Download Chapter 24: Digestive System

Chapter 24: The Digestive System
BIO 211 Lecture
Instructor: Dr. Gollwitzer
1
• Today in class we will:
– Describe the general functions of the digestive system
– Discuss how enzymes work
– Discuss the movement of digestive materials through the
digestive tract
– Discuss the mechanisms that control digestive functions
– Begin our discussion on the major functions of the organs,
regions and anatomical structures of the digestive tract
• Oral cavity
– Tongue
– Saliva
– Mastication
• Pharynx
– Deglutition
– Swallowing reflex
• Esophagus
• Stomach
– Secretory structures and secretions
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Components and Primary Functions
Figure 24-1
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Digestive System
• Provides fuel that keeps body running
– Glucose
– ATP
• Provides building blocks needed for cell growth
and repair
– Monosaccharides (carbohydrates, CHOs)
– Amino acids (proteins)
– Monoglycerides and fatty acids (lipids)
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Functions of the Digestive System
•
•
•
•
•
•
Ingestion
Mechanical processing
Digestion
Secretion
Absorption
Excretion
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Functions of the Digestive System
• Ingestion
– Entry of materials into digestive tract via mouth
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Functions of the Digestive System
• Mechanical processing
– = Crushing/shearing of food
– Makes materials easier to move along GI tract
– Increases surface area of available to enzymes
– May/may not be required before ingestion
• Not for liquids
• For solids
– Teeth- tear, mash food (mastication)
– Tongue – squashes, compacts food
– Stomach and intestines – swirl, mix, churn
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Functions of the Digestive System
• Digestion
– = Chemical breakdown of food into small organic
fragments that can be absorbed by digestive
epithelium
– Simple molecules absorbed intact, e.g., glucose
– Larger molecules must be broken down by
digestive enzymes (e.g., polysaccharides, proteins,
triglycerides)
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Functions of the Digestive System
• Secretion
– = Release of water, acids, enzymes, buffers, salts
into digestive tract
– By:
• Digestive tract epithelium
• Glandular organs, e.g., pancreas
– Into digestive tract
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Functions of the Digestive System
• Absorption
– Movement of organic substrates, electrolytes
(ions), vitamins, water
– Across digestive epithelium and into interstitial
fluid around digestive tract
– Surface area greatly increased by folds in lining of
digestive tract
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Functions of the Digestive System
• Excretion
– Digestive tract and glandular organs  waste
products into lumen of tract
– Waste mixes with indigestible residue of digestive
process
– Leaves body as feces eliminated through
defecation
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Lining of the Digestive Tract
• Protects surrounding tissues against:
– Corrosion from digestive acids and enzymes
– Mechanical stresses, abrasion
– Bacteria swallowed with food that are found
naturally in digestive tract
12
Enzymes
• Are catalysts:
– Proteins that lower the activation energy of a
chemical reaction
– Not changed or used up in the reaction
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How Enzymes Work
Figure 2–21
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Movement of Digestive Materials
• By smooth muscle layers of digestive tract
• Pacesetter cells
– Smooth muscle cells in:
• Muscularis mucosae
• Muscularis externa
– Contract spontaneously
– Trigger rhythmic waves of contraction that
spread through digestive tract
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Figure 24-3
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Movement of Digestive Materials
• Muscularis externa
– Involved in:
• Peristalsis
• Segmentation
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Movement of Digestive Materials
• Peristalsis
– Waves of muscular contractions
– Move bolus (small, oval mass of digestive contents)
along GI tract
– Process:
• Circular muscles behind bolus contract, while circular
muscles ahead of bolus relax
• Longitudinal muscles ahead of bolus contract (shortens
adjacent segments)
• Bolus forced forward
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Figure 24-4, 7th edition
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Movement of Digestive Materials
• Mass movements
– Powerful peristaltic contractions
– Move GI contents from transverse colon through
rest of large intestine ( bowel movements)
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Movement of Digestive Materials
• Segmentation
– Regional mechanical processing (e.g., haustral
churning in large intestine)
– Cycles of contractions
• Churn and break up the bolus
• Mix contents with intestinal secretions
– Not a set pattern so does not push materials
along tract
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Control of Digestive Functions
• Regulated by 3 types of mechanisms
– Neural
– Hormonal
– Local
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Figure 24-5, 7th edition
