Digestive systems perform four basic digestive processes

1
Digestive systems perform four basic digestive
processes

• Motility
• Secretion
• Digestion
• Absorption

2
1. Motility
Muscular contractions within the gut tube and
move forward the contents of the digestive
tract smooth muscle in the digestive tract
walls maintain a constant low level of
contraction tone tone is important
for -maintaining a steady pressure on
contents -preventing the walls from remaining
permanently stretched
3
Two types of digestive motility

• Propulsive movements
• propel (push) the contents through digestive
  tract
• at varying speeds, with the rate of propulsion
• dependent on the functions of different regions
• Mixing movements
• a. mix food with digestive juices
• b. facilitate absorption by increasing
  interactions
• between contents and surfaces of tract

4
Contraction sites and control
smooth muscle most of digestive tract skeletal
muscle ends of tract acts of chewing,
rumination, defecation motor cortex control
(voluntary) smooth muscle-contraction
complex autonomic (involuntary)
5
2. Secretion
a number of digestive juices are secreted into
the digestive tract lumen by specific exocrine
glands, after appropriate neuronal or hormonal
stimulation digestive secretion consists of
water, electrolytes, and specific important
organic constituents (enzymes, bile salts or
mucus) secretory cells extract materials from
blood secretion requires energy (active
transport and synthesis) digestive secretions
are reabsorbed back into blood (may not be in
original forms)
6
3. Digestion
carbohydrates, proteins, and fats are large
molecules unable to cross plasma membrane -gt
need to be broken down into smaller absorbable
units by digestive enzymes before entering
blood digestion begins at anterior tracts and
is accomplished by enzymatic hydrolysis digest
ive enzymes are specific in the bonds they can
hydrolyze
7
Fig. 14-2a, p.615
8
stepwise enzymatic breakdown of foodstuffs
Table 14-1, p.616
9
Dietary molecules

• Carbohydrates
• monosaccharides
• polysaccharides
• starch enzymes are most widely used
• cellulose most abundant organic molecules in
• biosphere (major part of plant)
• chitin
• glycogen

10
Fig. 14-2b, p.615
11
Table sugar Milk sugar
Table 14-1a, p.616
12
Dietary molecules
Neutral fats
Also, nucleic acids -gt pancreatic nucleases -gt
nucleotides
Table 14-1b, p.616
13
4. Absorption
after digestion, absorption occurs in the middle
to posterior sections of tracts small units of
nutrients, water, vitamins, and electrolytes are
transferred from lumen into blood or body
cavity specialized transporters in epithelial
cells carry hydrophilic units through membrane
surface area enhances absorption (ex. villi)
14
Digestive system Digestive tract and accessory
digestive organs
digestive (gastrointestinal) tract mouth,
pharynx (throat), esophagus, stomach, small
intestine, large intestine, anus -gt continuous
tube but regional differences accessory
organs salivary glands, exocrine pancreas, the
biliary system (liver and gallbladder) -gt
connected with the digestive tract with ducts
15
Regulation of digestive function

• digestive motility and secretion are carefully
  regulated
• to maximize digestion and absorption
• Autonomous smooth-muscle function
• some smooth-muscle cells are pacesetter
  cells
• interstitial cells of Cajal that display
  rhythmic,
• spontaneous variations in membrane potential
• slow-wave potential, or basic electrical rhythm
  (BER),
• or pacesetter potential
• slow-wave oscillations cyclical variations in
  Ca2
• release from ER and Ca2 uptake by mitochondria

16
Regulation of digestive function

• digestive motility and secretion are carefully
  regulated
• to maximize digestion and absorption
• Autonomous smooth-muscle function
• rhythmic cycles of muscle contraction depends
  on
• mechanical, nervous, and hormonal factors
• gap junctions transmit electrical activity to
• contractile smooth muscle cells
• rate of contraction depends on pacesetter
  cells of
• each organ

17
Regulation of digestive function

• Intrinsic nerve plexuses digestive systems
  intramural
• nervous sytem (enteric nervous system)
• a. myenteric plexus
• b. submucous plexus
• located entirely within the digestive tract
  wall and run
• its entire length
• influence all facets of digestive tract
  activity
• -gt sensory neurons (input), neurons that
  innervate
• muscle or exocrine/endocrine cells
  (output),
• interneurons
• -gt acetylcholine vs. NO and vasoactive
  intestinal peptide

