Your stomach is located at the end of your esophagus and is the terminus for swallowed food and drink. The stomach receives chewed food and continues to mechanically and chemically break it down into smaller pieces, creating more surface area for your small intestine to absorb nutrients.
Your stomach is an acidic environment with a low pH of between 1 and 3. Parietal cells in the wall of the stomach secrete hydrochloric acid (HCl). If your esophageal sphincter; basically the lid to your stomach; isn’t closed properly, HCl will creep into your esophagus resulting in heartburn. HCl has a few different jobs. One, is to kill bacteria or other potentially dangerous pathogens you may have unknowingly ingested with your food. Another is to convert pepsinogen into pepsin. Pepsinogen is released from chief cells in your stomach wall. HCl, chemically changes pepsinogen into pepsin and is essential because pepsin doesn’t function in an environment with a pH greater than 5. Pepsin begins protein digestion by breaking it down into peptide chains. Peptide chains are made up of amino acids. Your small intestine absorbs amino acids into your circulatory system for distribution to the rest of your body. It is important to note there is a layer of mucus protecting your stomach from being chemically broken down by pepsin and HCl.
Parietal cells in your stomach wall also secrete intrinsic factor; a substance whose only job – as far as scientists know – is to facilitate the absorption of vitamin B-12. Intrinsic factor cannot do its job in the acidic environment of your stomach; it works best in a pH of 7 – close to water – but is used later in your ileum to absorb vitamin B-12 into your circulatory system after bile from your gallbladder has neutralized the acidic chyme (what your partially digested food is called when it enters your small intestine). B-12 is vital for your red blood cells to carry oxygen. People who lack intrinsic factor, cannot absorb vitamin B-12 and suffer from pernicious anemia.
Chymosin, or rennin, is secreted by the chief cells in your stomach wall and is responsible for the breakdown of a specific peptide bond: phenylalanine and methionine, through a complicated chemical process that I won’t detail here. Interestingly, rennin is the active ingredient in rennet which is used the cheese production; compelling some vegetarians into eating cheese without rennet.
Gastric lipase (“lip” means fat and “ase” means breakdown) is secreted by the chief cells to begin fat digestion in your stomach by hydrolyzing (“hydro” means water and “lyzing” means breaking apart; so the breaking apart of a molecule using water) fat molecules into fatty acid chains. Further fat digestion happens in the small intestine with the addition of pancreatic lipase.
G-cells in the wall of your stomach secrete gastrin, a hormone responsible for stimulating the release of HCl from the parietal cells. Gastrin is a chemical messenger that travels in your bloodstream and is released when your stomach is distended from having recently eaten, or when directed to be released by your brain in response to the sight or smell of food. Gastrin stimulates the release of HCl and pepsinogen. It enhances the strength of your stomach contractions to aid in mechanical digestion and causes the pyloric sphincter to relax or contract, controlling movement of chyme that moves into duodenum, the first segment of the small intestine. Your duodenum can only process a certain amount of chyme at a time, so your pyloric sphincter opens and closes to allow small packets of chyme to enter at regular intervals.
In summary, your stomach breaks your food into smaller pieces and mixes it with all of the above secretions in mechanical digestion. Parietal cells secrete hydrochloric acid and intrinsic factor. Chief cells secrete pepsinogen, chymosin and gastric lipase. Mucus cells secrete mucus to protect the stomach wall from the acidic chyme. Gastrin is the hormone responsible for mobilizing the whole process.