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Neural Mechanisms
• Control
– Movement of materials along digestive tract
• Local peristaltic movements
– Control small segments (via short reflex to myenteric plexus)
• Large-scale peristaltic waves
– Control movement from one region of digestive tract to another
(via long reflex to CNS)
– Digestive gland secretions
• Exocrine secretory cells  buffers, acids, enzymes
• Enteroendocrine cells  hormones
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Hormonal Mechanisms
• Via hormones produced by digestive tract
• Hormones
– Are peptides (e.g., gastrin, secretin, cholecystokinin CCK,
gastric inhibitory peptide GIP)
– Produced by enteroendocrine cells in stomach (gastrin) or
duodenum (secretin, CCK, GIP)
– Target cells in stomach, small intestine, liver, pancreas
– Reach targets via bloodstream
– Affect every aspect of digestive function
– Can stimulate or inhibit smooth muscle cells’ responses to
neural commands
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Local Mechanisms
• Via local factors, e.g.,
– pH, chemical messengers, e.g., prostaglandins,
histamine
• Coordinate response to local conditions
• Affect only that portion of tract, e.g.,
– Lamina propria of stomach releases histamine 
secretion of HCl by parietal cells
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Oral Cavity
• Tongue
– Functions
• Mechanical processing - compresses, abrades food
• Manipulation – assists in chewing, preparing material
for swallowing
• Sensory analysis - by touch, temperature, taste
receptors
• Secretion
– Mucins  mucous layer
– Lingual lipase  breaks down lipids
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Oral Cavity
• Saliva
– Produced by 3 pairs of salivary glands
• Submandibulars  70 %
• Parotids  25%
• Sublinguals  5%
– Components
• Water  99.4%
• Remaining 0.6%:
–
–
–
–
Electrolytes (Na, Cl, HCO3)
Buffers  pH near 7.0
Glycoproteins (mucins)  lubricating action
Antibodies and digestive enzymes – help control oral bacteria
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Oral Cavity
• Saliva (continued)
– Functions
• Lubricates mouth and materials in it
• Dissolves chemicals that stimulate taste buds
• Initiates digestion
– Of complex carbohydrates via salivary amylase
– Of lipids via lingual lipase (from tongue glands)
– Control of secretions
• By ANS
• Secretions increased by:
– Chewing with empty mouth
– Smell of/thinking about food
– Irritating/unpleasant stimuli, nausea
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Oral Cavity
• Mastication (chewing)
– Combination of mandibular movements by muscles of
mastication (elevation/depression, protraction/ retraction,
medial/lateral movement)
– Forces food back and forth across oral cavity; also involves
muscles of cheeks, lips, tongue
– Ingested material shredded and moistened with salivary
secretions
– Tongue compacts food into bolus
– Compact, moist bolus easy to swallow
• No nutrients absorbed in oral cavity
• But lipid-soluble drugs can dissolve, e.g.,
nitroglycerine
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Pharynx
• Shared passage for food and air
• Connects nasal and oral passages to
respiratory and digestive systems, respectively
• Lymphoid tissue (tonsils) contributes to body’s
defense system
• Muscles cooperate with muscles of oral cavity
and esophagus to initiate deglutition
(swallowing)
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Pharynx
• Deglutition
– Complex process
– Control
• Initiation voluntary but proceeds involuntarily
• Also occurs unconsciously as result of saliva collecting
at back of mouth
– Occurs approximately every 40 seconds
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Pharynx
• Deglutition (continued)
– Swallowing reflex
• Begins when bolus stimulates tactile receptors on
posterior palate
• Info relayed to swallowing center in brain  motor
commands to:
– Larynx  epiglottis covers entrance to airway (glottis)
– Uvula and soft palate  block backward passage
– Pharyngeal muscles  propel bolus into esophagus
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Figure 24-11
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Esophagus
• Primary function
– Carries solid food and liquids to stomach from oral cavity
and pharynx
• Bolus pushed through esophagus/toward stomach by
peristaltic wave (under CNS control)
• Approach of bolus triggers esophageal sphincter and
bolus enters stomach
• Transit time through esophagus
– Liquids = 2 sec
– Bolus = 9 sec
– Dry or poorly lubricated bolus much slower, may need
second peristaltic wave to push into stomach
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Stomach
• Primarily a holding tank
– Food saturated with gastric juices, exposed to
stomach acids, and digestive effects of enzyme
pepsin
– Preliminary digestive steps occur here
• Can distend a lot as it receives food because of
rugae (temporary folds)
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Stomach
• 4 major functions
– Storage of ingested food
– Mechanical breakdown of ingested food
– Disruption of chemical bonds in food through
actions of acids/enzymes
– Production of intrinsic factor
• Glycoprotein required for vitamin B12 absorption in
small intestine
– Essential for hematopoiesis and synthesis of bone proteins
