b. Lubrication. Although the saliva that is formed in the mouth does produce
the enzyme ptyalin that aids in digestion, saliva functions as a more valuable aid in
other ways. Saliva is of great importance as a lubricant. Saliva moistens the food and
makes it easy to swallow. Without saliva or some other liquid, it would be impossible to
swallow food. Saliva lubricates and keeps the mouth soft; this plays a very important
role in the mechanics of speech.
216. DIGESTION IN THE STOMACH
The three major functions of the stomach are to store food, mix it with gastric
secretion until chyme (semifluid substance) is formed, and to slowly release the chyme
into the small intestine at a rate suitable for digestion and absorption in the small
intestine. The stomach plays only a small part in the actual breakdown of food into
simpler substances. The following types of cells are located in the gastric glands found
in the stomach's mucous coat, a membrane forming its inner lining.
a. Parietal Cells. Parietal cells are the cells that secrete hydrochloric (HCl) acid
into the stomach. Gastric juice contains between 0.4 and 0.5 percent hydrochloric acid.
(The stomach wall is not digested by this acid because it is coated with a protective
layer of mucus, which also serves as a lubricant to aid in the passage of food.)
b. Chief Cells. Chief cells secrete digestive enzymes into the stomach.
(1) Pepsin. Pepsin, which begins the digestion of proteins, is the main
enzyme secreted by the chief cells. Pepsin is produced inside the chief cells in an
inactive form called pepsinogen. It is not until the pepsinogen enters the stomach and
comes into contact with hydrochloric acid, that the pepsinogen is activated into active
(2) Gastric lipase and rennin. Gastric lipase and rennin are secreted in
such small quantities that they are of little importance. Rennin is an enzyme that aids in
the digestion of casein, one of the proteins in milk. Gastric lipase begins the digestion
217. PANCREATIC SECRETION
a. Hormone Regulation. When food stimulates the upper part of the small
intestine, two hormones, secretin and pancreozymin, are released which cause
secretion by the pancreas. Secretin is a polypeptide hormone found in the mucosa of
the upper small intestine. When chyme enters the intestine, it stimulates the secretion
of secretin, which is absorbed by the blood. The hormone pancreozymin is also
released by the mucosa of the upper small intestine; this secretion is particularly
activated by the presence of proteoses and peptones. Pancreozymin also passes to
the pancreas by way of the blood. Stimulation of the pancreas by pancreozymin causes
the production of enzymerich pancreatic fluid; stimulation by secretin causes the
production of watery, enzymepoor pancreatic fluid, which contains bicarbonate to
neutralize acid from the stomach.