18
Regulation of digestive function
3. Extrinsic nerves nerve fibers from autonomic
nervous system that innervate different
digestive organs influence digestive motility
and secretion by regulate intrinsic plexuses,
gastrointestinal hormone secretion, smooth
muscle and glands directly fight-or-flight
sympathetic system vs. rest-and- digest
parasympathetic system autonomic nervous system
can modify only digestive system -gt specific
activation of extrinsic innervation leads to
coordination of activity between dif. regions
19
Regulation of digestive function
4. Receptor activation three different types
sensory receptor a. chemoreceptors b.
mechanoreceptors (pressure receptors) c.
osmoreceptors receptors active through neural
reflexes hormone secretion digestive
systems effector cells are activated smooth
muscle cells, exocrine gland cells, endocrine
gland cells
20
Fig. 14-5, p.619
21
Mouth obtaining and receiving food
mouth, or oral cavity, is ringed by muscular
lips -gt prehension palate hard palate vs. soft
palate -gt separates mouth from nasal
cavity uvula sealing off nasal passage during
swallowing tongue floor of mouth, consists of
voluntarily controlled skeletal muscle taste
buds (also in soft palate, throat, linings of
cheeks) pharynx (throat) rear of oral
cavity -gt link between mouth and esophagus -gt
common passageway for both digestive system and
the respiratory system
22
Mouth mastication
motility of the mouth that involves slicing,
tearing, grinding and mixing food by
teeth teeth incisors, canines, molar purposes
of mastication break food, mix food with
saliva, stimulate taste buds taste buds
stimulation reflexly increases salivary,
gastric, pancreatic, and bile secretion food in
the mouth -gt activates mechanoreceptors -gt
medullar oblongata has a chewing center -gt
activates motor neurons that open the oral cavity
23
Saliva aids in mastication lubricates food
saliva is secreted by salivary glands outside of
mouth salivary glands produce serous product
(water enzyme) and mucus saliva 99.5 water
and 0.5 electrolytes and protein functions a.
facilitate swallowing moistening and
lubrication b. salivary amylase starch -gt
maltose c. antibacterial action lysozyme and
rinsing away bacterial food source d. solvent
for molecules that stimulate taste buds e. keeps
oral cavity moist f. rich in bicarbonate buffers
-gt neutralization
24
Salivary secretion simple and conditioned
reflexes
salivary secretion in vertebrates is the only
digestive secretion entirely under neural
control (all others also require
hormones) basal secretion low level, stimulated
by parasympathetic (0.5 ml/min, for moisture
max 5 ml/min) enhanced stimulation by two
different salivary reflexes 1. Simple
(unconditioned) reflex 2. Acquired (conditioned)
reflex learned response based on previous
experience
25
Cerebral cortex
Other inputs
Salivary center in medulla
Conditioned reflex
extrinsic
Autonomic nerves
Pressure receptors and chemoreceptors in mouth
Simple reflex
Salivary glands
Salivary secretion
Fig. 14-6, p.622
26
Salivary secretion simple and conditioned
reflexes
both sympathetic and parasympathetic
stimulation (autonomic nervous system) increases
salivary secretion, but quantity and
consistency of saliva changes with food (sensory
evaluation of food) parasympathetic increases
blood flow, prompt and abundant flow of watery
saliva rich in enzymes sympathetic reduces
blood flow -gt output of smaller volume but rich
in mucus (stress situations)
Mouth digestion is minimal and no absorption
27
Pharynx, esophagus, and crop
motility associated with the pharynx and
esophagus is swallowing or deglutition (food
from mouth to stomach) swallowing begins when
bolus is moved into esophagus pharyngeal
pressure receptors -gt swallowing center in
medulla -gt muscles involved in swallowing swallo
wing a sequentially programmed all-or-none
reflex highly coordinated activities are
initiated in a regular pattern over a period of
time
28
Swallowing oropharyngeal esophageal stages
when in pharynx, bolus must be prevented from
entering the mouth, nasal passages, and
trachea a. tongue against hard palate b. uvula
is elevated against the throat, sealing
off nasal passages c. elevation of larynx and
tight closure of glottis (vocal folds across
the laryngeal opening) d. epiglottis e.
respiration in humans is briefly inhibited
during swallowing f. pharyngeal muscles
contract to force bolus into esophagus
29
Fig. 14-7, p.624
30
Nasal passages
Hard palate
Soft palate
Uvula
Pharynx
Epiglottis
Esophagus
Bolus
Trachea
Tongue
Glottis at entrance of larynx
(a)
Fig. 14-7a, p.624
31
Swallowing center inhibits respiratory center in
brain stem
Elevation of uvula prevents food from entering
nasal passages
Position of tongue prevents food from re-entering
mouth
Epiglottis is pressed down over closed glottis as
auxiliary mechanism to prevent food from
entering airways
Tight apposition of vocal folds across glottis
prevents food from entering respiratory airways
(viewed from above)
(b)
Fig. 14-7b, p.624
32
Esophagus muscular tube guarded by sphincters
at both ends
smooth muscle tube -gt links pharynx and
stomach lies in thoracic cavity, penetrates
diaphragm guarded at both ends by sphincter
valves pharyngoesophageal sphincter gastroesoph
ageal (cardiac) sphincter the oropharyngeal
stage of swallowing requires opening and closing
of pharyngoesophageal sphincter
pharyngoesophageal sphincter prevents large
volume of gas from entering digestive tract
(called eructation or belching)