• Only essential function of stomach
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Stomach
• Produces chyme
– Viscous, acidic soupy mixture of partially digested
food
– Formed from ingested substances + stomach gland
secretions
• Has extra layers of muscle to assist in
churning/mixing
– Muscularis mucosae – outer, circular layer
– Muscularis externa – inner, oblique layer
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Stomach
• Secretory sheet
– Simple columnar epithelium that covers surface
– Produces alkaline mucous layer
• Protects epithelial cells against acids and enzymes in
gastric lumen
• Gastric pits
– Openings to gastric glands
– Open onto gastric surface
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Figure 24-13
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Gastric Glands
• In stomach fundus and body
•  gastric juice with enzymes and acid
• 2 types of secretory cells
– Parietal cells
– Chief cells
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Parietal Cells
• Produce
– Intrinsic factor
• Required for absorption of vitamin B12
– Required for synthesis of bone proteins and hematopoiesis
– HCl
•
•
•
•
•
•
Keeps stomach contents at pH 1.5 – 2
Does not digest chyme
Kills most microorganisms in food
Denatures protein and inactivates most enzymes in food
Breaks down plant cell walls and CT in meat
Low pH essential for production of pepsin – protein
digesting enzyme
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HCl
• Very strong acid
• H+ and Cl- transported separately into stomach
lumen by different mechanisms
– Otherwise would destroy cell
• H2O + CO2  H2CO3  HCO3- + H+
• H+ actively transported into lumen of gastric
gland in stomach
• HCO3- moves into interstitial fluid in exchange
for Cl- which diffuses across cell and exits into
lumen of gastric gland in stomach
• H+ and Cl-  HCl in stomach lumen
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Figure 24-14
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Chief Cells
• Produce
– Enzymes important for digesting milk in
newborns
• Rennin – coagulates milk proteins
• Gastric lipase – initiates digestion of milk fats
– Pepsinogen
• Inactive proenzyme
• Converted to pepsin (active proteolytic/
protein-digesting enzyme) by HCl
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Pyloric Glands
• In stomach pylorus
• 2 Types of glands
– Mucus-secreting cells/glands  mucous secretion
– Enteroendocrine glands  hormones
e.g., G cells  gastrin that stimulates:
• Secretion by parietal and chief cells
• Contractions of gastric wall that mix/stir gastric
contents
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Figure 24-13
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Peptic Ulcers
• Result from:
– Excessive acid production
– Inadequate production of alkaline mucus that
protects epithelium from acid
• Digestive acids and enzymes erode:
– Lining of stomach (gastric ulcer)
– Proximal small intestine (duodenal ulcer)
• 80% caused by gastric bacterial infection by
Heliobacter pylori (H. pylori)
– Treat with antibiotic drugs that inhibit acid
production
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• Today in class we will:
– Continue our discussion on the major functions of the organs,
regions and anatomical structures of the digestive tract
• Stomach
– Regulation of gastric secretory activity
» Cephalic, gastric and intestinal phases, associated events,
mechanisms and hormones
– Digestion and absorption
• Small intestine
–
–
–
–
–
–
–
–
Digestion, nutrient absorption and associated structures
Surface characteristics
Intestinal secretions and associated structures
Intestinal movements
Emesis
Associated glandular organs (Pancreas, liver and gallbladder)
Mechanisms that coordinate digestive glands
Coordination of absorption
49
Regulation of Gastric Secretory Activity
• Gastric secretions
– HCl
– Enzymes
• Controlled via production of:
– Hormones (e.g., G cells  Gastrin)
– HCl (by parietal cells)
– Enzymes (e.g., chief cells  pepsinogen)
• Involves three overlapping phases
– Cephalic phase
– Gastric phase
– Intestinal phase
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Regulation of Gastric Activity
Figure 24-15
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Regulation of Gastric Secretory Activity
• Phases named for location of control center
• Cephalic phase
– Controlled by CNS (brain and spinal cord)
• Gastric phase
– Regulated by short reflexes of ANS
• i.e.,enteric nervous system (ENS) within parasympathetic sytem
– Involves submucosal and myenteric plexuses
– Coordinated in stomach wall
• Intestinal phase
– Regulated by intestinal hormones (e.g. CCK, GIP, secretin)
from enteroendocrine cells
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Cephalic Phase
• Directed by CNS
• Prepares stomach to receive food
• Begins when you see, smell, think about food
– Exaggerated by anger/hostility
– Inhibited by anxiety, stress, fear
• Short duration (minutes)
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Cephalic Phase
• Stimulates CNS  ANS (parasympathetic) 
vagus nerve (CN X)  submucosal plexus of
stomach  innervates mucous, parietal, chief, G
cells of stomach
• Increases production of gastric juice by
stimulation of:
– Mucous cells  mucus
