digestive system introduction
TRANSCRIPT
![Page 1: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/1.jpg)
1
SHS112The Digestive System
(Based on Marieb & Hoehn 8th Ed, Chapter 23)
Arthur Jenkins PhD41.312
Office Hours: Tue 10:30-12:30, Thu 12:30-2:[email protected]
9/9/10 Copyright © 2010 Pearson Edcation, Inc.
Digestive System
• Two groups of organs
1. Alimentary canal (gastrointestinal or GI tract)
• Digests and absorbs food
• Mouth, pharynx, esophagus, stomach,small intestine, and large intestine
Copyright © 2010 Pearson Edcation, Inc.
Digestive System
2. Accessory digestive organs
• Teeth, tongue, gallbladder
• Digestive glands
• Salivary glands
• Liver
• pancreas
Copyright © 2010 Pearson Education, Inc. Figure 23.1
Mouth (oral cavity)Tongue
Esophagus
LiverGallbladder
Anus
DuodenumJejunumIleum
Small intestine
Parotid glandSublingual glandSubmandibulargland
Salivaryglands
Pharynx
StomachPancreas(Spleen)
Transverse colonDescending colonAscending colonCecumSigmoid colonRectumVermiform appendixAnal canal
Largeintestine
![Page 2: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/2.jpg)
2
Copyright © 2010 Pearson Edcation, Inc.
Digestive Processes
• Six essential activities
1. Ingestion
2. Propulsion
3. Mechanical digestion
4. Chemical digestion
5. Absorption
6. Defecation
Copyright © 2010 Pearson Education, Inc. Figure 23.2
FoodIngestion
PropulsionEsophagus
Stomach
PharynxMechanicaldigestion
Chemicaldigestion
• Chewing (mouth)• Churning (stomach)• Segmentation (small intestine)
Smallintestine Largeintestine
Defecation Anus
Feces
Bloodvessel
Lymphvessel
Absorption
• Swallowing (oropharynx)• Peristalsis (esophagus, stomach, small intestine, large intestine)
Mainly H2O
Copyright © 2010 Pearson Education, Inc. Figure 23.3
Frommouth
(b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs.
(a) Peristalsis: Adjacent segments ofalimentary tract organs alternately contractand relax, which moves food along the tractdistally.
Copyright © 2010 Pearson Edcation, Inc.
GI tract regulatory mechanisms
1. Mechanoreceptors and chemoreceptors
• Respond to stretch, changes in osmolarityand pH, and presence of substrate and endproducts of digestion
• Initiate reflexes that
• Activate or inhibit digestive glands
• Stimulate smooth muscle to mix andmove lumen contents
![Page 3: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/3.jpg)
3
Copyright © 2010 Pearson Edcation, Inc.
GI tract regulatory mechanisms
2. Intrinsic and extrinsic controls
• Enteric nerve plexuses (gut brain) initiateshort reflexes in response to stimuli in the GItract
• Long reflexes in response to stimuli inside oroutside the GI tract involve CNS centers andautonomic nerves
• Hormones from cells in the stomach andsmall intestine stimulate target cells in thesame or different organs
Copyright © 2010 Pearson Education, Inc. Figure 23.4
External stimuli(sight, smell, taste,
thought of food)
Central nervous systemand extrinsic autonomic nerves
Afferent impulses Efferent impulses
Long reflexes
Internal(GI tract)stimuli
Chemoreceptors,osmoreceptors, ormechanoreceptors
Local (intrinsic)nerve plexus(“gut brain”)
Effectors:Smooth muscle
or glands
Gastrointestinalwall (site of shortreflexes)
Response:Change in
contractile orsecretory activityLumen of the
alimentary canal
Short reflexes
Copyright © 2010 Pearson Edcation, Inc.
Blood Supply: Splanchnic Circulation
• Arteries
• Hepatic, splenic, and left gastric
• Inferior and superior mesenteric
• Hepatic portal circulation
• Drains nutrient-rich blood from digestiveorgans
• Delivers it to the liver for processing
Copyright © 2010 Pearson Edcation, Inc.
Histology of the Alimentary Canal
• Four basic layers (tunics)
• Mucosa
• Submucosa
• Muscularis externa
• Serosa
![Page 4: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/4.jpg)
4
Copyright © 2010 Pearson Education, Inc. Figure 23.6
Glands in submucosa
Submucosa
LumenMucosa-associatedlymphoid tissue
Duct of gland outsidealimentary canal
Gland in mucosa
NerveArteryVein
Lymphaticvessel Mesentery
Intrinsic nerve plexuses• Myenteric nerve plexus• Submucosal nerve plexus
Mucosa• Epithelium• Lamina propria• Muscularis mucosae
Muscularis externa• Longitudinal muscle • Circular muscleSerosa• Epithelium• Connective tissue
Copyright © 2010 Pearson Edcation, Inc.
Mucosa
• Lines the lumen
• Functions
• Secretes mucus, digestive enzymes andhormones
• Absorbs end products of digestion
• Protects against infectious disease
• Three sublayers: epithelium, lamina propria,and muscularis mucosae
Copyright © 2010 Pearson Edcation, Inc.
Mucosa
• Epithelium
• Simple columnar epithelium and mucus-secreting cells
• Mucus
• Protects digestive organs from enzymes
• Eases food passage
• May secrete enzymes and hormones (e.g., instomach and small intestine)
Copyright © 2010 Pearson Edcation, Inc.
