After going through the lower esophageal sphincter, food enters the stomach. The stomach is involved in both chemical and mechanical digestion. Mechanical digestion occurs as the stomach churns and grinds food into a semisolid substance called chyme (partially digested food).

The lining of the stomach is made up of different layers of tissue. The mucosa is the outermost layer (closest to stomach cavity) as shown in the figure below.

The stomach has four major regions: the cardia, fundus, body, and pylorus. The addition of an inner oblique smooth muscle layer gives the muscularis the ability to vigorously churn and mix food.

The mucosa is not a flat surface. Instead, its surface is lined by gastric pits. Gastric pits are indentations in the stomach’s surface that are lined by four different types of cells.

The following video is a nice introduction to gastric pits and talks about chief and parietal cells.

The gastric juice in the stomach chiefly breaks down down protein structure; however, it also affects other macronutrients. As the stomach churns, it mixes gastric juices and foodstuff. The resulting “sludge” called chyme is passed through the pyloric sphincter to the small intestine for nutrient absorption.