– Chief cells  pepsinogen ( pepsin)
– Parietal cells  HCl
• Stimulates G cells  gastrin
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Regulation of Gastric Activity
Figure 24-15
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Gastric Phase
•
•
•
•
Regulated by short reflexes of ENS
Begins with arrival of food in stomach
Long duration (3-4 hours)
Purpose of this phase
– Enhances secretions started in cephalic stage
– Homogenizes and acidifies chyme
– Initiates protein digestion by pepsin
• 3 mechanisms
– Neural
– Hormonal
– Local
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Gastric Phase
• Neural mechanism
– Short reflexes triggered by:
• Stomach distention that stimulates stretch receptors
• Increased pH of gastric contents that stimulates
chemoreceptors (also stimulated by proteins, alcohol, and
caffeines)
– Receptors stimulate ENS (submucosal and myenteric
plexuses) which then stimulates:
•
•
•
•
Mucous cells  mucus
Chief cells  pepsinogen
Parietal cells  HCl
Mixing waves in muscularis externa
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Gastric Phase
• Hormonal mechanism
– G cells  gastrin, stimulated by:
• ENS (see previous)
• Partially digested proteins in chyme
– Gastrin (via system circulation) stimulates:
• Chief cells  pepsinogen
• Parietal cells  HCL
• Mixing waves in muscularis externa of stomach and
intestinal tract
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Gastric Phase
• Local mechanism
– Filling stomach stimulates mast cells in CT of lamina
propria  histamine which stimulates parietal cells
 HCl
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Regulation of Gastric Activity
Figure 24-15
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Intestinal Phase
• Begins when chyme enters small intestine (after several
hours of gastric mixing)
• Long duration (hours)
• Purpose of this phase
– Controls rate of gastric emptying (entry of chyme into
duodenum)
• Pylorus contracts
• Small quantity of chyme squirts through pyloric sphincter into small
intestine
– Ensures small intestine functions efficiently (secretion,
digestion, absorption)
– Triggers events that affect/coordinate activities of stomach
(generally inhibits its activity), intestinal tract, pancreas, liver,
gallbladder
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Intestinal Phase
• Neural mechanisms
– Chyme distends duodenum, stimulates
• Stretch receptors
• Chemoreceptors
– Receptors trigger
• Enterogastric reflex (to myenteric plexus)
– Inhibits gastrin production and gastric contractions
– Contracts pyloric sphincter
– Prevents further discharge of chyme into duodenum
• Local reflexes  increase mucous production  protects
duodenal lining from acids/enzymes
62
Intestinal Phase
• Hormonal mechanisms
– Presence of lipids and carbohydrates  secretion of
hormones by enteroendocrine cells in duodenum
• Cholecystokinin (CCK)  decreases gastric acid/enzyme
secretion
• Gastric inhibitory peptide (GIP)  decreases gastric
secretions, contraction
– Partially digested proteins stimulate G cells in
duodenum  gastrin  stomach  increase acid
and enzyme production (feedback mechanism)
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Intestinal Phase
• Hormonal mechanisms (continued)
– pH < 4.5 stimulates enteroendocrine cells  secretin

• Inhibits chief cells
• Inhibits parietal cells
• Stimulates pancreas  buffers that protect duodenum by
neutralizing acid in chyme (inc pH)
• Stimulates liver  bile secretion
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Regulation of Gastric Activity
Figure 24-15
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Digestion in Stomach
• At pH > 4.5 (first 1-2 hours post meal)
– Proteins – preliminary digestion by pepsin
– Carbohydrates (by salivary amylase) and lipids (by
lingual lipase) – digestion permitted until pH<4.5
• At pH < 2
– Pepsin increases  protein digestion begun but
not completed (limited time)
– Complex proteins  smaller peptides and
polypeptide chains
66
Absorption in Stomach
• No nutrients
– Epithelial cells covered in mucous, so not directly exposed
to chyme
– Epithelial cells lack specialized transport mechanisms
– Gastric lining impermeable to water
– Digestion not completed
• Alcohol
– Lipid soluble; diffuses through mucous, enters lipid
membranes of epithelial cells
• Some drugs
– Aspirin - lipid soluble
• Prolonged use  gastric bleeding
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Small Intestine
• Plays key role in:
– Digestion
– Nutrient absorption
– Water absorption
68
Small Intestine
• Digestion
– Completed in SI (small intestine)
• Starts in mouth (lipids and CHOs) and stomach (proteins)
– Most enzymes for digestion come from accessory
organs
• Pancreas  digestive enzymes and buffers
• Liver  bile = buffers and bile salts (emulsify lipids and
facilitate digestion/absorption of lipids)  stored in
gallbladder  SI
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Small Intestine
• Nutrient absorption
– SI absorbs 90% of nutrients (remainder absorbed in
LI)
– Epithelial surface adapted for absorption
• Surface area increased 600X by:
– Plicae (transverse folds)
– Villi with microvilli
• Extensive capillary network in villi
–  Hepatic portal circulation  liver
» Adjusts nutrient concentrations of blood before it reaches