Mucosa
• Lamina propria
• Loose areolar connective tissue
• Capillaries for nourishment and absorption
• Lymphoid follicles (part of MALT)
• Muscularis mucosae: smooth muscle thatproduces local movements of mucosa
![Page 5: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/5.jpg)
5
Copyright © 2010 Pearson Edcation, Inc.
Submucosa and Muscularis Externa
• Submucosa
• Dense connective tissue
• Blood and lymphatic vessels, lymphoidfollicles, and submucosal nerve plexus
Copyright © 2010 Pearson Edcation, Inc.
Submucosa and Muscularis Externa
• Muscularis externa
• Responsible for segmentation and peristalsis
• Inner circular and outer longitudinal layers
• Myenteric nerve plexus
• Sphincters in some regions
Copyright © 2010 Pearson Edcation, Inc.
Serosa
• Visceral peritoneum
• Replaced by the fibrous adventitia in theesophagus
• Retroperitoneal organs have both an adventitiaand serosa
Copyright © 2010 Pearson Education, Inc. Figure 23.6
Glands in submucosa
Submucosa
LumenMucosa-associatedlymphoid tissue
Duct of gland outsidealimentary canal
Gland in mucosa
NerveArteryVein
Lymphaticvessel Mesentery
Intrinsic nerve plexuses• Myenteric nerve plexus• Submucosal nerve plexus
Mucosa• Epithelium• Lamina propria• Muscularis mucosae
Muscularis externa
• Longitudinal muscle • Circular muscleSerosa• Epithelium• Connective tissue
![Page 6: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/6.jpg)
6
Copyright © 2010 Pearson Edcation, Inc.
Enteric Nervous System
• Intrinsic nerve supply of the alimentary canal
• Submucosal nerve plexus
• Regulates glands and smooth muscle in themucosa
• Myenteric nerve plexus
• Controls GI tract motility
Copyright © 2010 Pearson Edcation, Inc.
Enteric Nervous System
• Linked to the CNS via afferent visceral fibers
• Long ANS fibers synapse with entericplexuses
• Sympathetic impulses inhibit secretion andmotility
• Parasympathetic impulses stimulate
Copyright © 2010 Pearson Edcation, Inc.
Mouth
• Oral (buccal) cavity
• Bounded by lips, cheeks, palate, and tongue
• Oral orifice is the anterior opening
• Lined with stratified squamous epithelium
Copyright © 2010 Pearson Education, Inc. Figure 23.7a
UvulaSoft palate Palatoglossal arch
Palatine tonsil
Hard palate
Oral cavity
Tongue
Lingual tonsilOropharynx
EpiglottisHyoid bone
Laryngopharynx
Esophagus
Trachea(a) Sagittal section of the oral cavity and pharynx
![Page 7: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/7.jpg)
7
Copyright © 2010 Pearson Edcation, Inc.
Salivary Glands
• Extrinsic salivary glands (parotid, submandibular,and sublingual)
• Intrinsic (buccal) salivary glands are scattered in theoral mucosa
• Secretion (saliva)
• Cleanses the mouth
• Moistens and dissolves food chemicals
• Aids in bolus formation
• Contains enzymes that begin the breakdown of starch
Copyright © 2010 Pearson Edcation, Inc.
Composition of Saliva
• Secreted by serous and mucous cells
• 97–99.5% water, slightly acidic solution containing
• Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3–
• Salivary amylase and lingual lipase
• Mucin
• Metabolic wastes—urea and uric acid
• Lysozyme, IgA, defensins, and a cyanide compoundprotect against microorganisms
Copyright © 2010 Pearson Edcation, Inc.
Control of Salivation
• Intrinsic glands continuously keep the mouth moist
• Extrinsic salivary glands produce secretions when
• Ingested food stimulates chemoreceptors andmechanoreceptors in the mouth
• Salivatory nuclei in the brain stem send impulsesalong parasympathetic fibers in cranial nerves VIIand IX
• Strong sympathetic stimulation inhibits salivation andresults in dry mouth (xerostomia)
Copyright © 2010 Pearson Edcation, Inc.
Pharynx
• Oropharynx and laryngopharynx
• Allow passage of food, fluids, and air
• Stratified squamous epithelium lining
• Skeletal muscle layers: inner longitudinal,outer pharyngeal constrictors
![Page 8: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/8.jpg)
8
Copyright © 2010 Pearson Edcation, Inc.
Esophagus
• Flat muscular tube from laryngopharynx tostomach
• Pierces diaphragm at esophageal hiatus
• Joins stomach at the cardiac orifice
Copyright © 2010 Pearson Edcation, Inc.
Digestive Processes: Mouth
• Ingestion
• Mechanical digestion
• Mastication is partly voluntary, partly reflexive
• Chemical digestion (salivary amylase andlingual lipase)
• Propulsion
• Deglutition (swallowing)
Copyright © 2010 Pearson Edcation, Inc.
Deglutition
• Involves the tongue, soft palate, pharynx,esophagus, and 22 muscle groups
• Buccal phase
• Voluntary contraction of the tongue
• Pharyngeal-esophageal phase
• Involuntary
• Control center in the medulla and lower pons
Copyright © 2010 Pearson Edcation, Inc. Figure 23.13
Tongue
Trachea
Pharynx
Epiglottis
Glottis
Bolus of food
Epiglottis
Esophagus
Uvula
Bolus
Bolus
Relaxed muscles
Circular musclescontract
Bolus of food
Longitudinal musclescontract
Stomach
Relaxedmuscles
Gastroesophagealsphincter opens
Gastroesophagealsphincter closed
Upper esophageal sphincter iscontracted. During the buccal phase, thetongue presses against the hard palate,forcing the food bolus into the oropharynxwhere the involuntary phase begins.
