693
CHAPTER SEVENTEEN
Digestive System
sic feedback response, cells of the intestinal mucosa secrete
more of it into the bloodstream, and the gallbladder con-
tinues to be able to contract. The bile ducts widen in some
areas, but the end of the bile duct narrows as it approaches
the small intestine. As long as gallstones do not become
entrapped in the ducts, the gallbladder generally functions
well into the later years.
PRACTICE
67
Describe the ef
ects oF aging on the teeth.
68
What conditions might be caused by the slowing oF peristalsis in
the digestive tract that occurs with aging?
stimulation slows peristalsis, ultimately causing constipa-
tion. Compounding this common problem is a loss of elas-
ticity in the walls of the rectum and declining strength and
responsiveness of the internal and external sphincters.
The accessory organs to digestion age too, but not neces-
sarily in ways that affect health. Both the pancreas and the
liver are large organs with cells to spare, so a decline in their
secretion abilities does not usually hamper digestion. Only
10% of the pancreas and 20% of the liver are required to
digest foods. However, the liver may not be able to detoxify
certain medications as quickly as it once did. The gallblad-
der becomes less sensitive to cholecystokinin, but in a clas-
2. Tongue
a. The tongue is a thick, muscular organ that mixes
food with saliva and moves it toward the pharynx.
b. The rough surface of the tongue handles food and
has taste buds.
c. Lingual tonsils are located on the root of the tongue.
3. Palate
a. The palate comprises the roof of the mouth and
includes hard and soft portions.
b. The soft palate, including the uvula, closes the
opening to the nasal cavity during swallowing.
c. Palatine tonsils are located on either side of the
tongue in the back of the mouth.
d. Tonsils consist of lymphatic tissues.
4. Teeth
a. Two sets of teeth develop in sockets of the
mandibular and maxillary bones.
b. There are twenty primary and thirty-two secondary
teeth.
c. Teeth mechanically break food into smaller pieces,
increasing the surface area exposed to digestive
actions.
d. Different types of teeth are adapted to handle
foods in different ways, such as biting, grasping, or
grinding.
e. Each tooth consists of a crown and root and is
composed of enamel, dentin, pulp, nerves, and
blood vessels.
f. A tooth is attached to the alveolar process by the
periodontal ligament.
17.4
SALIVARY GLANDS (PAGE 660)
Salivary glands secrete saliva, which moistens food,
helps bind food particles, begins chemical digestion of
carbohydrates, makes taste possible, helps cleanse the
mouth, and regulates pH in the mouth.
1. Salivary secretions
a. Salivary glands include serous cells that secrete
digestive enzymes and mucous cells that secrete
mucus.
b. Parasympathetic impulses stimulate the secretion
of serous fl
uid.
CHAPTER SUMMARY
17.1
INTRODUCTION (PAGE 652)
Digestion is the process of mechanically and chemically
breaking down foods so that they can be absorbed. The
digestive system consists of an alimentary canal and several
accessory organs that carry out the processes of ingestion,
propulsion, digestion, absorption, and defecation.
17.2
GENERAL CHARACTERISTICS OF THE
ALIMENTARY CANAL (PAGE 652)
Regions of the alimentary canal perform speciF c functions.
1. Structure of the wall
a. The wall consists of four layers.
b. These layers include the mucosa, submucosa,
muscular layer, and serosa.
2. Movements of the tube
a. Motor functions include mixing and propelling
movements.
b. Peristalsis is responsible for propelling movements.
c. The wall of the tube undergoes receptive relaxation
just ahead of a peristaltic wave.
3. Innervation of the tube
a. The tube is innervated by branches of the
sympathetic and parasympathetic divisions of the
autonomic nervous system.
b. Parasympathetic impulses generally increase
digestive activities; sympathetic impulses generally
inhibit digestive activities.
c. Sympathetic impulses contract certain sphincter
muscles, controlling movement through the
alimentary canal.
17.3
MOUTH (PAGE 656)
The mouth is adapted to receive food and begin preparing
it for digestion. It also serves as an organ of speech and
sensory perception.
1. Cheeks and lips
a. Cheeks form the lateral walls of the mouth.
b. Lips are highly mobile and have a variety
of sensory receptors useful in judging the
characteristics of food.
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