33
Esophagus muscular tube guarded by sphincters
at both ends
peristaltic waves push the food through
esophagus esophageal stage of swallowing -gt
swallowing center initiates a primary
peristaltic wave that sweeps from the beginning
to the end of esophagus peristalsis ringlike
contractions of the circular smooth muscle that
more progressively forward with a stripping
motion unsuccessful swallowing bolus distends
esophagus -gt pressure receptors in the walls
-gtintrinsic nerve plexuses initiates a second,
more forceful peristaltic wave (distension of
esophagus also increases saliva secretion)
34
Esophagus muscular tube guarded by sphincters
at both ends
except during swallowing, gastroesophageal
sphincter remains contracted, as a barrier
between stomach and esophagus -gt reducing
reflux of acidic contents into esophagus
(otherwise heartburn) sphincter relaxes reflexly
as the peristaltic wave sweeps down
esophagus achalasia damage to the myenteric
nerve plexus in the gastroesophageal sphincter
-gt failure to relax and delay in food passage to
stomach mucus is secreted throughout the length
of the entire digestive tract protective for
esophagus
35
Stomach or midgut Food storage and nonsalivary
digestion
initial digestion (after minor salivary
digestion) of protein stomach muscular,
saclike chamber can be divided into three
sections fundus food storage and adaptation
to volume change corpus food is mixed with
gastric secretion antrum mixing and expulsion
of food into intestine pyloric sphincter
barrier between stomach and duodenum
36
Esophagus
Fundus
Smooth muscle
Gastroesophageal sphincter
Body
Stomach folds
Pyloric sphincter
Oxyntic mucosa
Pyloric gland area
Antrum
Duodenum
Fig. 14-8, p.627
37
Stomach or midgut Food storage and nonsalivary
digestion
the stomach performs three main functions 1.
store food so the food can enter small intestine
at a rate for optimal digestion and
absorption 2. secretes HCl and enzymes for
chemical digestion 3. through mixing, food is
pulverized and mixed with gastric
secretions to produce thick, liquid mixture
called chyme
38
Stomach or midgut Food storage and nonsalivary
digestion
gastric filling stomach can accommodate
significant changes in volume (empty 50 ml,
after meal 1 liter) the interior of stomach
has many deep folds -gt during a meal, folds
become smaller and flatten out as
stomach relaxes receptive relaxation
accommodate extra volume of food without rise in
stomach pressure (triggered by eating)
39
Stomach or midgut Food storage and nonsalivary
digestion
pacesetter cells in the upper fundus generate
slow-wave potentials -gt peristaltic
waves peristaltic waves spread over the fundus
and body to the antrum and pyloric sphincter
(weaker to stronger) food from esophagus is
stored in the body without being mixed (the
fundus area contains only gas)
40
Stomach or midgut Food storage and nonsalivary
digestion
gastric mixing takes place in the antrum, by
strong peristaltic contractions -gt producing
chyme tonic contraction of the pyloric
sphincter keeps it almost, but not completely
closed (for water and fluids passage) chyme is
pushed through the sphincter by strong
antral peristaltic contractions retropusion
41
Stomach or midgut Food storage and nonsalivary
digestion
antral peristaltic contractions drive gastric
emptying during digestive phase of gastric
motility, only particles less than 2 mm in
diameter can escape thru sphincter intensity of
antral peristalsis can be influenced by
gastric and duodenal factors housekeeping
function of antrum interdigestive
motility complex (hourly interval), emptying
remaining particles between meals
42
Fig. 14-9, p.629
43
Stomach
1
Esophagus
Gastroesophageal sphincter
Pyloric sphincter
Duodenum
3
4
Direction of movement of peristaltic contraction
2
Movement of chyme
Peristaltic contraction
Gastric emptying
Fig. 14-9a, p.629
44
6
5
Peristaltic contraction
Gastric mixing
Fig. 14-9b, p.629
45
Table 14-2, p.629
46
Duodenal factors that influence gastric emptying
enterogastric reflex neural response intrinsic
nerve plexuses autonomic nerves hormonal
response enterogastrones (endocrine) secretin
in response to acidity in dueodenum cholecystokin
in (CCK) gastric inhibitory peptide fat, acid,
hypertonicity, distension
47
Other factors that influence gastric motility
hunger may increase peristaltic contractions
over stomach (parasympathetic) emotions such
as stress intense pain inhibit digestive tract
motility (increased sympathetic activity)
48
Vomiting Forceful expulsion of gastric contents
though mouth
caused by irritation or distension of stomach
and duodenum chemical agents, such as drugs
toxins (either by acting in upper GI tract or
chemoreceptor trigger zone in the
brain) stomach, esophagus ( sphincter), and
pyloric sphincter are all relaxed contraction
of respiratory muscles (diaphragm and abdominal
muscle -gt stomach is squeezed consequences?
49
Secretion of gastric juice
rate and amount dependent on the frequency of
feeding cells are located in the gastric mucosa
(lining of stomach) 1. oxyntic mucosa (body and
fundus) 2. pyloric gland area (PGA, in
antrum) on luminal surface there are deep
pockets formed by infoldings of the gastric
mucosa the invaginations are called gastric
pits, gastric glands lie at the base
50
Table 14-3, p.632
51
Oxyntic mucosa
Stomach lumen
Pyloric gland area
Table 14-3a, p.632
52
Gastric pit
Mucosa
Submucosa
Table 14-3b, p.632
53
Secretion of gastric juice Exocrine secretory
cells in the oxyntic mucosa pits

