Physiological Pharmaceutics

78 Physiological PharmaceuticsFigure 5.3 Structure of a gastric pitlayer over the lining epithelium of the stomach cavity. This protective layer is approximately140 µm thick in humans and is a defence mechanism to prevent the stomach from beingdigested by its own proteolytic enzymes (Figure 5.4). The barrier is enhanced by thesecretion of bicarbonate into the surface layer from the underlying mucosa. As the hydrogenions diffuse across the mucus layer from the lumen, they meet bicarbonate secreted from theunderlying mucosa, thus setting up a pH gradient. The mucus is continually digested fromthe luminal surface and is continually being replaced from beneath. It has been estimatedthat the turnover time of the mucus layer, i.e. from production to digestion, is in the orderof 4 to 5 hours. However, it may be slower since any interaction with the mucosa causesit to secrete copious amounts of mucus, making accurate measurements problematic.Figure 5.4 Mucus degradation in the stomach