Chapter 23
The Digestive System
Table 23.2
Overview of the Functions of the Gastrointestinal Organs
Mouth and
accessory organs
Ingestion: food is voluntarily placed into
oral cavity
Propulsion: voluntary (buccal) phase of
deglutition (swallowing) initiated by
tongue; propels food into pharynx
Mechanical breakdown: mastication
(chewing) by teeth and mixing movements
by tongue
Digestion: salivary amylase in saliva,
produced by salivary glands, begins
chemical breakdown of starch
Mouth serves as a receptacle; most functions
performed by associated accessory organs.
Mucus in saliva helps dissolve foods so they
can be tasted and moistens food so that
tongue can compact it into a bolus that can
be swallowed. Saliva cleanses and lubricates
oral cavity and teeth.
Pharynx and
Propulsion: peristaltic waves move food
bolus to stomach, thus accomplishing invol-
untary (pharyngeal-esophageal) phase of
Primarily food chutes; mucus produced helps
to lubricate food passageways.
Mechanical breakdown and propulsion:
peristaltic waves mix food with gastric juice
and propel it into the duodenum
Digestion: pepsin begins the digestion of
Absorption: absorbs a few fat-soluble
substances (aspirin, alcohol, some drugs)
Also stores food until it can be moved into
the duodenum. Hydrochloric acid produced
is a bacteriostatic agent and activates
protein-digesting enzymes. Mucus produced
helps lubricate and protect stomach from
self-digestion. Intrinsic factor produced is
required for intestinal absorption of vitamin
Small intestine
and associated
accessory organs
(liver, gallbladder,
Mechanical breakdown and propulsion:
segmentation by smooth muscle of the
small intestine continually mixes contents
with digestive juices and, along with short-
distance peristaltic waves, moves food
along tract, allowing sufficient time for
digestion and absorption
Digestion: digestive enzymes delivered
from pancreas and brush border enzymes
attached to microvilli membranes complete
digestion of all classes of foods
Absorption: breakdown products of
carbohydrate, protein, fat, and nucleic acid
digestion, plus vitamins, electrolytes, and
water, are absorbed by active and passive
Small intestine is highly modified for
digestion and absorption (circular folds, villi,
and microvilli). Alkaline mucus produced
by intestinal glands and bicarbonate-
rich juice ducted in from pancreas help
neutralize acidic chyme and provide proper
environment for enzymatic activity. Bile
produced by liver emulsifies fats and
enhances (1) fat digestion and (2) absorption
of fatty acids, monoglycerides, cholesterol,
phospholipids, and fat-soluble vitamins.
Gallbladder stores and concentrates bile,
releasing it to small intestine in response to
hormonal signals.
Large intestine
Digestion: some remaining food
residues are digested by enteric bacteria
(which also produce vitamin K and some
B vitamins)
Absorption: absorbs most remaining water,
electrolytes (largely NaCl), and vitamins
produced by bacteria
Propulsion: propels feces toward rectum by
haustral churning and mass movements
Defecation: reflex triggered by rectal
distension; eliminates feces from body
Temporarily stores and concentrates residues
until defecation can occur. Copious mucus
produced by goblet cells eases passage of
feces through colon.
*The colored boxes beside the functions correspond to the color coding of digestive functions (gastrointestinal tract activities) illustrated in Figure 23.2.
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