• Mucous cells secretes mucus
• Chief and parietal cells enzyme precursor
  pepsinogen
• Parietal (or oxyntic) cells HCl and intrinsic
  factor
• Also
• Stem cells parent cells of all new cells of
  gastric mucosa
• Surface epithelial cells viscous, alkaline mucus
  on surface

54
In oxyntic mucosa
Surface epithelial cells
Gastric pit
Mucous cells
Chief cells
Gastric gland
Parietal cells
Enterochromaffin- Like (ECL) cells
Table 14-3c, p.632
55
Secretion of gastric juice Endocrine and
paracrine secretory cells

• Enterochromaffin-like (ECL) cells histamine
• G cells (of PGA) gastrin (into blood)
• D cells (pylorus and duodenum) somatostatin
• the gastric glands of PGA primarily secret mucus
  and
• a small amount of pepsinogen

56
In pyloric gland area
D cells
G cells
Table 14-3d, p.632
57
Table 14-3e, p.632
58
Exocrine products and role in digestion

• Hydrochloric acid secretion
• parietal cells actively transport H (and Cl)
  ions against
• huge concentration gradient into lumen
• -gt requires energy, mitochondria
• secreted H is not from plasma but metabolic
  process
• -gt breakdown of water
• transport H-K ATPase in the luminal membrane
• carbonic anhydrase combine OH with CO2
• -gtHCO3
• basolateral border HCO3-Cl carrier

59
Fig. 14-10, p.633
60
Exocrine products and role in digestion

• Hydrochloric acid secretion
• functions
• does not digest anything
• activates pepsin
• breakdown of large food particles
• denatures protein
• kills microorganisms

61
Exocrine products and role in digestion
2. Pepsinogen/pepsin stored in chief cells
cytoplasm within secretory vesicles known as
zymogen granules released by exocytosis HCl
conversion into pepsin, pepsin also cleavages
pepsinogen -gt autocatalytic (self-activating)
process pepsin cleaves proteins into peptide
fragments, works best at low pH mucus
protects gastric mucosa from mechanical
injury, pepsin digestion, low pH
62
Fig. 14-11, p.634
63
Exocrine products and role in digestion

• Intrinsic factor
• secretory product of parietal cells
• important for vitamin B12 absorption (must be
• combined with intrinsic factor before being
  absorbed)
• absorption intrinsic factor-vit. B12 complex
  with a
• special receptor located in the terminal ileum
  (last
• portion of small intestine) -gt
  receptor-mediated
• endocytosis
• vitamin B12 is important for red blood cell
  formation

64
Regulation of parietal and chief cells
4 chemical messengers influence gastric juice
secretion Ach, gastrin, histamine, and
somatostatin parietal cells have receptors for
each messenger Ach, gastrin, histamine
stimulatory on HCl secretion somatostatin
inhibit HCl secretion Ach, gastrin also
increases pepsinogen secretion (effect on chief
cells)
65
Regulation of parietal and chief cells
Ach neurotransmitter from intrinsic nerve
plexus, acts on parietal, chief, G, and ECL
cells gastrin from G cells, into blood in
response to protein products in stomach and to
Ach, major gastrointestinal hormone, main factor
for HCl secretion -gt parietal and chief cells
acidic gastric juice -gt stimulate ECL cells to
release histamine -gt HCl -gt trophic (growth
promoting) to mucosa of stomach and small
intestine
66
Regulation of parietal and chief cells
histamine a paracrine released from ECL cells
in response to gastrin and Ach -gt parietal
cells for HCl secretion somatostatin a
paracrine released from D cells in response to
acid -gt inhibit secretion by parietal, G and ECL
cells -gt negative feedback effect to turn off
HCl secretion
67
Table 14-3e, p.632
68
Control of gastric secretion involves three phases
rate of gastric secretion can be increased
by 1. Factors arising before food reaches
stomach -gt cephalic phase 2. Factors due to
presence of food in stomach -gt gastric
phase 3. Factors originating in the duodenum
after food has left stomach -gt intestinal
phase
69
Control of gastric secretion involves three phases

