach wall. Gastric juice is very acidic (pH 2) due to an abundance of HCl. Pepsin is the most important digestive enzyme in gastric juice, and it is secreted in an inactive form that prevents digestion of the cells secreting it.
Once it is released into the stomach, pepsin is activated by the strong acidity of gastric juice. Pepsin acts on proteins and breaks these complex molecules into shorter amino acid chains called peptides. However, peptides are still much too large to be absorbed and require further digestion in the small intestine.
Gastric juice contains a substance known as intrinsic factor that is essential for the absorption of vitamin B12 by the small intestine.
The gastric juice of infants contains two unique enzymes that help to improve the digestion of milk proteins and lipids. Rennin (ren-in) curdles milk proteins, which keeps them in the stomach longer and makes them more easily digested by pepsin. Gastric lipase acts on triglycerides and breaks them into fatty acids and monoglycerides. Except for a few substances such as water, minerals, some drugs, and alcohol, little absorption occurs in the stomach.
When semiliquid chyme passes from the stomach into the duodenum, the first part of the small intestine, secretions from the pancreas and liver are emptied into the duodenum. The secretions from these accessory organs play important roles in digestion within the small intestine.