Food is movedthrough the esophagusto the stomach byperistalsis.
The gastroesophagealsphincter opens, and foodenters the stomach.
The uvula and larynx rise to prevent foodfrom entering respiratory passageways. Thetongue blocks off the mouth. The upperesophageal sphincter relaxes, allowing foodto enter the esophagus.
The constrictor muscles of thepharynx contract, forcing foodinto the esophagus inferiorly. Theupper esophageal sphinctercontracts (closes) after entry.
1 2
4
3
5
![Page 9: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/9.jpg)
9
Copyright © 2010 Pearson Edcation, Inc.
Stomach: Gross Anatomy
• Cardiac region (cardia)
• Surrounds the cardiac orifice
• Fundus
• Dome-shaped region beneath the diaphragm
• Body
• Midportion
• Pyloric region: antrum, pyloric canal, and pylorus
• Pylorus is continuous with the duodenum through thepyloric valve (sphincter)
Copyright © 2010 Pearson Edcation, Inc. Figure 23.14a
Cardia
Esophagus
Pyloric sphincter(valve) at pylorus
Pyloriccanal
Pyloricantrum
Rugae ofmucosa
Body
Lumen
Serosa
Fundus
Lessercurvature
Greatercurvature
Muscularisexterna • Longitudinal layer • Circular layer • Oblique layer
(a)
Duodenum
Copyright © 2010 Pearson Edcation, Inc.
Stomach: Gross Anatomy
• ANS nerve supply
• Sympathetic via splanchnic nerves and celiacplexus
• Parasympathetic via vagus nerve
• Blood supply
• Celiac trunk
• Veins of the hepatic portal system
Copyright © 2010 Pearson Edcation, Inc.
Stomach: Microscopic Anatomy
• Four tunics
• Muscularis and mucosa are modified
• Muscularis externa
• Three layers of smooth muscle
• Inner oblique layer allows stomach to churn,mix, move, and physically break down food
![Page 10: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/10.jpg)
10
Copyright © 2010 Pearson Edcation, Inc. Figure 23.15a
Mucosa
Surfaceepithelium
Lamina propria
Muscularismucosae
Oblique layer
Circular layer
Longitudinallayer
Serosa
(a) Layers of the stomach wall (l.s.)Stomach wall
Muscularis externa(contains myentericplexus)
Submucosa(contains submucosalplexus)
Copyright © 2010 Pearson Edcation, Inc.
Stomach: Microscopic Anatomy
• Mucosa
• Simple columnar epithelium composed ofmucous cells
• Layer of mucus traps bicarbonate-rich fluidbeneath it
• Gastric pits lead into gastric glands
Copyright © 2010 Pearson Edcation, Inc. Figure 23.15b
(b) Enlarged view of gastric pits and gastric glands
Mucous neck cells
Parietal cell
Surface epithelium(mucous cells)
Gastric pits
Chief cell
Enteroendocrine cell
Gastric pit
Gastric gland
Copyright © 2010 Pearson Edcation, Inc.
Gastric Glands
• Cell types
• Mucous neck cells (secrete thin, acidic mucus)
• Parietal cells
• Chief cells
• Enteroendocrine cells
![Page 11: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/11.jpg)
11
Copyright © 2010 Pearson Edcation, Inc. Figure 23.15c
(c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland
Pepsinogen
Mitochondria
PepsinHCl
Chief cell
Enteroendocrinecell
Parietal cell
Copyright © 2010 Pearson Edcation, Inc.
Gastric Gland Secretions
• Glands in the fundus and body produce most of the gastricjuice
• Parietal cell secretions
• HCl
• → pH 1.5–3.5 denatures protein in food, activates pepsin, andkills many bacteria
• Intrinsic factor
• Glycoprotein required for absorption of vitamin B12 in smallintestine
Copyright © 2010 Pearson Edcation, Inc.
Gastric Gland Secretions
• Chief cell secretions
• Inactive enzyme pepsinogen
• Activated to pepsin by HCl and by pepsin itself(a positive feedback mechanism)
Copyright © 2010 Pearson Edcation, Inc.
Gastric Gland Secretions
• Enteroendocrine cells
• Secrete chemical messengers into the laminapropria
• Paracrines
• Serotonin and histamine
• Hormones
• Somatostatin and gastrin
![Page 12: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/12.jpg)
12
Copyright © 2010 Pearson Edcation, Inc.
Mucosal Barrier
• Layer of bicarbonate-rich mucus
• Tight junctions between epithelial cells
• Damaged epithelial cells are quickly replacedby division of stem cells
Copyright © 2010 Pearson Edcation, Inc.
Homeostatic Imbalance
• Gastritis: inflammation caused by anythingthat breaches the mucosal barrier
• Peptic or gastric ulcers: erosion of thestomach wall
• Most are caused by Helicobacter pyloribacteria
Copyright © 2010 Pearson Edcation, Inc. Figure 23.16
Bacteria
Mucosalayer ofstomach
(a) A gastric ulcer lesion (b) H. pylori bacteria
Copyright © 2010 Pearson Edcation, Inc.
Digestive Processes in the Stomach
• Physical digestion
• Denaturation of proteins
• Enzymatic digestion of proteins by pepsin(and rennin in infants)
• Secretes intrinsic factor required forabsorption of vitamin B12
• Lack of intrinsic factor → pernicious anemia
• Delivers chyme to the small intestine
![Page 13: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/13.jpg)
13
Copyright © 2010 Pearson Edcation, Inc.