• Cephalic phase
• gastric secretion occurs in response to
  food-related
• stimuli acting in the brain -gt feedforward
  activation
• vagal stimulation of intrinsic plexuses and G
  cells in
• PGA
• 2. Gastric phase
• may be independent from the cephalic phase
• stimuli acting in the stomach (peptide
  fragments)
• can trigger release of gastrin

70
Control of gastric secretion involves three phases

• Intestinal phase
• occurs with the entry of food enter small
  intestine
• excitatory component
• products of protein digestion in the duodenum
• triggers release of intestinal gastrin -gt
  stomach
• inhibitory component
• enteroglucagon is released into blood when the
  pH
• of intestine is too low -gt shut off gastric
  secretion

71
Table 14-4, p.635
72
Gradual inhibition of gastric secretion
as food enters duodenum, stimuli for enhanced
gastric secretion (protein) is withdrawn in
response to low gastric pH after food leaves
stomach, somatostatin is released -gt gastric
secretion lowers same stimuli that inhibit
gastric motility also inhibit gastric secretion
73
Table 14-5, p.636
74
Gastric mucosal barrier of stomach
mucosal linings provide barriers a. luminal
membranes of mucosal cells are impermeable
to H (acid cant enter cells) b. lateral edges
of these cells are joined near luminal
borders by tight junctions -gt gastric mucosal
barrier the entire stomach lining is replaced
every three days peptide ulcer protective
mechanisms fail
75
Two separate digestive processes in stomach
food in the body of stomach still semi-solid
due to weak mixing -gt very little protein
digestion (only on then surface
carbohydrates digestion continues in the
interior of food mass (by salivary
amylase) digestion by gastric juice is done in
the antrum, due to mixing with HCl and
pepsin fat digestion has not begun in the
stomach
76
Pancreas mixture of exocrine and endocrine
tissue
98 of the pancreas has exocrine function
acini (exocrine) secrete pancreatic juice
enzymatic secretion and aqueous alkaline
secretion islets of Langerhans
(endocrine) most important pancreatic hormones
are insulin and glucagon
77
Bile duct from liver
Stomach
Duodenum
Hormones (insulin, glucagon)
Blood
Endocrine portion of pancreas (islets of
Langerhans)
Acinar cells secrete digestive enzymes
Duct cells secrete aqueous NaHCO3 solution
Exocrine portion of pancreas (acinar and duct
cells)
The glandular portions of the pancreas are
grossly exaggerated.
Fig. 14-12, p.638
78
Exocrine pancreatic secretion

• Proteolytic enzymes (for protein digestion)
• trypsinogen activates by enterokinase in the
  luminal
• border of mucosal cells (autocatalytic
  activation)
• chymotrypsinogen and procarboxypeptidase
• activated by trypsin
• protein can be digested into amino acids or
  peptides
• 2. Pancreatic amylase
• polysaccharides to disaccharides

79
Exocrine pancreatic secretion

• Pancreatic chitinase
• chitin chains of glucose molecules
• Pancreatic lipase
• principal enzyme that digests fat
• hydrolyzes dietary triglycerides into
  monoglycerides
• and free fatty acids (absorbable)
• pancreas is the only significant source of
  lipase
• steatorrhea excessive undigested fat in the
  feces

80
Exocrine pancreatic secretion

• Pancreatic aqueous alkaline secretion
• pancreatic enzymes function best in neutral or
• slightly alkaline condition
• acidic chyme from stomach must be neutralized
• -gt allow optimal digestion and prevent acid
  damage
• alkaline (NaHCO3-rich) fluid largest component
  
• of pancreatic secretion

81
Hormonal regulation of pancreatic secretion

• parasympathetic stimulated pancreatic secretion
  occurs
• during the cephalic and gastric phases
• major stimulation occurs during intestinal
  phase, when
• chyme is in the small intestine -gt
  enterogastrones are
• released
• Secretin
• stimulated by acid in the duodenum
• secretin stimulates duct cells in pancreas
• a mechanism for maintaining neutrality of chyme

