After food passes through the mouth where it is chewed and saliva and enzymes are added to it, it reaches the stomach. Once in the stomach, food churns via peristalsis two to four times per minute. This propels food toward the antrum. Food is held in to the stomach because the pyloric sphincter is closed. Food continues to churn and when they are small enough through the process of propulsion and by further breakdown by stomach acid and chyme, the pyloric sphincter opens and allows the mixture of partially digested food and chemicals to pass through. Several complex chemical processes then take place, including the destruction of microbes, the exchange of potassium, the generation of bicarbonate ions, and the diffusion of waste products into capillaries.
The specific chemical processes that occur are the parietal cells secreting intrinsic factors that are necessary for the absorption of vitamins and the secretion of hydrochloric acid that breaks down and denatures proteins and creates pepsin, an enzyme that breaks down the amino acids of protein. “Pepsin is most effective in the very acidic environment of the stomach; it becomes inactive at a higher pH (Tortoria & Derrickson, 2013, p. 905). Additionally, chief cells secrete both hydrochloric acid and gastric lipase, the pre-curser to gastric acid. G cells signal the parietal cells to make hydrochloric acid and to keep the esophageal sphincter closed and they also cause the pyloric sphincter to relax. All of these chemicals work in close relationship to one another, opening and closing chemical chain reactions to keep the contents of the stomach churning and further breaking down the food that has been ingested.
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The entire process takes approximately two to four hours. Starchy foods pass through this process more quickly than protein-rich foods, and high fat foods take the longest amount of time to process.
- Tortoria, G.T. & Derrickson, B. (2013). Principles of anatomy & physiology. Hoboken, NJ: Wiley.