systemic circulation
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Intestinal Villi
• Fingerlike projections of mucosa
• Covered by simple columnar epithelium with microvilli (brush
border)
• Interior contains:
–
–
–
–
Lamina propria
Capillary network; originates in vascular network in submucosa
Nerve endings
Lacteals (lymphatic capillaries)
• Transport materials that cannot enter blood capillaries (i.e., FAs)
– Smooth muscle cells
– Move back and forth
• Exposed to liquefied intestinal contents by contractions of:
– Smooth muscle cells in villi
– Muscularis mucosae in mucosa
• Movement also squeezes lacteals and helps move lymph out of
villi
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Figure 24-17
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Small Intestine
• Surface characteristics
– Vary over length of SI and parallel absorptive activity
– Duodenum
• Receives chyme, neutralizes acids, primary site for digestion
• Few plicae, small villi
– Jejunum
• Primary site for absorption
• Prominent plicae and villi
• Plicae and villi gradually decrease in size distally
– Ileum
• Little/no nutrient absorption
• No plicae
73
Intestinal Secretions/Structures
• Intestinal juice
– Watery fluid
– Source
• Osmosis of water out of mucosa into
concentrated chyme
• Secreted by intestinal cells/glands
– Functions
• Moistens chyme
• Assists in buffering acids
• Liquefies digestive enzymes from pancreas and
products of digestion
74
Intestinal Secretions/Structures
• Goblet cells
– Unicellular exocrine structures
– Interspersed between columnar epithelial cells
– Secrete mucins onto intestinal surfaces
• Intestinal glands
– At base of villi
– Contain enteroendocrine cells  hormones that
coordinate digestive functions
75
Intestinal Secretions/Structures
• Duodenal glands
– Produce:
• Mucus
– Protects epithelium from enzymes and acidity of
chyme
– Contains buffers that elevate pH of chyme (over length
of duodenum, pH goes from 1-2 to 7-8)
• Urgastrone
– Inhibits gastric acid production by stomach
– Stimulates division of epithelial stem cells of digestive
tract; epithelial cells replaced every 3-7 days
76
Intestinal Secretions/Structures
• Brush border enzymes
– Membrane proteins on surface of intestinal
microvilli
– Break down materials that come in contact with
brush border
– Released into lumen by disintegrated epithelial cells
that shed at intestinal surface
– e.g., enterokinase  activates key pancreatic
proenzyme, trypsinogen
77
Intestinal Movements
• Stimulation of stretch receptors in stomach accelerates
movement along/through SI
• Chyme moved through duodenum by short, local
reflexes
• Gastroenteric reflex
– Stimulates motility and secretion along entire SI
• Gastroileal reflex
– Triggers relaxation of ileocecal valve (at entrance to large
intestine)
• Digestive tract hormones enhance or suppress reflexes
78
Emesis (Vomiting)
• Chemical or mechanical irritation (of pharynx,
esophagus, stomach, proximal small intestine):
– Increases digestive fluid secretion, including saliva
(buffers stomach acid, protects teeth)
– Triggers vomiting reflex (vomiting center in medulla
oblongata)
– Intestinal contents eliminated as quickly as possible
• Pylorus relaxes
• Contents of duodenum and proximal jejunum discharged into
stomach by strong peristaltic waves toward the stomach (rather than
toward the ileum)
• Vomiting occurs as stomach regurgitates its contents
through esophagus and pharynx
79
Glandular Organs Associated
With Small Intestine
• Pancreas
• Liver
• Gall bladder
80
Pancreas
• Endocrine (1%)
– Cells located in pancreatic islets
• Alpha cells  glucagon  increase blood glucose
• Beta cells  insulin  decrease blood glucose
• Exocrine (99%)
– Acinar cells  pancreatic enzymes
– Epithelial cells  water and ions that assist in
diluting and buffering acids in chyme
– Enzymes + water + ions = pancreatic juice
81
Pancreas Exocrine Secretory Activity
• Controlled by duodenal hormones triggered by
chyme
– Secretin  pancreatic secretion of watery, buffer
solution (pH 7.5 – 8.8) that helps raise pH
– CCK  production and secretion of pancreatic
enzymes
(also occurs during cephalic stage before food
reaches stomach; head start important so enzymes
can be synthesized before chyme reaches
duodenum)
82
Pancreas Exocrine Secretory Activity
• Pancreatic enzymes
– Pancreatic amylase
• Breaks down starches (similar to salivary amylase)
– Pancreatic lipase
• Breaks down complex lipids  FAs that can be absorbed
(similar to lingual lipase)
– Nucleases
• Break down nucleic acids (DNA, RNA)
– Proteolytic enzymes
• 70% of pancreatic enzymes
• Digest proteins; attack peptide bonds
– Proteases – break apart large protein complexes
– Peptidases – break small peptide chains into individual amino
acids
83
Pancreatic Proteolytic Enzymes
• Secreted as inactive proenzymes
– Proenzymes converted to active enzymes after they
reach small intestine
– Protects secretory cells from destruction by own
enzyme products
84
Pancreatic Proteolytic Enzymes
Proenzyme
Enzyme
Catalyst
Active Enzyme
Trypsinogen
Enterokinase
(brush border of
duodenum)
Trypsin
Trypsin
Chymotrypsinogen
Chymotrypsin
Procarboxypeptidase Trypsin
Carboxypeptidase
Proelastase
Elastase
Trypsin
85
Liver
• Can’t live without your liver
• >200 functions!