82
Hormonal regulation of pancreatic secretion
2. CCK (cholecystokinin) regulates pancreatic
digestive enzyme secretion released from
duodenal mucosa in response to nutrients in
the lumen (fat) CCK stimulates pancreatic
acinar cells all three types of pancreatic
enzymes are packaged together in the zymogen
granules -gt released together (adaptive
response?) Both secretin and CCK exert trophic
effects on exocrine pancreas
83
(secretin carried by blood)
(CCK carried by blood)
Fig. 14-13, p.640
84
Biliary system liver and gallbladder
liver is the largest and most important
metabolic organ in vertebrates -gt biochemical
factory perform a wide variety of functions a.
bile salts secretion b. metabolic processing of
major nutrients after absorption c.
detoxification or degradation of body wastes
and foreign compounds d. synthesis of
plasma proteins for blood clotting or
transport of hormones and cholesterol e. storage
of glycogen, fats, iron, copper, and many
vitamins
85
Biliary system liver and gallbladder
perform a wide variety of functions f.
activation of vitamin D (with kidneys) g.
removal of bacteria and worn-out RBC
(macrophages) h. excretion of cholesterol and
bilirubin (breakdown product of RBC) i.
gluconeogenesis (converting noncarbohydrates
into glucose) hepatocyte single cell type
for the wide variety of jobs
86
Liver blood flow
Hepatic portal system
Fig. 14-14, p.641
87
Liver lobules
liver is organized into functional units
lobules (hexagonal arrangements of tissue with
a central vein) channels a. vessels hepatic
artery, branch of portal vein, bile
duct b. sinusoids capillary network c. hepatic
vein d. bile canaliculs -gt bile duct each
hepatocyte is in contact with a sinusoid and a
bile canaliculus
88
Fig. 14-15, p.642
89
Branch of hepatic portal vein
Central vein
Bile canaliculi
Cords of hepatocytes (liver cells)
Bile duct
Branch of hepatic artery
Hepatic portal vein
To hepatic duct
Sinusoids
Hepatic artery
(a)
Fig. 14-15a, p.642
90
Branch of hepatic portal vein
Branch of hepatic artery
Bile duct
Connective tissue
Kupffer cell
Bile canaliculi
Sinusoids
Cords of hepatocytes (liver cells)
Hepatic plate
Central vein
Fig. 14-15b, p.642
91
Bile secretion
sphincter of Oddi bile duct-duodenum
junction prevents bile from entering until
during digestion when closed, bile is diverted
back to gallbaldder gallbladder can concentrate
bile 10-to-20-fold gallbladder storage and
concentration of bile between meals
92
Fig. 14-16, p.642
93
Bile secretion enterohepatic circulation
bile consists of an aqueous alkaline fluid
(similar to pancreatic NaHCO3 secretion, by
duct cells) and other organic constituents
bile salts, cholesterol, lecithin, bilirubin
(from liver) bile is important for digestion and
absorption of fats, through emulsification
effect of the bile salts bile salts derivatives
of cholesterol (conjugated with taurine or
glycine) bile salts are reabsorbed back into
blood by active- transport in terminal ileum -gt
hepatic portal system -gt liver (enterohepatic
circulation)
94
Bile salts aid fat digestion and
absorption Detergent action of bile salts
detergent action convert large fat globules
into a lipid emulsion that consists of small fat
droplets -gt increase surface area for pancreatic
lipase attack bile salts emulsify large fat
droplets for better digestion bile salts
molecule large fat droplets -gt intestinal mixing
-gt small droplets -gt lipid emulsion -gt lipase
and colipase food in duodenum triggers release
of CCK -gt contraction of gallbladder
relaxation of sphincter of Oddi
95
Lipid emulsion
Fig. 14-17, p.644
96
Bile salts aid fat digestion and
absorption Micellar formation
bile salts (and cholesterol and lecithin) are
important for fat absorption through micellar
formation in micelle, both bile salts and
lecithin aggregate in small clusters (1/106 the
size of emulsified lipid droplet) micelles
provide a vehicle for carrying water-insoluble su
bstances through the watery luminal
contents products of fat digestion and
fat-soluble vitamins
97
Fig. 14-18, p.645
98
Bile secretion bilirubin
waste product excreted by by the liver
breakdown of worn out RBC RBC -gt removed from
blood by macrophages -gt degraded by Kupffer
cells to yield heme -gt converted into biliverdin
-gt transformed into bilirubin -gt released into
lobule sinusoid -gt extracted from blood by
hepatocytes -gt excreted into bile small amount
of bilirubin is absorbed by the small intestine
and exreted in the urine jaundice accumulation
of bilirubin
99
Small intestine
site where most digest and absorption of
nutrients takes place in vertebrates a coiled
tube that lies within the abdominal cavity,
divided into three segments duodenum, jejunum,
and ileum digestive tract wall has four
layers a. mucosa b. submucosa c. muscularis
externa d. serosa
100
Fig. 14-19, p.646
101
Digestive tract wall
mucosa has three layers 1. Mucous membrane,
inner epithelial layer that is a surface for
protection and secretion/absorption
(exocrine, endocrine, and epithelial cells) 2.
Lamina propria thin middle layer of connective
tissue (contains small blood vessels, lymph
vessels and nerve fibers, and gut-associated
lymphoid tissue, or GALT) 3. Muscularis
mucosa outer layer of smooth muscles
102
Body wall
Peritoneum
Mesentery
Outer longitudinal muscle
Muscularis externa
Inner circular muscle
Serosa
Mucous membrane
Mucosa
Lamina propria
Submucosa
Muscularis mucosa
Lumen
Duct of large accessory digestive gland (i.e.,
liver or pancreas) emptying into digestive
tract lumen
Myenteric plexus
Submucous plexus
(a)
Fig. 14-19a, p.646
103
Digestive tract wall
submucosa thick layer of connective tissue that
provides distensibility and elasticity -gt
contains large blood and lymph vessels -gt nerve
network called submucous plexus muscularis
externa major smooth muscle coat of digestive
tube -gt inner circular layer constricts
tube -gt outer longitudinal layer shortens the
tube -gt contraction propulsive and mixing
movements -gt nerve network between the two
layers myenteric plexus
104
Body wall
Peritoneum
Mesentery
Outer longitudinal muscle
Muscularis externa
Inner circular muscle
Serosa
Mucous membrane
Mucosa
Lamina propria
Submucosa
Muscularis mucosa
Lumen
Duct of large accessory digestive gland (i.e.,
liver or pancreas) emptying into digestive
tract lumen
Myenteric plexus
Submucous plexus
(a)
Fig. 14-19a, p.646
105
Digestive tract wall
serosa outer connective tissue covering the
tract function secretes a watery serous fluid
that lubricates and prevents friction between
the digestive organs and surrounding
vscera serosa is continuous with the mesentery
throughout the tract -gt fixation and position
support, freedom for mixing and propulsive
movements
106
Mucous lining of small intestine Well adapted to
its absorptive functions

