Digestion in the Stomach
Digestion in the stomach takes place by the action of various enzymes and the movements, which are as follows-
- As food enters the stomach, waves of peristalsis pass over the stomach every 15 to 25 seconds. Fundus has a storage function.
- Each peristaltic wave moves gastric contents from the body of the stomach down into the antrum, a process known as propulsion.
- Most food particles in the stomach are forced back into the body of the stomach, this process referred to as retropulsion.
- These movements of gastric contents are mixed with gastric juice reduced to a liquid called chyme. They can pass through the pyloric sphincter, known as gastric emptying. Gastric emptying is a slow process, only about 3 mL of chyme moves through the pyloric sphincter at a time.
- Foods remain in the fundus for about an hour without becoming mixed with gastric juice. During this time, digestion by salivary amylase from the salivary glands continues. churning action mixes chyme with acidic gastric juice, inactivating salivary amylase and activating lingual lipase which starts to digest triglycerides into fatty acids and diglycerides.
- Parietal cells secrete hydrochloric acid (HCl).
- Gastrin secreted by G cells (enteroendocrine), stimulates parietal cells to secrete more HCl in the presence of histamine.
- The strongly acidic fluid of the stomach kills many microbes in food. HCl denatures proteins in food and stimulates the secretion of hormones that promote the flow of bile and pancreatic juice.
- A protein-digesting enzyme in the stomach is pepsin, which is secreted by chief cells. Pepsin breaking down a protein chain of many amino acids into smaller peptide fragments.
- Pepsin is effective in the acidic environment of the stomach. Pepsin is secreted in an inactive form called pepsinogen. Pepsinogen is not converted into active pepsin until it comes in contact with hydrochloric acid secreted by parietal cells.
- Stomach epithelial cells are protected from gastric juices by a layer of alkaline mucus secreted by surface mucous cells and mucous neck cells.
- Gastric lipase, which splits triglycerides in fat molecules into fatty acids and monoglycerides.
- A small number of nutrients are absorbed in the stomach. Mucous cells of the stomach absorb some water, ions, certain drugs, and alcohol.
- Within 2 to 4 hours after eating a meal, the stomach has emptied its contents into the duodenum. Foods rich in carbohydrates spend the least time in the stomach, high-protein foods remain longer, and emptying is slowest after a meal containing large amounts of triglycerides.