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Chapter 26
Physiology of the Digestive System
Overview of Digestive Function
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Primary function of digestive system—to bring essential nutrients into
the internal environment so that they are available to each cell of the
body
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Mechanisms used to accomplish primary function of digestive system
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Ingestion—food is taken in
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Digestion—breakdown of complex nutrients into simple nutrients
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Motility of gastrointestinal (GI) wall—physically breaks down large chunks of food
material and moves food along the tract
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Secretion of digestive enzymes allows chemical digestion
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Absorption—movement of nutrients through the GI mucosa into the internal
environment
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Elimination—excretion of material that is not absorbed
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Regulation—coordination of the various functions of the digestive system
Digestive tract is functionally an extension of external environment—
material does not truly enter body until it is absorbed into internal
environment
Mechanical Digestion
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Movements of the digestive tract
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Change ingested food from large particles into minute
particles, facilitating chemical digestion
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Churn contents of the GI lumen to mix with digestive juices
and come in contact with the surface of the intestinal
mucosa, facilitating absorption
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Propel food along the alimentary tract, eliminating digestive
waste from the body
Mechanical Digestion
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Mastication
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Reduces size of food particles
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Mixes food with saliva in preparation for swallowing
Deglutition (Figure 26-2)
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Oral stage (mouth to oropharynx)
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Voluntarily controlled
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Formation of a food bolus in the middle of the tongue
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Tongue presses bolus against the palate and food is then moved into the oropharynx
Pharyngeal stage (oropharynx to esophagus)
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Involuntary movement
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To propel bolus from pharynx to esophagus, the mouth, nasopharynx, and larynx must be
blocked
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Combination of contractions and gravity move bolus into esophagus
Esophageal stage (esophagus to stomach)
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Involuntary movement
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Contractions and gravity move bolus through esophagus and into stomach
Mechanical Digestion
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Peristalsis and segmentation
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Two main types of motility produced by the smooth muscle of GI
tract
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Can occur together, in an alternating fashion
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Peristalsis
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Wavelike ripple of the muscle layer of a hollow organ
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Progressive motility that produces forward movement of matter along
the GI tract
Segmentation
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Mixing movement
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Digestive reflexes cause a forward-and-backward movement with a
single segment of the GI tract
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Helps break down food particles, mixes food and digestive juices, and
brings digested food in contact with intestinal mucosa to facilitate
absorption
Mechanical Digestion
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Regulation of motility
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Gastric motility
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Emptying the stomach takes approximately 2 to 6 hours
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Stomach - food is churned and mixed with gastric juices to form
chyme
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Chyme is ejected about every 20 seconds into the duodenum
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Gastric emptying is controlled by hormonal and nervous mechanisms
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Hormonal mechanism—fats in duodenum stimulate release of gastric
inhibitory peptide, which acts to decrease peristalsis of gastric muscle and
slows passage of chyme into duodenum
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Nervous mechanism—enterogastric reflex; receptors in duodenal mucosa
are sensitive to presence of acid and to distention; impulses over sensory
and motor fibers in vagus nerve cause a reflex inhibition of gastric
peristalsis
Mechanical Digestion
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Regulation of motility
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Intestinal motility includes peristalsis and segmentation
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Segmentation in duodenum and upper jejunum mixes chyme with
digestive juices from pancreas, liver, and intestinal mucosa
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Rate of peristalsis picks up as chyme approaches end of jejunum,
moving it through rest of small intestine into large intestine
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After leaving stomach, normally takes approximately 5 hours for
chyme to pass all the way through small intestine
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Peristalsis—regulated in part by intrinsic stretch reflexes; stimulated
by cholecystokinin (CCK)
Chemical Digestion
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Changes in chemical composition of food as it travels
through the digestive tract
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Result of hydrolysis
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Digestive enzymes
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Extracellular, organic (protein) catalysts
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Principles of enzyme action
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Specific in their action
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Function optimally at a specific pH
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Most enzymes catalyze a chemical reaction in both directions
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Enzymes are continually being destroyed or eliminated from the body
and must continually be synthesized
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Most digestive enzymes are synthesized as inactive proenzymes
Chemical Digestion
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Carbohydrate digestion
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Carbohydrates are saccharide compounds
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Polysaccharides are hydrolyzed by amylases to form
disaccharides
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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
intestinal lumen
Chemical Digestion
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Protein compounds are made up of twisted chains of
amino acids
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Proteases catalyze hydrolysis of proteins into intermediate
compounds and, finally, into amino acids
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Main proteases: pepsin in gastric juice, trypsin in pancreatic
juice, peptidases in intestinal brush border
Chemical Digestion
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Fat digestion
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Fats must be emulsified by bile in small intestine before
