Pepsinogen is an inactive enzyme that is released into the stomach lumen by chief cells. Parietal cells are responsible for secreting hydrochloric acid. This acid will cleave the pepsinogen and make it an active enzyme, pepsin, which can then cleave peptide bonds and begin protein digestion.

Goblet cells secrete mucus to protect the epithelium of the stomach from the acid in the lumen. G cells secrete gastrin, which promotes the secretion of hydrochloric acid and pepsinogen.

Goblet cells secrete mucous, which protects the stomach lining. Chief cells secrete pepsinogen, which must be converted to pepsin by acid in order to carry out its function of breaking down proteins. G-cells secrete gastrin, increase secretion from the other cells, and induce muscle contractions. Parietal cells secrete both and intrinsic factor.

Hydrochloric acid does not break peptide bonds. Instead, it is responsible for breaking down the quaternary and tertiary structure of the protein, leaving the peptide bonds more accessible for pepsin to break. Pepsin is responsible for breaking the bonds between the amino acids, which is known as proteolysis. Lastly, the acidic environment of the stomach kills many microbes, preventing infection.

Since the stomach releases ions into the stomach, it releases bicarbonate into its venous supply, which can feed into the pancreas. This allows the pancreas to release the bicarbonate into the lumen to help neutralize the stomach acid. Remember, the venous blood leaving the pancreas is acidic in nature and can help provide the stomach with a supply of ions for its functionality.

Stratified squamous epithelium protects tissues in areas that are prone to abrasion. This lining consists of many layers and is typically located on the areas near/associated with the mouth, excretory system, and the esophagus. The stomach does not have a stratified squamous epithelial lining, as it instead uses simple columnar epithelium.

The stomach stores ingested food, denatures proteins, initiates protein digestion, and mechanically breaks down food. The absorption of triglycerides, or lipids, takes place in the small intestine.

Liquids exit the stomach more quickly than solids, and isotonic solutions exits more quickly than hypertonic or hypotonic solutions. As such, isotonic liquids will exit the stomach most quickly during gastric emptying.

Solids exit the stomach in the general order of carbohydrates, followed by proteins, followed by fats.