Chapter 26 Physiology of the
Digestive System
OVERVIEW OF DIGESTIVE FUNCTION
• Primary function of the digestive system: to bring essential
nutrients into the internal environment so they are available
to every cell in the body
• Mechanisms used to accomplish the primary function of the
digestive system (Figure 26-1, Table 26-1)
– Ingestion: food is taken in
– Digestion: breakdown of complex nutrients into simple
nutrients
– Motility of the gastrointestinal (GI) wall: physically breaks
down large chunks of food material and moves food along
the tract
– Secretion of digestive enzymes allows chemical digestion
– Nutrients are absorbed into the internal environment
through the GI mucosa
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– Material that is not absorbed is eliminated
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Digestion
• All the processes that chemically and
mechanically break down food into nutrients
• Begins immediately after food is ingested
• 2 types: Mechanical and Chemical
– Mechanical – movement of the GI tract which:
• Changes the physical state of food to facilitate chemical
digestion
• Churning of the contents in the GI lumen so they are
well mixed with the GI juices
• Propelling food through the digestive tract
Mechanical Digestion
• Mastication – reducing the food particle size
through chewing, and mixes food with saliva
• Deglutition – swallowing
1. Oral Stage (moth to oropharynx)
2. Pharyngeal Stage (oropharynx to esophagus)
3. Esophageal Stage (esophagus to stomach)
DIGESTION: MECHANICAL
– Deglutition: process of swallowing; complex process
requiring coordinated, rapid movements (Figure 26-2)
• Oral stage (mouth to oropharynx): voluntarily controlled;
formation of a food bolus in the middle of the tongue; tongue
presses bolus against the palate and food is then moved into the
oropharynx
• Pharyngeal stage (oropharynx to esophagus): involuntary
movement; to propel bolus from the pharynx to the esophagus,
the mouth (tongue), nasopharynx (soft palate), and larynx
(epiglottis) must be blocked; a combination of contractions and
gravity move bolus into esophagus
• Esophageal stage (esophagus to stomach): involuntary movement;
contractions and gravity move bolus through esophagus and into
stomach
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DIGESTION: MECHANICAL (cont.)
– Peristalsis and segmentation: two main types of motility
produced by the smooth muscle of the GI tract; can occur
together in an alternating fashion
• Peristalsis: wavelike ripple of the muscle layer of a hollow organ;
progressive motility that produces forward movement of matter
along the GI tract (Figures 26-3 and 26-4)
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DIGESTION: MECHANICAL (cont.)
• Segmentation: mixing movement; digestive reflexes cause a forward-andbackward movement with a single segment of the GI tract; helps break
down food particles, mixes food and digestive juices, and brings digested
food in contact with intestinal mucosa to facilitate absorption (Figure 265)
DIGESTION: MECHANICAL (cont.)
– Regulation of motility
• Gastric motility:
– emptying the stomach takes approximately 2 to 6 hours
– while in the stomach, food is churned (propulsion and
retropulsion) and mixed with gastric juices to form
chyme
– chyme is ejected approximately every 20 seconds into
the duodenum
– gastric emptying is controlled by hormonal and nervous
mechanisms (Figure 26-6)
» Hormonal mechanism: fats in duodenum stimulate
the release of gastric inhibitory peptide (GIP), which
decreases peristalsis of gastric muscle and slows
passage of chyme into duodenum
» Nervous mechanism: receptors in the duodenal
mucosa are sensitive to presence of acid and
distention; impulses over sensory and motor fibers
in the vagus nerve cause a reflex inhibition of gastric
peristalsis; enterogastric reflex
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DIGESTION: MECHANICAL (cont.)
• Intestinal motility includes peristalsis and segmentation
– Segmentation in duodenum and upper jejunum
mixes chyme with digestive juices from the
pancreas, liver, and intestinal mucosa
– Rate of peristalsis picks up as chyme approaches
end of jejunum, moving it through the rest of the
small intestine into the large intestine
– Chyme normally takes approximately 5 hours to
pass through the small intestine
– Peristalsis is regulated in part by intrinsic stretch
reflexes but also is stimulated by the hormone
cholecystokinin (CCK)
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DIGESTION
• Chemical digestion: changes in chemical composition of food as it
travels through the digestive tract; these changes are the result of
hydrolysis
– Digestive enzymes
• Extracellular, organic (protein) catalysts
• Principles of enzyme action
– Specific in their action (Figure 26-7)
– Function optimally at a specific pH (changing pH changes the
shape of the enzyme)
– Most enzymes catalyze a chemical reaction in both directions
– Enzymes are continually being destroyed or eliminated from
the body and must continually be synthesized
– Most digestive enzymes are synthesized as inactive
proenzymes
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DIGESTION: Chemical Digestion (cont’d)
– Carbohydrate digestion (Figure 26-9)
• Carbohydrates are saccharide compounds
• Polysaccharides are hydrolyzed by amylases to form disaccharides
• Carbohydrate digestion begins in the mouth and continues in the small
intestine with amylase
• Final steps of carbohydrate digestion are catalyzed by sucrase, lactase,
and maltase, which are found in the cell membrane of epithelial cells
covering the villi that line the intestinal lumen
– Protein digestion (Figure 26-10)
• Protein compounds are composed of twisted chains of amino acids
• Proteases catalyze hydrolysis of proteins into amino acids
• Main proteases: pepsin in gastric juice, trypsin in pancreatic juice,
peptidases in intestinal brush border
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DIGESTION: Chemical Digestion (cont’d)
– Fat digestion (Figure 26-12)
• Since fats are insoluble in water, they must be
emulsified by bile in small intestine before being
digested (Figure 26-11)
• Bile is made of lecithin and bile salts
• Lecithin and bile salts form micelles around fat
droplets, making them more water soluble and easier
to break down
