Proteins need pepsin in order to begin digestion, and that enzyme is only produced in the stomach — not the mouth.

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.

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.

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.

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. 

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.

The portal vein carries blood from the small intestine to the liver. It has an extremely high nutrient content because it is fed with blood by the superior and inferior mesenteric veins along the small intestine. The small intestine absorbs most of the nutrients from the food that has been ingested into these vessels that take the blood directly to the liver to remove toxins prior to being distributed to the rest of the body. Essentially all nutrients that enter the body must first pass through the portal vein and liver before being distributed to other tissues.

The liver has a variety of functions including carbohydrate metabolism and storage, as well as blood detoxification. Most hepatocytes have a developed smooth endoplasmic reticulum to facilitate detoxification or wastes absorbed from the blood. Glucose is also taken from the blood and dimerized to create glycogen. Digested proteins are also carried to the protein for metabolism. Once fully dissociated, the amino acids of digested proteins can be used to build new molecules.

Although the liver produces bile, it does not store it. Bile storage is the function of the gall bladder.