Download Digestive Functions of the Stomach

Digestive Functions of the Stomach
The stomach participates in virtually all-digestive activities with the
exception of ingestion and defecation. In addition to mechanical and
chemical digestion, the stomach absorbs some fat-soluble substances,
including alcohol and aspirin.
Mechanical Digestion
A few minutes after food enters the stomach, mixing waves begin at
intervals of about 15 to 25 seconds. Mixing waves are a gentle type of
peristalsis that soften and moisten food. They also combine food with
gastric juice and create chyme. Initial gentle mixing waves are followed
by more intense waves that break down food into smaller pieces and
further mix with digestive juice, starting at the body of the stomach and
increasing in force as they reach the pylorus.
The pylorus, which holds around 30 mL (1 ounce) of chyme, acts as a
filter, permitting only liquids and small food particles to pass through
the mostly, but not fully, closed pyloric sphincter. When food enters the
pylorus, periodic mixing waves force about 3 mL of chyme through the
pyloric sphincter and into the duodenum, a process called gastric
emptying. The rest of the chyme is pushed back into the body of the
stomach, where it continues mixing. This process is repeated when the
next mixing waves force more chyme into the duodenum. These forward
and backward movements accomplish most of the mixing in the
stomach.
Both the stomach and the duodenum regulate gastric emptying. The
presence of chyme in the duodenum activates receptors that inhibit
gastric emptying. This prevents additional chyme from being released by
the stomach before the duodenum is ready to process it.
Chemical Digestion
Some foods sit in the fundus of the stomach for an hour or so before
they are mixed with gastric juice. During this time, the digestive activity
of salivary amylase continues. Mixing waves soon take over however,
combining chyme with acidic gastric juice, which inactivates salivary
amylase and activates lingual and gastric lipase. Lingual lipase then
starts breaking down triglycerides into fatty acids and diglycerides.
The digestion of protein begins in the stomach, primarily by pepsin.
During infancy, gastric glands also produce rennin, an enzyme that
helps digest milk protein. Lingual lipase secreted by the salivary glands
may also participate in triglyceride digestion in the stomach. Parietal
cells secrete hydrogen ions and chloride ions into the stomach lumen,
effectively forming HCl, which denatures dietary proteins.
The contents of the stomach are completely emptied into the duodenum
within two to four hours after you eat a meal. Carbohydrate-rich foods
are the quickest to leave the stomach. High-protein foods exit a little
more slowly. Fatty meals with high triglyceride content remain in the
stomach the longest. Enzymes in the small intestine digest fats slowly.
So when the duodenum is processing fatty chyme, food can stay in the
stomach for six hours or longer.