Stomach

Below the esophagus, the stomach works to digest proteins in the food (Figure 6.1). The connection between the stomach and the esophagus is called the cardiac sphincter. The stomach is an organ that can be described as a blender made of smooth muscle, which turns the bolus of food into a paste called chyme. The muscularis consists of three distinct layers. The positioning of these layers allows the stomach to constrict in all directions. The first layer is arranged in a circular pattern, making it look similar to a donut. The second layer lies flat on top, in a longitudinal pattern. The third layer lies diagonally to the other two.

The stomach is a section of the digestive tube that is "J" shaped. The largest part of the stomach, the body, is where most of the stomach's digestive activity occurs. The portion of the stomach above the body but below the connection to the esophagus is called

Diaphragm

Diaphragm

Rugae Stomach

Pyloric sphincter Gastric rugae

Fundus

Cardiac sphincter Serosa

Longitudinal muscle

Circular muscle

Oblique muscle

Pyloric sphincter Gastric rugae

Figure 6.1 The stomach is below the diaphragm, with a connection to the esophagus called the cardiac sphincter and a connection to the duodenum of the small intestine called the pyloric sphincter. Three layers of smooth muscle make up most of the wall of the stomach. Folds of the mucosa called rugae increase the surface area of the organ.

the fundus. When food enters the stomach, some stays in the fundus while the rest of the food is mixed with stomach fluids in the body. While the food is in the fundus, salivary amylase continues to break up starch. As the food in the body of the stomach leaves to enter the small intestine, more food is brought from the fundus to continue the digestive process. In this way, the fundus acts like a storeroom for excess food until there is space in the body of the stomach. An empty stomach is about the size of a fist, while a full one is considerably larger, especially after a large meal. When the stomach is empty, large folds called rugae are created in the mucosa that can be seen without magnification. The folds of the rugae increase the surface area of the inside of the stomach.

The mucosa of the stomach has several adaptations not found anywhere else in the digestive tract. The epithelial cells extend into the underlying layers of the mucosa to form depressions called gastric pits (Figure 6.2). These pits are lined with a mixture of columnar epithelial cells and special cells that secrete chemicals to aid in digestion. Goblet cells secrete mucus to protect the stomach lining from other secretions, especially the hydrochloric acid secreted by another type of cell, the parietal cells. The acid aids digestion by indiscriminately breaking up larger compounds into smaller pieces. The acid digests everything, including bacteria and medications. Parietal cells also secrete a chemical called intrinsic factor that is necessary for the absorption of vitamin B12 in the small intestine. If intrinsic factor is not available, the vitamin will not be absorbed and a syndrome called pernicious anemia will result.

A third type of cell found in the epithelia of the gastric pits is the chief cell. This cell secretes a chemical called pepsinogen. When pepsinogen comes in contact with hydrochloric acid, it changes into an active enzyme called pepsin. Pepsin begins the digestion of proteins by breaking large, complex proteins into smaller pieces that will be further broken apart in the small intestine. If hydrochloric acid is not present, pepsin will not be formed from pepsinogen and the digestion of protein does not begin.

The fourth specialized epithelial cell, called the G cell, secretes a hormone called gastrin that is primarily responsible for stimulating the other three types of cells. Stomach fluids are produced when the G cells are active.

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