• Pancreatic lipase is the main fat-digesting enzyme
– Residues of digestion: some compounds of food
resist digestion and are eliminated as feces
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Secretion
• Saliva is made up of:
– Water – helps liquify food into chyme
– mucus – protects mucosa and lubricates food for
easy passage
– salivary amylase – carbohydrate digesting enzyme
that works best in a neutral pH
– Sodium bicarbonate – helps maintain neutral pH
for amylase
Secretion
• Gastric juice
– Made up of water, mucus and digestive enzymes
secreted by gastric glands within gastric pits
– Chief cells: secrete inactive pepsinogen which is
activated by HCl to form pepsin
– Parietal cells: secrete HCl
• Chloride shift and Proton pump help stomach become
more acidic and blood more basic
• Produce IF which binds to B12 and facilitates its
absorption in the SI
Secretion
• Pancreatic juice
– Made up of water and inactive prozymes
(zymogens)
• Trypsinogen is activated by enterokinase to become
trypsin (protein digesting)
• Trypsin activates chymotrypsin (protein), lipases (fats),
nucleases (DNA, RNA) and amylase (starch)
• “reverse chloride shift” secretes H+ into the blood and
bicarbonate into GI lumen; neutralizes the pH of the
chyme
Secretion
• Bile
– Secreted by the liver and stored in the gallbladder
– Stimulated by CCK
– Delivered to the duodenum via the cystic duct into
the common bile duct
– Contains cholesterol, bile pigments and products
of detoxification for removal through feces
• Intestinal juice
– Basic, mucus solution that buffers and lubricates
materials in the intestine
CONTROL OF DIGESTIVE GLAND
SECRETION
• Digestive glands secrete when food is present in the
digestive tract or when it is seen, smelled, or imagined
• Nervous and hormonal mechanisms control the flow of
digestive juices
• Salivary secretion
– Only reflex mechanisms control the secretion of
saliva
– Chemical and mechanical stimuli come from the
presence of food in the mouth
– Olfactory and visual stimuli come from the smell
and sight of food
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CONTROL OF DIGESTIVE GLAND SECRETION (cont.)
• Gastric secretion: three phases (Figure 26-18)
– Cephalic phase: called psychic phase because mental factors
activate the mechanism (sight, smell, taste and thought of food)
in the medulla oblongata
• parasympathetic fibers in branches of the vagus nerve
conduct stimulating efferent impulses to the glands to
produce gastrin (by G cells in the stomach)
– Gastric phase: when products of protein digestion reach the
pyloric portion of the stomach, they stimulate release of gastrin
• Gastrin accelerates secretion of gastric juice, ensuring
enough enzymes are present to digest food; also secreted
with stomach distension
– Intestinal phase: various mechanisms seem to adjust gastric
secretion as chyme passes to and through the intestinal tract;
endocrine reflexes involving gastric inhibitory peptide, secretin,
and CCK inhibit gastric secretions
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CONTROL OF DIGESTIVE GLAND
SECRETION (cont.)
• Pancreatic secretion: stimulated by several hormones
released by intestinal mucosa
– Secretin, stimulated by acid in the duodenum, evokes production of
pancreatic fluid low in enzyme content but high in bicarbonate to
neutralize chyme
– Cholecystokinin (CCK): several functions
• Causes increased exocrine secretion from the pancreas
• Opposes gastrin, thus inhibiting gastric hydrochloric acid secretion
• Stimulates contraction of the gallbladder so that bile is ejected
into the duodenum
• Bile secretion: bile secreted continually by the liver; secretin and CCK
stimulate ejection of bile from the gallbladder
• Intestinal secretion: little is known about how intestinal secretion is
regulated; suggested that the intestinal mucosa is stimulated to release
hormones that increase the production of intestinal juice
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ABSORPTION
• Process of absorption
– Passage of substances through the intestinal
mucosa into the blood or lymph (Figure 26-19)
– Most absorption occurs in the small intestine due
to the large surface area created by the villi and
microvilli
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Absorption
• Mechanisms
– Water is absorbed through osmosis
– Movement through secondary active transport
• Na+ is pumped out of the cell creating a concentration
gradient allowing Na+ to move out of the GI lumen
• Glucose is too large and hydrophilic to move easily
through the membrane; must be transported via Na+
cotransport
• Amino Acids move similarly to glucose
• After absorption, nutrients move through the hepatic
portal system to the liver
• Liver stores excess nutrients and the rest leave via the
hepatic vein into systemic circulation
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Lipid Absorption
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Include triglycerides, phospholipids, steroids, fatsoluble vitamins
Bile salts and lecithin surround fatty acid and
glycerol to form micelles
Micelles approach the brush border, simple lipid
molecules (FA) are released to pass through the
membrane
Inside the cell, the FA reunite to form large fat
molecules and finally form chylomicrons
Absorbed by lacteals and move through lymphatic
system
Water soluble chylomicrons allow fat to travel
through the lymph and enter blood stream.
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Absorption
• Fat soluble vitamins
– A,D,E,K depend on bile salts for absorption
• Water soluble vitamins require transport
• Most drugs are lipid soluble and can pass
through the membrane
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ELIMINATION
• Definition: the expulsion of feces from the digestive tract; act
of expelling feces is called defecation
• Defecation occurs as a result of a reflex brought about by
stimulation of receptors in the rectal mucosa that is produced
when the rectum is distended (Figure 26-23)
• Constipation: contents of the lower part of the colon and
rectum move at a slower than normal rate; extra water is
absorbed from the feces, resulting in a hardened stool
• Diarrhea: result of increased motility of the small intestine,
causing decreased absorption of water and electrolytes and a
watery stool
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