pancreatic juices, and drains into the DUODENUM,
the first part of the SMALL INTESTINE. INSULINis the
best known of the pancreatic products, though the
pancreas synthesizes a number of other important
hormones, DIGESTIVE ENZYMES, and juices essential
for digestion. Digestive hormones orchestrate and
synchronize the multitude of gastrointestinal
functions.
Mechanical preparation: the mouthThe diges-
tive journey begins with the mouth. Each mouth-
ful of food passes between the crushing force of
the TEETH, which can exert up to 3,500 pounds per
square inch, at least two dozen times. The taste,
texture, and smell of the food induce the SALIVARY
GLANDSto release saliva, a watery liquid that con-
tains the digestive enzyme amylase. Amylase
begins to break down carbohydrates in the food
into the sugar molecules that form them, getting a
head start on extracting from food the body’s most
significant fuel source. The lips and cheeks hold
the food in the mouth while the tongue pushes
the food under the teeth and against the palate
(roof of the mouth). These actions grind the food
and mix the particles with saliva to form a paste-
like wad called a bolus. Finally the tongue pushes
the bolus to the back of the THROATfor swallowing.
A small projection of cartilaginous tissue that
hangs at the back of the throat, the epiglottis,
closes across the TRACHEA(windpipe) to direct the
bolus of food down the ESOPHAGUS.
Wavelike contractions—PERISTALSIS—propel the
bolus down the esophagus, a muscular tube about
12 inches long. The MUSCLEstructure of the esoph-
agus changes along its length, transitioning from
striated muscle tissue that responds to voluntary
control to smooth muscle tissue, completely under
direction of the autonomic NERVOUS SYSTEM.
Esophageal peristalsis thrusts the swallowed food
bolus toward the STOMACHwith such vigor that the
food continues to its destination even if the person
is upside down. A ring of muscle, the lower
esophageal sphincter, opens to pass the bolus into
the stomach then closes to keep it there.
Liquefication: the stomachThe pouchlike stom-
ach can expand to five or six times its empty size to
accommodate the meals that come its way. This is
where the process of digestion gets under way; the
stomach digests more than half the carbohydrates
and about 20 percent of the protein that a meal
contains. The stomach resides below the
DIAPHRAGM, its upper portion resting under the apex
of the HEARTand near the SPLEEN, just under the left
ribs, and its lower portion beneath the liver.
The inside of the stomach is the gastric mucosa,
a thick layer of mucous membrane. A network of
deep folds, called rugae, gives the gastric mucosa a
furrowed appearance. At the bottom of the folds
of the rugae are the gastric glands, which produce
hydrochloric acid and digestive enzymes (gastric
juices). Near the top of the folds are the cells that
produce the mucus that protects the stomach’s lin-
ing from the gastric juices. When the stomach
expands with food the rugae flatten, spreading the
mucus and expanding the surface area of the
stomach for thorough mixing of food with gastric
juices. The gastric mucosa also secretes the diges-
tive hormone gastrin.
Three layers of muscle give the stomach
STRENGTHand FLEXIBILITY. The innermost layer wraps
obliquely, or diagonally, around the gastric
mucosa. The middle layer encircles the oblique
muscle. The outer layer, the longitudinal muscle,
envelopes the stomach lengthwise. Among them,
these muscles give the stomach the ability to con-
tract and convolute with considerable force as well
as the ability to expand and contract for the vol-
ume of food it contains. These layers of muscle
also give the stomach the ability to squeeze its
contents downward into the small intestine for
the rest of the digestive journey.
Food, in a semiliquid state after initial prepara-
tion in the mouth, enters at the top of the stom-
ach, called the fundus, and flows downward
across the rugae. Gastric juices immediately begin
working to dissolve food particles, breaking them
down into more basic compounds that the small
intestine can digest. At the same time the stomach
muscles contract, producing powerful contortions
that further mix and liquefy the stomach’s con-
tents. The combined chemical and mechanical
actions produce chyme, a somewhat soupy solu-
tion the stomach then sends to the small intestine.
The lower portion of the stomach is the pylorus,
from the Greek for “gatekeeper.” Chyme exits the
stomach through the pyloric sphincter, which
relaxes to permit passage into the duodenum.
Chyme trickles from the stomach over four to six
hours.The Gastrointestinal System 3