Illustration of a section of the stomach wall. The surface epithelial cells (pink, top) produce an alkaline mucous that protects the surface of the stomach from its acid contents. The stomach surface is lined with glandular mucosa that has numerous invaginations, called gastric pits, where the gastric glands (ovals) open to the lumen. The gastric glands secrete enzymes and acid to digest the food. Beneath this is the submucosa, which contains blood vessels (red and blue) and lymph vessels (green). At the bottom are layers of smooth muscle.