• Surface area of mucosa
• inner surface is arranged in circular folds
• increase area 3x
• microscopic projection from this folded
  surface
• villi (increase another 10x)
• -gt surface of villi epithelial and mucous
  cells
• -gt villi are dynamic and can change dimension
• microvilli (or brush border) of epithelial
  cells
• increase area another 20x
• -gt each cell has 3k-6k microvilli, longest in
  jejunum
• -gt membrane space within microvilli, called
• glycocalyx (carbohydrate filaments with
  enzymes)

107
Villus
(b)
Fig. 14-19b, p.646
108
Epithelial cell
Capillaries
Mucous cell
Central lacteal
Crypt of Lieberkühn
Arteriole
Venule
Lymphatic vessel
(c)
Fig. 14-19c, p.646
109
Microvilli
(d)
Fig. 14-19d, p.646
110
Mucous lining of small intestine Well adapted to
its absorptive functions
2. Structure of a villi epithelial cells on the
villi surface tight junctions at lateral
borders (leakier than those in stomach) carriers
at the brush borders for nutrients and
electrolytes digestive enzymes at brush
borders capillary network each villus is
supplied by an arteriole terminal lymphatic
vessel each villus is supplied by a lymphatic
vessel (central lacteal) -gt transepithelial
transport of nutrients
111
Small intestine segmentation contractions
segmention small intestines primary method of
motility consists of oscillating, ringlike
contractions contractile rings do not sweep
like peristaltic waves serves the dual
functions of mixing chyme and enzymes and
exposing chyme to the mucosa
surface initiation and control pacesetter
cells produce BER smooth muscles
responsiveness and intensity of segmentation
contractions are influenced by distension of
intestine, gastrin, and extrinsic nerve
112
Small intestine segmentation contractions
initiation and control when food enters, both
duodenum and ileum segment simultaneously
duodenum (local distension) ileum (gastrin)
gastroileal reflex extrinsic nerves modify
strength of contractions chyme progress forward
slowly by segmentation, but segmentation
frequency declines along the length of small
intestine (12/min vs. 9/min) -gt slow progress
allows time for mixing and absorption (3-6
hours to move through small intestine)
113
Small intestine motility between meals
migrating motility complex, or intestinal
housekeeper weak, repetitive peristaltic
waves short peristaltic waves take about
100-140 mins to migrate from from stomach to
end of small intestine sweeping residual chyme,
mucosal debris, and bacteria towward
colon hormone called motilin may play a role in
regulation ceases when next meal arrives
114
Fig. 14-20, p.647
115
Ileocecal juncture
juncture between small and large
intestines valvelike folds of tissue protrude
from ileum into the lumen of cecum -gt ileocecal
valve the smooth muscle within the last
centimeters of ileal wall is thickened,
forming a sphincter -gt ileocecal sphincter
the sphincter is under neuronal control
(intrinsic plexuses) and hormonal control
(gastrin)
116
Fig. 14-21, p.648
117
Small intestine secretions
exocrine glands in small intestine mucosa
secrete 1.5L/day of succus entericus aqueous
salt and mucus digestive enzymes function
within the borders of epithelial cells
succus entericus protection and lubrication
H2O for enzymatic digestion (hydrolysis)
regulation of this secretion (after a meal) is
not clear, probably by the presence of chyme
118
Small intestine enzymes
digestion in small intestine lumen is done by
pancreatic enzymes, with fat digestion enhanced
by bile fat digestion is complete in small
intestine lumen, but not protein and
carbohydrate epitheial cell brush
border enterokinase activates pancreatic
trypsinogen disaccharidases aminopeptidases
119
Table 14-6, p.649
120
Small intestine primary role in absorption
normally, all products of carbo, protein, and
fat digestion, and most of electrolytes,
vitamins and water are absorbed by small
intestine (except calcium and iron) length of
small intestine and the types and density of
its transporters are matched to typical diets of
species most absorption occurs in the duodenum
and jejunum (very little in ileum) small
intestine has an abundant reserve
absorptive capacity vitamin B12 and bile salts
are absorbed only in terminal ileum (all others
throughout length of small intestine)
121
Small intestine rapid turnover of mucosal lining
crypts of Lieberkuhn secrete water and