• 3 General functions
– Metabolic regulation
– Hematological regulation
– Bile production
86
Liver: Metabolic Regulation
• Liver = primary organ involved in regulating
composition of blood
• All blood leaving absorptive surfaces of digestive tract
 hepatic portal system  liver
• Liver cells adjust blood composition before it reaches
systemic circulation
– Extract toxins
– Extract/replenish nutrients (e.g., CHOs, lipids)
• Excess removed and stored (glucose  glycogen, TGs and FAs
 lipids)
• Deficiencies corrected by:
– Mobilizing stored reserves (glycogen  glucose, lipids  FAs
and TGs
– Performing synthetic activities (e.g. glucose from AAs –
gluconeogenesis)
87
Liver: Metabolic Regulation
• CHO metabolism
– Hepatocytes stabilize blood glucose levels
– When blood glucose decreases
• Breaks down glycogen  glucose
• Synthesizes glucose from lipids or amino acids
(gluconeogenesis)
– When blood glucose increases
• Glucose removed from blood
– Stored as glycogen
– Used to synthesize lipids, stored in liver or other tissues
– Regulated by hormones (insulin, glucagon from
pancreas)
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Liver: Metabolic Regulation
• Lipid metabolism
– Hepatocytes regulate circulating TGs, FAs,
cholesterol
– When TGs and FAs decrease
• Breaks down lipid reserves and releases them into
bloodstream
– When TGs and FAs increase
• Removed for storage
89
Liver: Metabolic Regulation
• Amino acid metabolism
– Hepatocytes remove excess amino acids from
bloodstream
• Converted to lipids or glucose and stored
• Used to synthesize proteins
• Drug inactivation
– Removes and metabolizes circulating drugs; limits
duration of effects
• Storage of fat soluble vitamins (D, E, A, and K)
and B12
90
Liver: Metabolic Regulation
• Removal of waste products
– Toxins, various waste products removed from blood
for inactivation, storage, excretion
– e.g., when amino acids used to make CHOs or lipids
 NH2; creates ammonia (NH3, toxic waste
product); liver converts to urea (harmless and
excreted at kidneys)
• Conversion of Iron to ferretin (a protein-iron
complex) and storage
91
Liver: Hematological Regulation
• Liver = largest blood reservoir in body; receives
25% of cardiac output of blood
• Performs several hematological functions
– Removes old/damaged RBCs, cellular debris,
pathogens from bloodstream
– Synthesizes plasma proteins (e.g., for clotting,
transport)
– Removes/metabolizes circulating hormones
– Removes antibodies
– Removes or stores lipid-soluble toxins (e.g., DDT)
92
Liver: Bile Production
• Bile
– Primarily water, minor amounts of:
•
•
•
•
Ions
Bilirubin = pigment derived from hemoglobin
Cholesterol
Bile salts
– Combination of bile, FAs, and lipids
– Synthesized from cholesterol in the liver
– Emulsifying agents that break down lipids
– Excreted into:
• Gallbladder (in absence of chyme)
• Lumen of duodenum (in presence of chyme)
93
Liver: Bile Production
• Bile excretion
– Chyme enters duodenum
– Triggers production of intestinal hormone CCK
– CCK
• Relaxes hepatopancreatic sphincter
• Stimulates contractions in walls of gallbladder
• Pushes bile through common bile duct into duodenum
94
Function of Bile
• Emulsification
– Mechanical processing in stomach creates large drops
containing lipids (because lipids not water-soluble)
– Pancreatic lipase not lipid-soluble
• Can only interact with lipids at surface of droplet, not inside
– Bile salts
• Break droplets apart = emulsification
– Increases surface area available for enzymes
• Facilitate interaction between lipids and pancreatic lipase
• Promote lipid absorption by ileum
• Enter hepatic portal circulation and recycle through liver
(enterohepatic circulation = liver  intestine  liver)
95
Liver Damage or Disease
• Serious threat to life
• Liver has limited ability to regenerate after
injury; depends on normal vascular pattern
being established
• Cirrhosis
– Replacement of lobules by fibrous tissue
• Hepatitis
– Caused by viral infections
96
Gallbladder
• 2 Major functions
– Bile storage
• When bile cannot enter common bile duct (when
hepatopancreatic sphincter closed) it enters cystic duct
and is stored in gallbladder
– Bile modification
• While stored, much of water absorbed, bile salts
become more concentrated
97
Gallbladder
• Gallstones
– Form when bile is too concentrated  crystals on
insoluble minerals and salts
– If get too large can damage gallbladder wall or
block cystic or common bile duct
• Gallbladder
– May become infected, inflamed, perforated and
need to be surgically removed (cholecystectomy)
•  Dilute bile
98
Coordination of Digestive Glands
• Centered around duodenum
– Where acids neutralized and enzymes added)
• Involves combination of neural and hormonal
mechanisms
• Neural mechanisms
– Prepare digestive tract for activity, or