Regulation of Gastric Secretion
• Neural and hormonal mechanisms
• Stimulatory and inhibitory events occur in threephases:
1. Cephalic (reflex) phase: few minutes prior to foodentry
2. Gastric phase: 3–4 hours after food enters thestomach
3. Intestinal phase: brief stimulatory effect as partiallydigested food enters the duodenum, followed byinhibitory effects (enterogastric reflex andenterogastrones)
Copyright © 2010 Pearson Edcation, Inc. Figure 23.17
Presence of lowpH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty
Distension;presence offatty, acidic,partiallydigested foodin theduodenum
Briefeffect
Intestinal(enteric)gastrinreleaseto blood
Entero-gastricreflex
Release of intestinalhormones (secretin,cholecystokinin, vasoactiveintestinal peptide)
Localreflexes
Vagalnucleiin medullaPyloricsphincter
Stimulate
Inhibit
1
1
2
Stomachsecretoryactivity
Sight and thoughtof food
Stomachdistensionactivatesstretchreceptors
Stimulation oftaste and smellreceptors
Food chemicals(especially peptides and caffeine) and rising pHactivate chemoreceptors
Loss ofappetite,depression
Emotionalupset
Lack ofstimulatoryimpulses toparasym-patheticcenter
Cerebralcortex
Cerebral cortexConditioned reflex
Vagovagalreflexes
Localreflexes
Medulla
G cells
Hypothalamusand medullaoblongata
Vagusnerve
Vagusnerve
Gastrinreleaseto blood
Gastrinsecretiondeclines
G cells
Overridesparasym-patheticcontrols
Sympatheticnervoussystemactivation
1
11
1
2
2
2
Stimulatory events Inhibitory events
Cephalicphase
Gastricphase
Intestinalphase
Excessiveacidity (pH <2) in stomach
Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or irritants in the duodenum
Copyright © 2010 Pearson Edcation, Inc.
Regulation and Mechanism of HCl Secretion
• Three chemicals (ACh, histamine, andgastrin) stimulate parietal cells throughsecond-messenger systems
• All three are necessary for maximum HClsecretion
• Antihistamines block H2 receptors anddecrease HCl release
Copyright © 2010 Pearson Edcation, Inc. Figure 23.18
Stomach lumenChief cell
Parietal cell
Inter-stitialfluid
Carbonicanhydrase
Alkalinetide
HCO3–
Bloodcapillary
CO2
Cl–
CO2 + H2O
H2CO3
HCO3–- Cl–
antiporter
HCO3–
H+
Cl– Cl–l
K+ K+
H+
H+-K+
ATPase
HCI
![Page 14: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/14.jpg)
14
Copyright © 2010 Pearson Edcation, Inc.
Response of the Stomach to Filling
• Stretches to accommodate incoming food
• Reflex-mediated receptive relaxation
• Coordinated by the swallowing center of thebrain stem
• Gastric accommodation
• Plasticity (stress-relaxation response) ofsmooth muscle
Copyright © 2010 Pearson Edcation, Inc.
Gastric Contractile Activity
• Peristaltic waves move toward the pylorus atthe rate of 3 per minute
• Basic electrical rhythm (BER) initiated bypacemaker cells (cells of Cajal)
• Distension and gastrin increase force ofcontraction
Copyright © 2010 Pearson Edcation, Inc.
Gastric Contractile Activity
• Most vigorous near the pylorus
• Chyme is either
• Delivered in ~ 3 ml spurts to the duodenum, or
• Forced backward into the stomach
Copyright © 2010 Pearson Edcation, Inc. Figure 23.19
1 Propulsion: Peristaltic waves move from the fundus toward the pylorus.
2 3 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus.
Retropulsion: The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach.
Pyloricvalveclosed
Pyloricvalveclosed
Pyloricvalveslightlyopened
![Page 15: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/15.jpg)
15
Copyright © 2010 Pearson Edcation, Inc.
Regulation of Gastric Emptying
• As chyme enters the duodenum
• Receptors respond to stretch and chemicalsignals
• Enterogastric reflex and enterogastronesinhibit gastric secretion and duodenal filling
• Carbohydrate-rich chyme moves quicklythrough the duodenum
• Fatty chyme remains in the duodenum6 hours or more
Copyright © 2010 Pearson Edcation, Inc. Figure 23.20
Presence of fatty, hypertonic,acidic chyme in duodenum
Duodenal entero-endocrine cells
Chemoreceptors andstretch receptors
Enterogastrones(secretin, cholecystokinin,vasoactive intestinalpeptide)
Duodenalstimulidecline
Via shortreflexes
Via longreflexes
Entericneurons
Initial stimulus
Physiological response
Result
Contractile force andrate of stomachemptying decline
CNS centers sympathetic activity; parasympathetic activity
Stimulate
Inhibit
Secrete Target
Copyright © 2010 Pearson Edcation, Inc.
Small Intestine: Gross Anatomy
• Major organ of digestion and absorption
• 2–4 m long; from pyloric sphincter toileocecal valve
• Subdivisions
1. Duodenum (retroperitoneal)
2. Jejunum (attached posteriorly by mesentery)
3. Ileum (attached posteriorly by mesentery)
Copyright © 2010 Pearson Edcation, Inc. Figure 23.1
Mouth (oral cavity)Tongue
Esophagus
LiverGallbladder
Anus
DuodenumJejunumIleum
Small intestine
Parotid glandSublingual glandSubmandibulargland
Salivaryglands
Pharynx
StomachPancreas(Spleen)
Transverse colonDescending colonAscending colonCecumSigmoid colonRectumVermiform appendixAnal canal
Largeintestine
![Page 16: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/16.jpg)
16
Copyright © 2010 Pearson Edcation, Inc.
