Figure 50.12 Action in the Stomach (A) The human stomach stores ingested food and mixes it with digestive secretions. (B) Gastric pits are folds in the stomach lining. (C) Parietal cells in the gastric pits secrete hydrochloric acid, and chief cells secrete pepsinogen. HCl and autocatalysis convert inactive pepsinogen into active pepsin. (D) Parietal cells create a tremendous H+ concentration difference between the gut lumen and the extracellular fluid by actively transporting into the lumen of the gastric pit H+ ions produced through the catalytic action of carbonic anhydrase.