include chewing, mixing, and peristalsis, all of which
involve
muscle
contractions. The chemical processes
include secretions of fluids (saliva, gastric juices, pan-
creatic juices, bile) that consist mostly of water, mucus,
ions, bile salts, and digestive enzymes. The chemical se-
cretions and mechanical mixing work together to break
down food into simple carbohydrates, lipids, amino acids,
nucleic acids, and vitamins. These substances are ab-
sorbed mainly in the small intestine, but some are also
absorbed in the stomach and large intestine. What re-
mains of your breakfast after it passes through the large
intestine moves into the rectum and is excreted from the
body. For a summary of the functions of the digestive or-
gans, see Table 14.2
Summary of digestive organs and their functions Table 14.2
Organ
Functions
Mouth
See other listings in this table for the functions of the tongue, salivary glands, and teeth, all of which are in the mouth.
Additionally, the lips and cheeks keep food between the teeth during mastication, and buccal glands lining the mouth
produce saliva.
Tongue
Maneuvers food for mastication, shapes food into a bolus, maneuvers food for deglutition, detects taste and touch
sensations, and initiates digestion of triglycerides.
Salivary glands
Produce saliva, which softens, moistens, and dissolves foods; cleanses mouth and teeth; and initiates the digestion of
starch.
Teeth
Cut, tear, and pulverize food to reduce solids to smaller particles for swallowing.
Pharynx
Receives a bolus from the mouth and passes it into the esophagus.
Esophagus
Receives a bolus from the pharynx and moves it into the stomach. This requires relaxation of the upper esophageal
sphincter and secretion of mucus.
Stomach
Mixing waves soak food, mix it with the secretions of gastric glands (gastric juice), and reduce food to chyme. Gastric
juice activates pepsin and kills many microbes in food. Instrinsic factor aids absorption of vitamin B12 from the colon.
The stomach serves as a reservoir for food before releasing it into the small intestine.
Pancreas
Pancreatic juice buffers acidic gastric juice in chyme (creating the proper pH for digestion in the small intestine), stops
the action of pepsin from the stomach, and contains enzymes that digest carbohydrates, proteins, triglycerides, and
nucleic acids.
Liver
Produces bile, which is needed for the emulsification and absorption of lipids in the small intestine.
Gallbladder
Stores and concentrates bile and releases it into the small intestine.
Small intestine
Segmentations mix chyme with digestive juices; peristatic contractions propel chyme toward the ileocecal sphincter;
digestive secretions from the small intestine, panceas, and liver complete the digestion of carbohydrates, proteins,
lipids, and nucleic acids; circular folds, villi, and microvilli increase surface area for absorption; site where about 90%
of nutrients and water are absorbed.
Large intestine
Haustral churning, peristalsis, and mass peristalsis drive the contents of the colon into the rectum; bacteria produce
some B vitamins and vitamin K; absorption of some water, ions, and vitamins; defecation.
CONCEPT CHECK
1.
What
process begins physical digestion?
2.
Which
layer of the Gi tract is responsible for
absorption?
3.
What
are the conditions in the stomach that
help break down proteins?
4.
How
is the surface of the small intestine special-
ized for absorption?
5.
Where
is bile made, and what does it do?
6.
What
is the function of pancreatic juice?
7.
Which
substances are absorbed in the large
intestine?
8.
How
is the defecation reflex controlled?
9.
What
types of materials are absorbed into the
lacteals?
Let's Journey Through the Digestive System 415
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