Duodenum
• The bile duct and main pancreatic duct
• Join at the hepatopancreatic ampulla
• Enter the duodenum at the major duodenalpapilla
• Are controlled by the hepatopancreaticsphincter
Copyright © 2010 Pearson Edcation, Inc. Figure 23.21
Jejunum
Mucosawith folds
Cystic duct
DuodenumHepatopancreaticampulla and sphincter
Gallbladder
Right and lefthepatic ducts of liver
Bile duct and sphincter
Main pancreatic ductand sphincter
PancreasTail of pancreas
Head of pancreas
Common hepatic duct
Major duodenalpapilla
Accessory pancreatic duct
Copyright © 2010 Pearson Edcation, Inc.
Structural Modifications
• Increase surface area of proximal part fornutrient absorption
• Circular folds (plicae circulares)
• Villi
• Microvilli
Copyright © 2010 Pearson Edcation, Inc.
Structural Modifications
• Circular folds
• Permanent (~1 cm deep)
• Force chyme to slowly spiral through lumen
![Page 17: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/17.jpg)
17
Copyright © 2010 Pearson Edcation, Inc. Figure 23.22a
Vein carrying blood tohepatic portal vessel
MusclelayersCircularfoldsVilli
(a)
Lumen
Copyright © 2010 Pearson Edcation, Inc.
Structural Modifications
• Villi
• Motile fingerlike extensions (~1 mm high) ofthe mucosa
• Villus epithelium
• Simple columnar absorptive cells(enterocytes)
• Goblet cells
Copyright © 2010 Pearson Edcation, Inc.
Structural Modifications
• Microvilli
• Projections (brush border) of absorptive cells
• Bear brush border enzymes
Copyright © 2010 Pearson Edcation, Inc.
Intestinal Crypts
• Intestinal crypt epithelium
• Secretory cells that produce intestinal juice
• Enteroendocrine cells
• Intraepithelial lymphocytes (IELs)
• Release cytokines that kill infected cells
• Paneth cells
• Secrete antimicrobial agents (defensins and lysozyme)
• Stem cells
![Page 18: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/18.jpg)
18
Copyright © 2010 Pearson Edcation, Inc. Figure 23.22b
(b)
Absorptive cells
Lacteal
Intestinal crypt
Mucosaassociatedlymphoid tissue
MuscularismucosaeDuodenal gland Submucosa
EnteroendocrinecellsVenuleLymphatic vessel
Goblet cellBloodcapillaries
Vilus
Microvilli(brush border)
Copyright © 2010 Pearson Edcation, Inc. Figure 23.3b
(b)
Microvilli
Absorptivecell
Copyright © 2010 Pearson Edcation, Inc.
Submucosa
• Peyer’s patches protect distal part againstbacteria
• Duodenal (Brunner’s) glands of theduodenum secrete alkaline mucus
Copyright © 2010 Pearson Edcation, Inc.
Intestinal Juice
• Secreted in response to distension or irritationof the mucosa
• Slightly alkaline and isotonic with bloodplasma
• Largely water, enzyme-poor, but containsmucus
• Facilitates transport and absorption ofnutrients
![Page 19: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/19.jpg)
19
Copyright © 2010 Pearson Edcation, Inc.
Liver
• Largest gland in the body
• Four lobes—right, left, caudate, and quadrate
Copyright © 2010 Pearson Edcation, Inc.
Liver: Associated Structures
• Lesser omentum anchors liver to stomach
• Hepatic artery and vein at the porta hepatis
• Bile ducts
• Common hepatic duct leaves the liver
• Cystic duct connects to gallbladder
• Bile duct formed by the union of the above twoducts
Copyright © 2010 Pearson Edcation, Inc. Figure 23.21
Jejunum
Mucosawith folds
Cystic duct
DuodenumHepatopancreaticampulla and sphincter
Gallbladder
Right and lefthepatic ducts of liver
Bile duct and sphincter
Main pancreatic ductand sphincter
PancreasTail of pancreas
Head of pancreas
Common hepatic duct
Major duodenalpapilla
Accessory pancreatic duct
Copyright © 2010 Pearson Edcation, Inc.
Liver: Microscopic Anatomy
• Liver lobules
• Hexagonal structural and functional units
• Filter and process nutrient-rich blood
• Composed of plates of hepatocytes (livercells)
• Longitudinal central vein
![Page 20: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/20.jpg)
20
Copyright © 2010 Pearson Edcation, Inc. Figure 23.25a, b
(a) (b)Lobule Central vein Connectivetissue septum
Copyright © 2010 Pearson Edcation, Inc.
Liver: Microscopic Anatomy
• Portal triad at each corner of lobule
• Bile duct receives bile from bile canaliculi
• Portal arteriole is a branch of the hepatic artery
• Hepatic venule is a branch of the hepatic portalvein
• Liver sinusoids are leaky capillaries betweenhepatic plates
• Kupffer cells (hepatic macrophages) in liversinusoids
Copyright © 2010 Pearson Edcation, Inc. Figure 23.25c
(c)
Interlobular veins(to hepatic vein) Central vein
Sinusoids
Portal triad
Plates ofhepatocytes
Portal vein
Fenestratedlining (endothelial cells) of sinusoids
Bile duct (receivesbile from bile canaliculi)
Bile duct
Portal arteriolePortal venuleHepatic
macrophagesin sinusoid walls
Bile canaliculi
Copyright © 2010 Pearson Edcation, Inc.
