The stomach receives boluses of food from the esophagus and turns
them into chyme - a semi-liquid mass of partially digested food.
It mechanically breaks up food and partially digests proteins, but absorbs very little. Rather,
it serves as a temporary storage, releasing only a small amount of chyme into the intestine at a
time to ensure that the intestine has enough time to fully digest and absorb the nutrients.
The stomach is basically an expandable muscular sac. When empty, its inner surface has a wrinkled
appearance with multiple folds, called rugae. These folds flatten when the stomach is full.
The stomach has 4 regions: the cardia at the connection to the esophagus, the fundus above
and to the left of the cardia, the main body, and the pylorus above the pyloric sphincter.
Different regions differ in gastric secretions. The wall of the stomach consists of four layers:
mucosa, submucosa, a muscle layer called muscularis externa, and serosa.
The muscularis externa has 3 sublayers: outer longitudinal, middle circular, and inner oblique
muscle. The oblique muscle is unique to the stomach, as compared to other digestive organs.
It enables the churning motions required for mechanical digestion.
The mucosa is lined with a surface epithelium which folds into the underlying connective
tissue - the lamina propria – to form tiny holes called gastric pits.
At the bottom of these pits are gastric glands producing various secretions. There are four
major secretory cell types in these glands: - Mucous cells secrete an alkaline mucus that
protects the epithelium from gastric acid. This is one of several mechanisms that prevent the stomach
from digesting itself. Other protective measures include tight junctions between epithelial cells
to block gastric juice from seeping into the underlying tissue, and a rapid turnover of gastric
epithelial cells to quickly replace injured cells. - Parietal cells secrete hydrochloric acid and
intrinsic factor. Hydrochloric acid has several functions: it breaks up food,
denatures proteins, kills ingested bacteria, and activates the protein-digesting enzyme pepsin.
Intrinsic factor is required for the absorption of vitamin B12 in the intestine.
- Next are chief cells producing pepsinogen, the inactive form of pepsin, which becomes
activated by the acid produced by parietal cells. In infants, chief cells also produce
some other enzymes for digestion of milk. - Finally, there are enteroendocrine cells
secreting various hormones into the bloodstream. Notable hormones include: gastrin produced by
G-cells and somatostatin by D-cells in the pylorus region; and histamine by ECL-like cells in the
fundus. Gastrin increases secretions by gastric glands, promotes gastric emptying, and stimulates
intestinal motility. Histamine increases gastric acid production by parietal cells.
Somatostatin inhibits the release of gastrin,
reducing gastric secretions and motility. It also inhibits a large number of other hormones.
Gastric activity is under both neural and hormonal control; and occurs in 3 phases,
which overlap and can happen simultaneously: - The cephalic phase starts before food reaches
the stomach. It is triggered by the thought, smell, sight, or taste of food. These inputs act
on the hypothalamus, which relays the information to the medulla. Vagus nerve fibers from the
medulla then stimulate the enteric nervous system to prepare the stomach for digestion.
- The gastric phase is triggered by the arrival of food in the stomach. As the stomach fills,
stretch receptors in stomach wall activate the parasympathetic system to release acetylcholine,
which stimulates gastric secretions and motility. There is also a local reflex mediated via the
myenteric plexus. At the same time, partially digested proteins and the rising pH stimulate
the release of gastrin, which increases gastric acid production and stimulates gastric motility.
A negative feedback loop exists to wind down this phase as the stomach is emptying.
- The intestinal phase is triggered by the arrival of chyme in the duodenum. Initially,
the stretching and presence of partially digested food in the duodenum further stimulate gastric
activity via the vagovagal reflex and production of intestinal gastrin. Soon however, distension
of the duodenum inhibits gastric secretions and slows down stomach emptying via the enterogastric
reflex and several intestinal hormones, thus allowing time for the intestine to process chyme.