Function of the Digestive System—to break
down food into simple chemicals that can
be absorbed into the blood and lymph and
utilized by cells
Divisions of the Digestive System
- Alimentary tube—oral cavity, pharynx, esophagus,
stomach, small intestine, large intestine. Digestion
takes place in the oral cavity, stomach, and small
intestine. - Accessory organs—salivary glands, teeth, tongue,
liver, gallbladder, and pancreas. Each contributes to
digestion.
Types of Digestion
- Mechanical—breaks food into smaller pieces to
increase the surface area for the action of enzymes. - Chemical—enzymes break down complex organics
into simpler organics and inorganics; each enzyme
is specific for the food it will digest.
End Products of Digestion
- Carbohydrates are digested to monosaccharides.
- Fats are digested to fatty acids and glycerol.
3. Proteins are digested to amino acids.
4. Other end products are vitamins, minerals, and
water.
Oral Cavity—food enters by way of the
mouth- Teeth and tongue break up food and mix it with
saliva. - Tooth structure (see Fig. 16–2)—enamel covers the
crown and provides a hard chewing surface; dentin
is within the enamel and forms the roots; the pulp
cavity contains blood vessels and endings of the
trigeminal nerve; the periodontal membrane
produces cement to anchor the tooth in the jaw-
bone. - The tongue is skeletal muscle innervated by the
hypoglossal nerves. Papillae on the upper surface
contain taste buds (facial and glossopharyngeal
nerves). Functions: taste, keeps food between the
teeth when chewing, elevates to push food back-
ward for swallowing. - Salivary glands—parotid, submandibular, and sub-
lingual (see Fig. 16–3); ducts take saliva to the oral
cavity.
390 The Digestive System
STUDY OUTLINE
periodontal disease and loss of teeth. Secretions are
reduced throughout the digestive system, and the
effectiveness of peristalsis diminishes. Indigestion may
become more frequent, especially if the LES loses its
tone, and there is a greater chance of esophageal dam-
age. In the colon, diverticula may form; these are bub-
ble-like outpouchings of the weakened wall of the
colon that may be asymptomatic or become infected.
Intestinal obstruction, of the large or small bowel,
occurs with greater frequency among the elderly.
Sluggish peristalsis contributes to constipation, which
in turn may contribute to the formation of hemor-
rhoids. The risk of oral cancer or colon cancer also
increases with age.
The liver usually continues to function adequately
even well into old age, unless damaged by pathogens
such as the hepatitis viruses or by toxins such as alco-
hol. There is a greater tendency for gallstones to form,
perhaps necessitating removal of the gallbladder.
Inflammation of the gallbladder (cholecystitis) is also
more frequent in older adults. In the absence of spe-
cific diseases, the pancreas usually functions well,
although acute pancreatitis of unknown cause is some-
what more likely in elderly people.SUMMARY
The processes of the digestion of food and the absorp-
tion of nutrients enable the body to use complex food
molecules for many purposes. Much of the food we eat
literally becomes part of us. The body synthesizes pro-
teins and lipids for the growth and repair of tissues and
produces enzymes to catalyze all of the reactions that
contribute to homeostasis. Some of our food provides
the energy required for growth, repair, movement,
sensation, and thinking. In the next chapter we will
discuss the chemical basis of energy production from
food and consider the relationship of energy produc-
tion to the maintenance of body temperature.