Liver: Microscopic Anatomy
• Hepatocyte functions
• Process bloodborne nutrients
• Store fat-soluble vitamins
• Perform detoxification
• Produce ~900 ml bile per day
![Page 21: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/21.jpg)
21
Copyright © 2010 Pearson Edcation, Inc.
Bile
• Yellow-green, alkaline solution containing
• Bile salts: cholesterol derivatives that functionin fat emulsification and absorption
• Bilirubin: pigment formed from heme
• Cholesterol, neutral fats, phospholipids, andelectrolytes
Copyright © 2010 Pearson Edcation, Inc.
Bile
• Enterohepatic circulation
• Recycles bile salts
• Bile salts → duodenum → reabsorbed fromileum → hepatic portal blood → liver →secreted into bile
Bile secretion, release & recycling
• Bile salts are major stimulus for bile secretion
• Fat in duodenum -> CCK -> gall bladdercontraction and opens sphincter of Oddi
gall bladder
bile duct
liver
duodenum
terminal ileumbile salts
(via portal vein)
sphincter of Oddi
Copyright © 2010 Pearson Edcation, Inc.
The Gallbladder
• Thin-walled muscular sac on the ventralsurface of the liver
• Stores and concentrates bile by absorbing itswater and ions
• Releases bile via the cystic duct, which flowsinto the bile duct
![Page 22: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/22.jpg)
22
Copyright © 2010 Pearson Edcation, Inc.
Pancreas
• Location
• Mostly retroperitoneal, deep to the greatercurvature of the stomach
• Head is encircled by the duodenum; tail abutsthe spleen
Copyright © 2010 Pearson Edcation, Inc.
Pancreas
• Endocrine function
• Pancreatic islets secrete insulin and glucagon
• Exocrine function
• Acini (clusters of secretory cells) secretepancreatic juice
• Zymogen granules of secretory cells containdigestive enzymes
Copyright © 2010 Pearson Edcation, Inc. Figure 23.26a
Smallduct
Acinar cells
Basementmembrane
Zymogengranules
Roughendoplasmicreticulum
(a)
Copyright © 2010 Pearson Edcation, Inc.
Pancreatic Juice
• Watery alkaline solution (pH 8) neutralizeschyme
• Electrolytes (primarily HCO3–)
• Enzymes
• Amylase, lipases, nucleases are secreted inactive form but require ions or bile for optimalactivity
• Proteases secreted in inactive form
![Page 23: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/23.jpg)
23
Copyright © 2010 Pearson Edcation, Inc.
Pancreatic Juice
• Protease activation in duodenum
• Trypsinogen is activated to trypsin by brushborder enzyme enteropeptidase
• Procarboxypeptidase and chymotrypsinogenare activated by trypsin
Copyright © 2010 Pearson Edcation, Inc. Figure 23.27
Stomach
Pancreas
Epithelialcells
Trypsinogen(inactive)Chymotrypsinogen(inactive)Procarboxypeptidase(inactive)
Trypsin
Chymotrypsin
Carboxypeptidase
Membrane-boundenteropeptidase
Copyright © 2010 Pearson Edcation, Inc.
Regulation of Bile Secretion
• Bile secretion is stimulated by
• Bile salts in enterohepatic circulation
• Secretin from intestinal cells exposed to HCland fatty chyme
Copyright © 2010 Pearson Edcation, Inc.
Regulation of Bile Secretion
• Gallbladder contraction is stimulated by
• Cholecystokinin (CCK) from intestinal cellsexposed to proteins and fat in chyme
• Vagal stimulation (minor stimulus)
• CKK also causes the hepatopancreaticsphincter to relax
![Page 24: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/24.jpg)
24
Copyright © 2010 Pearson Edcation, Inc.
Regulation of Pancreatic Secretion
• CCK induces the secretion of enzyme-richpancreatic juice by acini
• Secretin causes secretion of bicarbonate-richpancreatic juice by duct cells
• Vagal stimulation also causes release ofpancreatic juice (minor stimulus)
Copyright © 2010 Pearson Edcation, Inc. Figure 23.28
Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells.
CCK (red dots) and secretin (yellow dots) enter the bloodstream.
CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3
–-rich pancreatic juice.
Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly.
CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum.
During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.
Slide 1
1
2
3
4
5
6
Copyright © 2010 Pearson Edcation, Inc.
Digestion in the Small Intestine
• Chyme from stomach contains
• Partially digested carbohydrates and proteins
• Undigested fats
• Slow delivery of hypertonic chyme
• Delivery of bile, enzymes, and bicarbonate from theliver and pancreas
• Mixing
Requirements for Digestion and Absorptionin the Small Intestine
Copyright © 2010 Pearson Edcation, Inc.
Motility of the Small Intestine
• Segmentation
• Initiated by intrinsic pacemaker cells
• Mixes and moves contents slowly and steadilytoward the ileocecal valve
• Intensity is altered by long and short reflexes
• Wanes in the late intestinal (fasting) phase
![Page 25: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/25.jpg)
25
Copyright © 2010 Pearson Edcation, Inc.