being digested (Figure 26-11)
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Pancreatic lipase is the main fat-digesting enzyme
Residues of digestion—some compounds of food
resist digestion and are eliminated as feces
Extra Slides Not Covered In Class
Secretion
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Saliva—secreted by salivary glands
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Mucus lubricates food and, with water,
facilitates mixing
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Amylase is an enzyme that begins digestion of
starches; small amount of salivary lipase released,
function uncertain
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Sodium bicarbonate increases the pH for optimal
amylase function
Secretion
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Gastric juice—secreted by gastric glands
Pepsin (secreted as inactive pepsinogen by chief cells)
is a protease that begins the digestion of proteins
 Hydrochloric acid (HCl, secreted by parietal cells)
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HCl decreases the pH of chyme for activation and optimal function of
pepsin (Figure 26-13)
Released actively into the gastric juice by H-K pumps (proton pumps)
Vesicles in the resting parietal cell move to the apical surface when the
cell becomes active—thus increasing the surface area for the process
of secretion (Figure 26-14)
Intrinsic factor (secreted by parietal cells) protects vitamin B12
and later facilitates its absorption (Figure 26-15)
Mucus and water lubricates, protects, and facilitates mixing
of chyme
Secretion
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Pancreatic juice—secreted by acinar and duct
cells of pancreas
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Proteases (e.g., trypsin and chymotrypsin) are
enzymes that digest proteins and polypeptides
Lipases are enzymes that digest emulsified fats
 Nucleases are enzymes that digest nucleic acids such
as DNA and RNA
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Amylase is an enzyme that digests starches
 Sodium bicarbonate increases the pH for optimal
enzyme function; its manufacture also helps restore
normal pH of blood (Figures 26-16 and 26-17)
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Secretion
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Bile—secreted by the liver; stored and concentrated
in gallbladder
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Lecithin and bile salts emulsify fats by encasing them in shells
to form tiny spheres called micelles
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Sodium bicarbonate increases pH for optimal enzyme function
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Cholesterol, products of detoxification, and bile pigments
(e.g., bilirubin) are waste products excreted by liver and
eventually eliminated in feces
Intestinal juice—secreted by cells of intestinal
exocrine cells
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Mucus and water lubricate and aid in continued mixing of chyme
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Sodium bicarbonate increases pH for optimal enzyme function
Control of Digestive Gland Secretion
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Salivary secretion
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Only reflex mechanisms control secretion of saliva
Chemical and mechanical stimuli come from presence of food in the mouth
Olfactory and visual stimuli come from the smell and sight of food
Gastric secretion—three phases (Figure 26-18)
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Cephalic phase—“psychic phase,” because mental factors activate
mechanism; parasympathetic fibers in branches of the vagus nerve
conduct stimulating efferent impulses to the glands; stimulate production
of gastrin (by G cells in the stomach)
Gastric phase—when products of protein digestion reach pyloric portion
of stomach, they stimulate release of gastrin; gastrin accelerates secretion
of gastric juice, ensuring enough enzymes are present to digest food
Intestinal phase—various mechanisms seem to adjust gastric secretion
as chyme passes to and through intestinal tract; endocrine reflexes
involving gastric inhibitory peptide, secretin, and CCK inhibit gastric
secretions
Control of Digestive Gland Secretion
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Pancreatic secretion stimulated by several hormones released
by intestinal mucosa
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Secretin evokes production of pancreatic fluid low in enzyme content
but high in bicarbonate
CCK—several functions
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Causes increased exocrine secretion from pancreas
Opposes gastrin, thus inhibiting gastric HCl secretion
Stimulates contraction of gallbladder so that bile is ejected
into duodenum
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Secretion of bile—secreted continually by liver; secretin and CCK
stimulate ejection of bile from gallbladder
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Intestinal secretion—little is known about how intestinal secretion
is regulated; suggested that intestinal mucosa is stimulated to
release hormones that increase production of intestinal juice
Absorption
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Process of absorption
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Passage of substances through intestinal mucosa into blood or lymph
(Figure 26-19)
Most absorption occurs in small intestine
Mechanisms of absorption
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For some substances such as water, absorption occurs by simple
diffusion or osmosis
Other substances are absorbed through more complex mechanisms
(Figures 26-20 and 26-21)
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Secondary active transport—how sodium is transported
Sodium cotransport (coupled transport)—how glucose is transported
Fatty acids, monoglycerides, and cholesterol are transported with the aid of
bile salts from lumen to absorbing cells of the villi
After food is absorbed, it travels to the liver via the portal system
In summary, most absorption occurs in the small intestine (Figure 26-22)
Elimination
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Elimination—expulsion of feces from digestive tract; act of
expelling feces is called defecation
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Defecation occurs as a result of a reflex brought about by
stimulation of receptors in the rectal mucosa that is
produced when rectum is distended (Figure 26-23)
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Constipation—contents of lower colon and rectum move
at a slower-than-normal rate; extra water is absorbed from
the feces, resulting in a hardened stool
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Diarrhea—result of increased motility of small intestine,
causing decreased absorption of water and electrolytes
and a watery stool
The Big Picture:
Digestion and the Whole Body
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Primary contribution of digestive system to overall
homeostasis is to provide a constant nutrient
concentration in the internal environment
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Secondary roles of digestive system
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Absorption of nutrients
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Teeth and tongue, along with respiratory system and
nervous system, important in producing spoken
language
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Gastric acids aid the immune system by destroying
potentially harmful bacteria
The Big Picture:
Digestion and the Whole Body
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To accomplish its functions, digestive system
needs other systems to contribute
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Regulation of digestive motility and secretion requires
nervous system and endocrine system
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Oxygen for digestive activity needs proper functioning
of respiratory and circulatory systems
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Integumentary and skeletal systems support and
protect digestive organs
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Muscular system is needed for ingestion, mastication,
deglutition, and defecation to occur normally