Stomach – Tutorial
Please read Unit 10 – Introduction to Digestive System Tissues and study the image slider of the four major layers of the digestive tract wall prior to completing the activities in this chapter.
Introduction to the Stomach
The stomach is a highly expandable saclike organ shaped like the letter j in the left upper quadrant of the abdominal cavity just inferior to the diaphragm and the left lobe of the liver. The stomach begins at the cardiac sphincter (lower esophageal sphincter) where the esophagus passes through the diaphragm and ends at the pyloric sphincter where the contents of the stomach enter the duodenum of the small intestine. The wall of the stomach is able to greatly stretch to accommodate a large meal eaten in a relatively short period of time, so the size and shape of the stomach vary greatly depending on the volume of its contents. When the stomach is empty, the mucosa of the stomach wall exhibits distinct folds known as rugae. As the stomach fills with food, the rugae stretch and flatten allowing the lumen of the stomach to greatly expand.
When a meal enters the lumen of the stomach it is temporarily stored and mixed with a variety of secretions from the stomach wall to create a thick, soupy, acidic mixture of partially digested food known as chyme. While chyme is held in the stomach, the acidic pH destroys most microbes that may have entered the stomach along with food. At the same time, three layers of smooth muscle (oblique, circular, and longitudinal) in the wall of the stomach produce powerful peristaltic contractions known as mixing waves that work to mechanically break apart solid foods. After a period of time, small amounts of chyme are allowed to exit the stomach through the pyloric sphincter into the small intestine where the majority of chemical digestion and absorption occur.
The lumen of the stomach is lined with simple columnar epithelium with numerous infoldings that penetrate the mucosa known as gastric pits. The simple columnar cells lining the surface (in contact with the lumen) and the cells lining the superficial portions of the gastric pits (close to the surface) produce massive amounts of alkaline mucous creating a blanket of mucus that protects the endothelium from acidic chyme in the lumen of the stomach.
As gastric pits penetrate deeper, they eventually split into two or more branches known as gastric glands that extend even deeper into the mucosa. The cells along the more superficial portion of the gastric glands are dominated by parietal cells that secrete hydrochloric acid (HCl) and intrinsic factor which aids the absorption of vitamin B12 in the small intestine. The cells lining the deepest portions of the gastric glands are primarily chief cells that secrete a substance known as pepsinogen. Pepsinogen is a proenzyme that is converted by HCl into an active enzyme known as pepsin. Pepsin starts the chemical digestion of proteins in the highly acidic environment of the stomach.
Tutorial: Use the image slider below to learn more about the structure and characteristics of the stomach.
Microscopy: Use the image slider below to learn how to use a microscope to study the stomach on a microscope slide.
Tutorial: Use the hotspot image below to learn more about the structure of the wall of the stomach on a low power image.
Tutorial: Use the hotspot image below to learn more about the structure and function of the stomach.