– Inhibit gastrointestinal activity
– Coordinate movement of materials through
digestive tract (via enterogastric, gastroenteric,
gastroileal reflexes)
99
Coordination of Digestive Glands
• Hormonal mechanisms
>>>(see Figure 24-23 on slide 101, Table 24-2 on slide 102)
• Hormones important to regulation of intestinal
and glandular function
– Gastrin
– Cholecystokinin (CCK)
– Gastric inhibitory peptide (GIP)
– Secretin
• Produced by duodenal enteroendocrine cells
100
Figure 24-23
101
102
Coordination of Absorption
• Transit time through SI approx 5 hours (lunch entering
when breakfast leaving)
• Absorption enhanced by movement of mucosa
–
–
–
–
Microvilli moved by microfilaments
Villi moved by smooth muscle cells
Groups of villi moved by muscularis externa
Plicae moved by muscularis mucosae and muscularis externa
• These movements
– Stir and mix intestinal contents
– Constantly change environment around each epithelial cell
– Enhance absorption
103
• Today in class we will:
– Conclude our discussion on the major functions of the organs,
regions and anatomical structures of the digestive tract
• Large intestine
– Histological features
– Movements
» Defecation reflex
– Adverse conditions/diseases
–
–
–
–
Discuss nutrients, nutrient digestion and nutrient absorption
Malabsorption of nutrients
Trace the chemical events in digestion
Effects of aging on the digestive system
104
Large Intestine
• Minor function (10%)
– Absorption of nutrients
• Major functions
– Absorption of water
– Absorption of useful compounds
• Organic molecules
• Bile salts – reabsorbed in cecum, transported in blood to
liver
• Vitamins generated by bacteria
– Vitamin K – blood clotting factors
– Biotin – glucose metabolism
– Vitamin B5 – synthesis of steroid hormone and
neurotransmitters
105
Large Intestine
• Major functions (continued)
– Compaction of intestinal contents into feces
• Reduced almost 90% (from 1400 mL to 150 mL)
• Feces
– 75% water, 5% bacteria, 20% indigestible materials, inorganic
matter, remains of epithelial cells
– Storage of fecal material prior to defecation
106
Large Intestine
• Histologic features
–
–
–
–
Thinner walls than SI
Lacks villi
Many goblet cells
Distinctive intestinal glands
• Deeper than in small intestine
• Dominated by goblet cells
– Mucosa does not produce enzymes; any digestion results
from enzymes introduced in SI or from bacterial action
– Mucus provides lubrication (as fecal matter becomes less
moist and more compact)
– Muscularis externa longitudinal layer reduced to 3
muscular bands of taeniae coli
107
Figure 24-25
108
Large Intestine
• Movements
– Reflexes move materials into cecum while you eat
– Peristaltic waves move material through colon
• Movement from cecum to transverse colon very slow;
allows hours for water absorption
• Segmentation movements (haustral churning)
– Mix LI contents
109
Large Intestine
• Mass movements
– Powerful peristaltic contractions
• Move contents from transverse colon through rest of large
intestine
– Occur 1-2 times per day
– Stimulus = distention of stomach and duodenum;
relayed over intestinal nerve plexuses
– Force feces into empty rectum  distension of rectal
wall (stretch receptors)  defecation reflex
• Involuntary (ANS)
– Defecation reflex  conscious urge to defecate
– Relaxation of internal anal sphincter (smooth muscle, controls
movement of feces into anorectal canal)
• Voluntary (CNS)
– Contraction of external anal sphincter (skeletal muscle)
110
Figure 24-25
111
Large Intestine
• Elimination of feces requires:
– Both internal and external anal sphincters must be
relaxed
– Conscious activities
• Opening external anal sphincter (except when internal
pressure sufficiently high)
• Increasing intra-abdominal pressure so fecal material
forced out of rectum
112
Large Intestine
• Adverse conditions/diseases
– Diarrhea
• Frequent, watery bowel movements
• Causes
– Colon mucosa can’t maintain normal levels of absorption
– Rate of fluid entry into colon exceeds absorptive capacity
– Bacterial, viral, protozoan infection of colon or small intestine
• May be life threatening due to fluid and ion loss
– Constipation
• Infrequent defecation
– Feces become dry,hard, compact, difficult to move, highly
abrasive
• Cause: fecal materials move through colon so slowly
excessive reabsorption of water occurs
– Related to inadequate dietary fiber and fluids, lack of exercise
113
Large Intestine
• Adverse conditions/diseases (continued)
– Diverticulosis
• Pockets (diverticula) form in mucosa (generally sigmoid
colon)
• Get forced outward (during defecation), push through
weak points in muscularis externa  subject to
recurrent infection/inflammation (diverticulitis)
– Colon and rectal cancers
• Best defense = early detection and prompt treatment
