3.3: Stomach

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After going through the lower esophageal sphincter, food enters the stomach. Our stomach is involved in both chemical and mechanical digestion. Mechanical digestion occurs as the stomach churns and grinds food into a semifluid substance called chyme (partially digested food).

The lining of the stomach is made up of different layers of tissue. The mucosa is the layer closest to stomach cavity as shown in the figure below.

Figure \(\PageIndex{1}\): The anatomy of the stomach1

Query \(\PageIndex{1}\)

The mucosa is not a flat surface. Instead, its surface is lined by gastric pits, as shown in the figure below.

Gastric pits are indentations in the stomach's surface that are lined by four different types of cells.

Figure \(\PageIndex{3}\): Blowup of mucosa to show the structure of gastric pits1

The following video is a nice introduction to gastric pits and talks about chief and parietal cells that are described in more detail below.

Video \(\PageIndex{1}\): Cells in the gastric pits of the stomach. https://www.youtube.com/watch?v=6hquzCXYlNg

At the bottom of the gastric pit are the G cells that secrete the hormone gastrin. Gastrin stimulates the parietal and chief cells that are found above the G cells. The chief cells secrete the zymogen pepsinogen and the enzyme gastric lipase. A zymogen is an inactive protein that must be cleaved or altered to form the active protein. The parietal cells secrete hydrochloric acid (\(\ce{HCl}\)), which lowers the pH of the gastric juice (water + enzymes + acid). Hydrochloric acid inactivates salivary amylase and catalyzes the conversion of pepsinogen to pepsin. Finally, the top of the pits are the neck cells that secrete mucus to prevent the gastric juice from digesting or damaging the stomach mucosa3. The table below summarizes the actions of the different cells in the gastric pits.

Table \(\PageIndex{1}\): Cells involved in the digestive processes in the stomach
Type of Cell	Secrete
Neck	Mucus
Chief	Pepsinogen and gastric lipase
Parietal	Hydrochloric acid
G	Gastrin

The figure below shows the action of all these different secretions in the stomach.

Figure \(\PageIndex{4}\): The action of gastric secretions in the stomach

To reiterate, the figure above illustrates that the neck cells of the gastric pits secrete mucus to protect the mucosa of the stomach from essentially digesting itself. Gastrin from G cells stimulates the parietal and chief cells to secrete hydrochloric acid and enzymes, respectively.

Query \(\PageIndex{2}\)

Hydrochloric acid in the stomach denatures salivary amylase and other proteins by breaking down the structure and, thus, the function of it. Hydrochloric acid also converts pepsinogen to the active enzyme pepsin. Pepsin is a protease, meaning that it cleaves bonds in proteins. It breaks down the proteins in food into individual peptides (shorter segments of amino acids). The other enzyme that is active in the stomach is gastric lipase. This enzyme preferentially cleaves the sn-3 position of triglycerides to produce 1,2-diglyceride and a free fatty acid, as shown below4. It is responsible for up to 20% of triglyceride digestion3.

Figure \(\PageIndex{5}\): Gastric Lipase action results in production of 1,2-diglyceride and a free fatty acid

The chyme will then leave the stomach and enter the small intestine via the pyloric sphincter (shown below).

Figure \(\PageIndex{6}\): Cross section of the stomach showing the pyloric sphincter5

Table \(\PageIndex{2}\): Summary of chemical digestion in the stomach
Chemical or Enzyme	Action
Gastrin	Stimulates chief cells to release pepsinogen Stimulates parietal cells to release \(\ce{HCl}\)
\(\ce{HCl}\)	Denatures salivary amylase Denatures proteins Activates pepsinogen to pepsin
Pepsin	Cleaves proteins to peptides
Gastric lipase	Cleaves sn-3 FA of triglycerides

Query \(\PageIndex{3}\)

Query \(\PageIndex{4}\)

References

https://en.Wikipedia.org/wiki/Stomac...u_stomach2.jpg
http://en.Wikipedia.org/wiki/File:Gray1055.png
Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
https://en.Wikipedia.org/wiki/Pyloru...e:Gray1050.png