Chapter 24: The Digestive System

1
Chapter 24 The Digestive System

• BIO 211 Lecture
• Instructor Dr. Gollwitzer

2

• 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

3
Components and Primary Functions
Figure 24-1
4
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)

5
Functions of the Digestive System

• Ingestion
• Mechanical processing
• Digestion
• Secretion
• Absorption
• Excretion

6
Functions of the Digestive System

• Ingestion
• Entry of materials into digestive tract via mouth

7
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

8
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)

9
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

10
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

11
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

12
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

13
Enzymes

• Are catalysts
• Proteins that lower the activation energy of a
  chemical reaction
• Not changed or used up in the reaction

14
How Enzymes Work
Figure 221
15
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

16
Figure 24-3
17
Movement of Digestive Materials

• Muscularis externa
• Involved in
• Peristalsis
• Segmentation

18
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

19
Figure 24-4, 7th edition
20
Movement of Digestive Materials

• Mass movements
• Powerful peristaltic contractions
• Move GI contents from transverse colon through
  rest of large intestine (? bowel movements)

21
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

22
Control of Digestive Functions

• Regulated by 3 types of mechanisms
• Neural
• Hormonal
• Local

23
Figure 24-5, 7th edition
24
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

25
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

26
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

27
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

28
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

29
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

30
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

31
Pharynx

• Shared passage for food and air
• Connects nasal and oral passages to respiratory
  and digestive systems, respectively
• Lymphoid tissue (tonsils) contributes to bodys
  defense system
• Muscles cooperate with muscles of oral cavity and
  esophagus to initiate deglutition (swallowing)

32
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

33
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

34
Figure 24-11
35
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

36
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)

37
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

38
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

39
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

40
Figure 24-13
41
Gastric Glands

• In stomach fundus and body
• ? gastric juice with enzymes and acid
• 2 types of secretory cells
• Parietal cells
• Chief cells

42
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

43
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

44
Figure 24-14
45
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

46
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

47
Figure 24-13
48
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

49

• 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

50
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

51
Regulation of Gastric Activity
Figure 24-15
52
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

53
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)

54
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

55
Regulation of Gastric Activity
Figure 24-15
56
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

57
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

58
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

59
Gastric Phase

• Local mechanism
• Filling stomach stimulates mast cells in CT of
  lamina propria ? histamine which stimulates
  parietal cells ? HCl

60
Regulation of Gastric Activity
Figure 24-15
61
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

62
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

63
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)

64
Intestinal Phase

• Hormonal mechanisms (continued)
• pH lt 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

65
Regulation of Gastric Activity
Figure 24-15
66
Digestion in Stomach

• At pH gt 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
  pHlt4.5
• At pH lt 2
• Pepsin increases ? protein digestion begun but
  not completed (limited time)
• Complex proteins ? smaller peptides and
  polypeptide chains

67
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

68
Small Intestine

• Plays key role in
• Digestion
• Nutrient absorption
• Water absorption

69
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

70
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

71
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

72
Figure 24-17
73
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

74
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

75
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

76
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

77
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

78
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

79
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

80
Glandular Organs Associated With Small Intestine

• Pancreas
• Liver
• Gall bladder

81
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

82
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)

83
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

84
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

85
Pancreatic Proteolytic Enzymes
Proenzyme Enzyme Catalyst Active Enzyme
Trypsinogen Enterokinase (brush border of duodenum) Trypsin
Chymotrypsinogen Trypsin Chymotrypsin
Procarboxypeptidase Trypsin Carboxypeptidase
Proelastase Trypsin Elastase
86
Liver

• Cant live without your liver
• gt200 functions!
• 3 General functions
• Metabolic regulation
• Hematological regulation
• Bile production

87
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)

88
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)

89
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

90
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

91
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

92
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)

93
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)

94
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

95
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)

96
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

97
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

98
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

99
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)

100
Coordination of Digestive Glands

• Hormonal mechanisms
• gtgtgt(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

101
Figure 24-23
102
(No Transcript)
103
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

104

• 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

105
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

106
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

107
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

108
Figure 24-25
109
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

110
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)

111
Figure 24-25
112
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

113
Large Intestine

• Adverse conditions/diseases
• Diarrhea
• Frequent, watery bowel movements
• Causes
• Colon mucosa cant 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

114
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)

115
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

116
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

117
Simple Sugars
Figure 211
118
Formation and Breakdown of Complex Sugars
Figure 212
119
Polysaccharides

• Chains of many simple sugars, e.g., glycogen

Figure 213
120
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

121
Lipids

• Fats, oils, waxes
• Building blocks
• Glycerol fatty acids (FAs)
• Combine via dehydration synthesis to form
  monoglycerides, diglycerides, triglycerides
• Digestion is reverse process

122
Glycerides

• Glycerides
• Fatty acids attached to a glycerol molecule
• Triglyceride
• 3 Fatty-acids
• Storage molecule

Figure 216
123
Combination Lipids
Figure 218a, b
124
Formation and Breakdown of Triglycerides
Figure 216
125
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

126
Proteins

• Most abundant, most important organic component
  in human body
• gt2M 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

127
Amino Acid
Figure 2-19
128
Formation and Breakdown of Peptides
Figure 220
129
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

130
Figure 24-27
131
Nutrient Absorption

• Absorbed nutrients
• Broken down to provide energy for synthesis of
  ATP
• Used to synthesize CHOs, lipids, proteins

132
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

133
Digestive Secretion and Absorption of Water
Figure 2428
134
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
• Ca2 - calcitriol, PTH
• Na - aldosterone

135
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

136
Malabsorption

• Abnormal nutrient absorption
• Results from
• Damage to
• Accessory glands (pancreatitis, cirrhosis)
• Intestinal mucosa (radiation, toxic compounds-
  chemotherapeutic agents, infection)
• Duct blockage
• Bile duct biliary obstruction
• Pancreatic pancreatic obstruction
• Genetic inability to produce gastric or
  intestinal enzymes (lactose intolerance)

137
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)