Motility of the Small Intestine
• Peristalsis
• Initiated by motilin in the late intestinal phase
• Each wave starts distal to the previous (themigrating motility complex)
• Meal remnants, bacteria, and debris aremoved to the large intestine
Copyright © 2010 Pearson Edcation, Inc. Figure 23.3a
From mouth
(a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.
Copyright © 2010 Pearson Edcation, Inc.
Motility of the Small Intestine
• Local enteric neurons coordinate intestinalmotility
• Cholinergic sensory neurons may activate themyenteric plexus
• Causes contraction of the circular muscleproximally and of longitudinal muscle distally
• Forces chyme along the tract
Copyright © 2010 Pearson Edcation, Inc.
Motility of the Small Intestine
• Ileocecal sphincter relaxes and admits chymeinto the large intestine when
• Gastroileal reflex enhances the force ofsegmentation in the ileum
• Gastrin increases the motility of the ileum
• Ileocecal valve flaps close when chyme exertsbackward pressure
![Page 26: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/26.jpg)
26
Copyright © 2010 Pearson Edcation, Inc.
Large Intestine
• Unique features
• Teniae coli
• Three bands of longitudinal smooth muscle in the muscularis
• Haustra
• Pocketlike sacs caused by the tone of the teniae coli
• Epiploic appendages
• Fat-filled pouches of visceral peritoneum
Copyright © 2010 Pearson Edcation, Inc.
Large Intestine
• Regions
• Cecum (pouch with attached vermiformappendix)
• Colon
• Rectum
• Anal canal
Copyright © 2010 Pearson Edcation, Inc. Figure 23.29a
Left colic(splenic) flexure
Transversemesocolon
Epiploicappendages
Descendingcolon
Teniae coli
Sigmoidcolon
Cut edge ofmesentery
External anal sphincter
Rectum
Anal canal(a)
Right colic(hepatic) flexureTransversecolon SuperiormesentericarteryHaustrum
Ascendingcolon IIeum
IIeocecal valve
Vermiform appendix
Cecum
Copyright © 2010 Pearson Edcation, Inc.
Colon
• Ascending colon and descending colon areretroperitoneal
• Transverse colon and sigmoid colon areanchored via mesocolons (mesenteries)
![Page 27: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/27.jpg)
27
Copyright © 2010 Pearson Edcation, Inc. Figure 23.30c
Transverse colon
Greater omentum
Descending colonJejunumMesentery
Transversemesocolon
SigmoidmesocolonSigmoid colon
Ileum
(c)Copyright © 2010 Pearson Edcation, Inc.
Rectum and Anus
• Rectum
• Three rectal valves stop feces from beingpassed with gas
• Anal canal
• The last segment of the large intestine
• Sphincters
• Internal anal sphincter—smooth muscle
• External anal sphincter—skeletal muscle
Copyright © 2010 Pearson Edcation, Inc. Figure 23.29b
(b)
Rectal valveRectum
Anal canal
Levator animuscle
Anus
Anal sinuses
Anal columns
Internal analsphincter
External analsphincter
Hemorrhoidalveins
Pectinate line
Copyright © 2010 Pearson Edcation, Inc.
Bacterial Flora
• Enter from the small intestine or anus
• Colonize the colon
• Ferment indigestible carbohydrates
• Release irritating acids and gases
• Synthesize B complex vitamins and vitamin K
![Page 28: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/28.jpg)
28
Copyright © 2010 Pearson Edcation, Inc.
Functions of the Large Intestine
• Vitamins, water, and electrolytes arereclaimed
• Major function is propulsion of feces towardthe anus
• Colon is not essential for life
Copyright © 2010 Pearson Edcation, Inc.
Motility of the Large Intestine
• Haustral contractions
• Slow segmenting movements
• Haustra sequentially contract in response todistension
Copyright © 2010 Pearson Edcation, Inc.
Motility of the Large Intestine
• Gastrocolic reflex
• Initiated by presence of food in the stomach
• Activates three to four slow powerful peristalticwaves per day in the colon (mass movements)
Copyright © 2010 Pearson Edcation, Inc.
Defecation
• Mass movements force feces into rectum
• Distension initiates spinal defecation reflex
• Parasympathetic signals
• Stimulate contraction of the sigmoid colon andrectum
• Relax the internal anal sphincter
• Conscious control allows relaxation ofexternal anal sphincter
![Page 29: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/29.jpg)
29
Copyright © 2010 Pearson Edcation, Inc. Figure 23.31
Impulses fromcerebral cortex(consciouscontrol)
Voluntary motornerve to externalanal sphincter
External analsphincter(skeletal muscle)
Internal anal sphincter(smooth muscle)
Sensorynerve fibers
Involuntary motor nerve(parasympathetic division)
Stretch receptors in wall
Rectum
Sigmoidcolon
3
1
2
Distension, or stretch, of therectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons.
A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter.
If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so that feces may pass.
Copyright © 2010 Pearson Edcation, Inc.
Chemical Digestion and Absorption ofCarbohydrates
• Digestive enzymes
• Salivary amylase, pancreatic amylase, andbrush border enzymes (dextrinase,glucoamylase, lactase, maltase, and sucrase)
Copyright © 2010 Pearson Edcation, Inc.
Chemical Digestion and Absorption ofCarbohydrates
• Absorption
• Secondary active transport (cotransport) withNa+
• Facilitated diffusion of some monosaccharides
• Enter the capillary beds in the villi
• Transported to the liver via the hepatic portalvein
Copyright © 2010 Pearson Edcation, Inc. Figure 23.32 (1 of 4)
Carbohydrate digestion
• Glucose and galactose are absorbed via cotransport with sodium ions.• Fructose passes via facilitated diffusion.• All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.
