In lecture, we covered the physiology of Gastric Secretion and Peptic Ulcer Disease. Here is the upper gastrointestinal mucosa, which is covered by a layer of mucus (orange). The surface mucous cells (green) releases bicarbonate into the mucus which neutralizes incoming protons. The mucus is impermeable to the drops of pepsin (grey) standing on its head. The pepsin are sad because they cannot play with the surface epithelium, creating craters (ulcers) that they can fill with gastric acid or grow their flagellated pets, H. Pylori.