• Screening test = blood in feces
• Definitive evaluation = visual inspection of lumen
(colonoscopy)
114
Nutrient Digestion
• Digestion
– Breakdown of large organic molecules into small
fragments that can be absorbed
– Occurs via hydrolysis with aid of digestive enzymes
– Source of digestive enzymes
• Secreted by salivary glands, tongue, stomach, pancreas,
and mixed into ingested material as it passes along
digestive tract
• Brush border enzymes attached to intestinal microvilli
115
Carbohydrates
• Sugars and starches
• Building blocks
– Simple sugars (monosaccharides)
– Made up of 3-7 carbon atoms
• Combine via dehydration synthesis to form
disaccharides, trisaccharides,
polysaccharides
• Digestion is reverse process
116
Simple Sugars
Figure 2–11
117
Formation and Breakdown of Complex
Sugars
Figure 2–12
118
Polysaccharides
• Chains of many
simple sugars, e.g.,
glycogen
Figure 2–13
119
Carbohydrate Digestion
• Polysaccharides (glycogen, starch)
– In presence of salivary and pancreatic amylases

• Tri- and disaccharides
– In presence of brush border enzymes
– e.g., maltase, sucrase, lactase 
• Monosaccharides
– e.g., glucose 
• Intestinal capillaries
120
Lipids
• Fats, oils, waxes
• Building blocks
– Glycerol + fatty acids (FAs)
• Combine via dehydration synthesis to form
monoglycerides, diglycerides, triglycerides
• Digestion is reverse process
121
Glycerides
• Glycerides
– Fatty acids
attached to a
glycerol molecule
• Triglyceride
– 3 Fatty-acids
– Storage molecule
Figure 2–16
122
Combination Lipids
Figure 2–18a, b
123
Formation and Breakdown of
Triglycerides
Figure 2–16
124
Lipid Digestion
• Lipids (TGs)
– In presence of lingual and pancreatic lipases 
• Monoglycerides, FAs
• Bile salts emulsify  micelles = lipid-bile
salt complex 
• TGs + proteins  chylomicrons 
• Lacteals
125
Proteins
• Most abundant, most important organic component in
human body
• >2M different proteins, 20% of BW
• Perform essential functions, e.g., support, metabolism,
movement, etc.
• Building blocks
– Amino acids (AAs)
• Form peptide bonds
• Combine via dehydration synthesis to form di/tripeptides, polypeptides, proteins
• Digestion is reverse process
126
Amino Acid
Figure 2-19
127
Formation and Breakdown of Peptides
Figure 2–20
128
Protein Digestion
• Proteins
– In presence of pepsin 
• Polypeptides
– In presence of other enzymes, e.g.,
• Trypsin, chymotripsin, elastase, carboxypeptidase
–
• Short peptides
– In presence of brush border enzymes (peptidases)
–
• Amino acids 
• Intestinal capillaries
129
Figure 24-27
130
Nutrient Absorption
• Absorbed nutrients
– Broken down to provide energy for synthesis of
ATP
– Used to synthesize CHOs, lipids, proteins
131
Water Absorption
• By small and large intestines
• Involves rapid, but passive flow of H2O
along osmotic gradients
• As intestinal cells absorb nutrients and
ions:
– Solute concentrations increase
– H20 moves into cells and surrounding tissues
• 9L/day enters intestinal tract, but only 150
mL lost in feces; remainder is absorbed
132
Digestive Secretion and Absorption of Water
Figure 24–28
133
Ion Absorption
• Each ion handled individually
• Rate of intestinal absorption of each is
tightly controlled
• Absorption transport mechanisms include
diffusion, active transport, carrier-mediated
transport
• Some absorption requires hormones
– Ca+2 - calcitriol, PTH
– Na+ - aldosterone
134
Vitamin Absorption
• Vitamins = organic compounds required in very small
quantities
• Fat-soluble vitamins (D, E, A, K)
– Easily absorbed by diffusion across digestive epithelium
• Water-soluble vitamins (9)
– e.g., B vitamins, C
– All except B12 easily absorbed by diffusion
• Required for synthesis of bone proteins and hematopoiesis
• Must be bound to intrinsic factor (glycoprotein secreted by parietal
cells of stomach); then can be absorbed by active transport
135
Malabsorption
• Abnormal nutrient absorption
• Results from:
– Damage to:
• Accessory glands (pancreatitis, cirrhosis)
• Intestinal mucosa (radiation, toxic compoundschemotherapeutic agents, infection)
– Duct blockage
• Bile duct – biliary obstruction
• Pancreatic – pancreatic obstruction
– Genetic inability to produce gastric or intestinal
enzymes (lactose intolerance)
136
Effects of Aging
• Epithelial stem cell division rate decreases
– Stratified epithelium (mouth, esophagus, anus) becomes
thinner, more fragile
– Digestive epithelium more susceptible to damage (e.g.
ulcers)
– Tissue repair less efficient
• Smooth muscle tone decreases
– Weakened muscular sphincters  esophageal reflux
– Motility decreases, peristaltic contractions weaker;
promotes constipation
– Sagging colon  diverticulosis
– Straining to defecate  hemorrhoids (swollen rectal
varicose veins)
• Cancer rates increase (colon, stomach; oral, pharyngeal
among smokers)
137