Starch and disaccharides
Oligosaccharidesand disaccharides
Lactose Maltose Sucrose
Glucose Fructose
Salivaryamylase
Mouth
Pancreaticamylase
Brush borderenzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)
Smallintestine
Smallintestine
Foodstuff
Galactose
Path of absorptionEnzyme(s)and source
Site ofaction
![Page 30: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/30.jpg)
30
Copyright © 2010 Pearson Edcation, Inc.
Chemical Digestion and Absorption ofProteins
• Enzymes: pepsin in the stomach
• Pancreatic proteases
• Trypsin, chymotrypsin, and carboxypeptidase
• Brush border enzymes
• Aminopeptidases, carboxypeptidases, anddipeptidases
• Absorption of amino acids is coupled to activetransport of Na+
Copyright © 2010 Pearson Edcation, Inc. Figure 23.33
Absorptiveepithelialcell
Apical membrane (microvilli)
Aminoacid carrier
Capillary
Lumen of intestine
Pancreaticproteases
Amino acids of protein fragmentsBrush border enzymes
Na+
Na+
1 Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.
2 The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis).
3 The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts.
Active transport
Passive transport
Copyright © 2010 Pearson Edcation, Inc. Figure 23.32 (2 of 4)
Protein digestion
• Amino acids are absorbed by cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+
and hydrolyzed to amino acids within the cells.
+
• Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.
Smallintestine
Smallintestine
Stomach
Foodstuff
Protein
Large polypeptides
Pepsin(stomach glands)in presence of HCl
Small polypeptides,small peptides
Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)
Amino acids(some dipeptidesand tripeptides)
Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)
Path of absorptionEnzyme(s)and source
Site ofaction
Copyright © 2010 Pearson Edcation, Inc.
Chemical Digestion and Absorption ofLipids
• Pre-treatment—emulsification by bile salts
• Enzymes—pancreatic lipase
• Absorption of glycerol and short chain fattyacids
• Absorbed into the capillary blood in villi
• Transported via the hepatic portal vein
![Page 31: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/31.jpg)
31
Copyright © 2010 Pearson Edcation, Inc.
Chemical Digestion and Absorption ofLipids
• Absorption of monoglycerides and fatty acids
• Cluster with bile salts and lecithin to formmicelles
• Released by micelles to diffuse into epithelialcells
• Combine with proteins to form chylomicrons
• Enter lacteals and are transported to systemiccirculation
Copyright © 2010 Pearson Edcation, Inc. Figure 23.34
Epithelialcells ofsmallintestine
Fat dropletscoated withbile salts
Fat globule
Lacteal
Bile salts
Micelles made up of fatty acids, monoglycerides,and bile salts
1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.
2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.
3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.
4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.
Copyright © 2010 Pearson Edcation, Inc. Figure 23.32 (3 of 4)
Fat digestion
Small intestine
Small intestine
Foodstuff
Unemulsifiedfats
Emulsification by the detergent action of bile salts ductedin from the liver
Pancreatic lipases
Monoglyceridesand fatty acids
Glyceroland
fatty acids
Path of absorptionEnzyme(s)and source
Site ofaction
• Fatty acids and monoglycerides enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis.
• The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.
Copyright © 2010 Pearson Edcation, Inc.
Vitamin Absorption
• In small intestine
• Fat-soluble vitamins (A, D, E, and K) arecarried by micelles and then diffuse intoabsorptive cells
• Water-soluble vitamins (vitamin C and Bvitamins) are absorbed by diffusion or bypassive or active transporters.
• Vitamin B12 binds with intrinsic factor, and isabsorbed by endocytosis
![Page 32: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/32.jpg)
32
Copyright © 2010 Pearson Edcation, Inc.
Vitamin Absorption
• In large intestine
• Vitamin K and B vitamins from bacterialmetabolism are absorbed
Copyright © 2010 Pearson Edcation, Inc.
Electrolyte Absorption
• Mostly along the length of small intestine
• Iron and calcium are absorbed in duodenum
• Na+ is coupled with absorption of glucose andamino acids
• Ionic iron is stored in mucosal cells with ferritin
• K+ diffuses in response to osmotic gradients
• Ca2+ absorption is regulated by vitamin D andparathyroid hormone (PTH)
Water Absorption
• 95% is absorbed in the small intestine byosmosis
• Net osmosis occurs whenever a concentrationgradient is established by active transport ofsolutes
• Water uptake is coupled with solute uptake
Copyright © 2010 Pearson Edcation, Inc.
Water fluxes
Fluid drunk
Saliva
Gastric Juice
Pancreatic Juice
Bile
Intestinal Juice
Sources of FluidsTotal = 9.5 l/day
Small Intestine
Large Intestine
AbsorptionTotal = 9.35 l/day
![Page 33: Digestive System Introduction](https://reader034.vdocuments.net/reader034/viewer/2022042518/5525eac2550346586f8b4a2f/html5/thumbnails/33.jpg)
33
Copyright © 2010 Pearson Edcation, Inc.
Malabsorption of Nutrients
• Causes
• Anything that interferes with delivery of bile orpancreatic juice
• Damaged intestinal mucosa (e.g., bacterialinfection)
Copyright © 2010 Pearson Edcation, Inc.
Malabsorption of Nutrients
• Gluten-sensitive enteropathy (celiac disease)
• Gluten damages the intestinal villi and brushborder
• Treated by eliminating gluten from the diet (allgrains but rice and corn)