864
UNIT 4
Maintenance of the Body
23
The Stomach
Identify structural modifications of the wall of the stomach
that enhance the digestive process.
Name the cell types responsible for secreting the various
components of gastric juice and indicate the importance of
each component in stomach activity.
Describe stomach structure and indicate changes in the
basic alimentary canal structure that aid its digestive
function.
Below the esophagus, the GI tract expands to form the
stomach
(see Figure 23.1), a temporary “storage tank” where chemical
breakdown of proteins begins and food is converted to a creamy
paste called
chyme
(kīm; “juice”). Te stomach lies in the upper
in 1 to 2 seconds. Just before the peristaltic wave (and food)
reaches the end of the esophagus, the gastroesophageal sphinc-
ter relaxes reflexively to allow food to enter the stomach. A±er
food entry, that sphincter closes, preventing regurgitation.
If we talk or inhale while swallowing, the various protective
mechanisms may be short-circuited and food may enter the res-
piratory passageways instead. Tis event typically triggers the
cough reflex to expel the food.
Check Your Understanding
22.
What role does the tongue play in swallowing?
23.
How are the respiratory passages blocked during
swallowing?
For answers, see Appendix H.
Tongue
Trachea
Pharynx
Epiglottis
Glottis
Bolus of food
Epiglottis
Esophagus
Uvula
Bolus
Bolus
Relaxed muscles
Circular muscles
contract
Upper
esophageal
sphincter
Bolus of food
Longitudinal muscles
contract
Stomach
Relaxed
muscles
Gastroesophageal
sphincter opens
Circular muscles contract
Gastroesophageal
sphincter closed
1
During the buccal phase, the upper
esophageal sphincter is contracted. The
tongue presses against the hard palate,
forcing the food bolus into the
oropharynx.
4
Peristalsis moves
food through the
esophagus to the
stomach.
5
The gastroesophageal
sphincter surrounding the
cardial oriface opens, and
food enters the stomach.
2
The pharyngeal-esophageal phase begins
as the uvula and larynx rise to prevent food
from entering respiratory passageways. The
tongue blocks off the mouth. The upper
esophageal sphincter relaxes, allowing food
to enter the esophagus.
3
The constrictor muscles of the
pharynx contract, forcing food into
the esophagus inferiorly. The upper
esophageal sphincter contracts
(closes) after food enters.
Figure 23.13
Deglutition (swallowing).
The process of swallowing consists of a voluntary
(buccal) phase (step
1
) and an involuntary (pharyngeal-esophageal) phase (steps
2
5
).
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