b. Function of the Stomach. The stomach, by mechanical (contractions and
peristalsis) and chemical action, plays an important part in digestion. Gastric juice
contains hydrochloric acid and several enzymes. The acid attacks proteins, which swell
to a gelatinous mass. Pepsin converts proteins to proteoses and peptones. Gastric
lipase hydrolyzes finely divided fats (as in egg yolk) to fatty acids and glycerine. The
food is reduced to a thin liquid mass (chyme). At intervals, the relaxation of the pyloric
sphincter allows chyme to enter the duodenum. The time required for gastric digestion
averages about 5 to 7 hours for a meal.
Figure 3-4. Anterior aspect of the stomach.
THE SMALL INTESTINE
The small intestine extends from the pylorus to the ileocecal valve where it joins
the large intestine. It is divided into the duodenum, the jejunum, and the ileum (figure
3-1). The small intestine is a primary site of both digestion and absorption.
a. The duodenum is the shortest part (8 to 10 inches) of the small intestine. It
begins at the termination of the pylorus and unites with the jejunum at the
duodenojejunal flexure. It is a C-shaped organ in the umbilical region of the abdomen,
extending from the level of the first lumbar vertebra to that of the fourth lumbar vertebra.
The first portion of the duodenum, the duodenal bulb or cap, is often the site of
duodenal ulcers. The accessory organs (liver and pancreas) of the digestive system
have a common opening into the duodenum (figure 3-5), adding their secretions to the
stomach juices that also continue to work in the duodenum.
b. The jejunum is the second portion of the small intestine. It extends from the
duodenojejunal flexure to the ileum. The jejunum is about 7-1/2 feet long and
constitutes about two-fifths of the small intestine.