electolytes for the succus entericus also
functions as nurseries new cells at bottom of
crypts migrate up the villi and replace turnover
cells on the top (100 million intestinal cells
are shed per minute) migration takes about three
days in human, cells also undergo changes
during migration Paneth cells in the crypts
defensive function for safe- guarding the stem
cells -gt produce lysozyme and defensins
(anti-microbial)
122
Small intestine water and electrolytes absorption
sodium active and passive absorption passive
diffusion through tight junctions movement
through cells involves two different
carriers entering epithelial cells passive, or
co-transported with glucose, amino acids, or
other nutrients actively pumped out at
basolateral border finally diffuse into the
capillaries
123
Small intestine water and electrolytes absorption
aborption of Cl, H2O, glucose and amino acids
from small intestine is linked to the
energy-dependent Na absorption H2O absorption
in the digestive tract depends on active carrier
that pumps Na into lateral space -gt osmotic
pressure
124
Small intestine carbohydrate absorption
glucose and galactose are both absorbed by
secondary active transport (co-transport with Na
at lumine border) co-transport depends on Na
concentration gradient created by energy-dep.
basolateral Na-K pump glucose leaves cells
through passive carrier in the basolateral
border (by concentration gradient) fructose
into blood by facilitated diffusion
(passive carrier-mediated transport)
125
Fig. 14-22, p.652
126
Small intestine protein absorption
both ingested and endogenous proteins are
absorbed amino acids secondary active
transport (with Na) (like monosaccharides, enter
capillary in villi) neonatal mammals active
process of endocytosis for uptake of whole
proteins (IgA class of antibodies) some large
protein molecules can be absorbed intact into
blood through a process called persorption
127
Fig. 14-23, p.653
128
Small intestine fat absorption
monoglycerides and free fatty acids passively
diffuse from the micelles through the lipid
component of the epithelial cell membranes (the
luminal side) -gt micelles are then free to pick
up more products bile salts are reabsorbed in
the terminal ileum by special active
transport once inside the cells, triglycerides
are formed -gt these droplets are coated with a
layer of lipoprotein (so water soluble
chylomicrons extruded by exocytosis, secreted
into central lacteals
129
Fig. 14-24, p.654
130
Fig. 14-24a, p.654
131
Fig. 14-24b, p.654
132
Small intestine vitamin absorption
water-soluble passive absorption with
water fat-soluble carried in micelles and
absorbed passively with fat digestion
products vitamin B12 combined with gastric
intrinsic factor for absorption by special
transport in terminal ileum
133
Absorbed nutrients pass thru liver
venules that leave GI tract empty into portal
vein (liver) -gt pass thru liver before entering
general circulation after metabolic processing
and detoxification in liver, venous blood return
to heart through vena cava fat (chylomicrons)
is absorbed in the central lacteal -gt lymphatic
system -gt thoracic duct, which empties
into venous system in the chest
134
Absorbed nutrients pass thru liver
liver does not act on fat until it is diluted in
the circulatory system and uptake by adipose
cells liver plays an important role in lipid
transport synthesizing three types of plasma
lipoproteins a. high-density lipoprotein b.
low-density lipoprotein c. very-low-density
lipoprotein (ab transport cholesterol for cell
membrane production c triglycerides for energy
storage in adipose tissue)
135
The only secretory product that escapes form the
body bilirubin
Table 14-7, p.655
136
Biochemical balance among middle GI
the acid-base balance of the body secretion
absorption stomach -gt pancreas -gt small
intestine between digestive tract lumen and
blood
137
Fig. 14-25, p.656
138
Diarrhea
highly fluid fecal matter, increased frequency
of defection beneficial but also damaging -gt
dehydration, loss of nutrient and secretions,
metabolic acidosis (due to loss of HCO3) most
common cause excessive intestinal
motility bacterial or viral infection, or
emotional stress no sufficient time for
intestinal absorption
139
Large intestine
Transverse colon
Haustra
Taeniae coli
Descending colon
Ascending colon
Ileocecal valve
Appendix
Sigmoid colon
Cecum
Rectum
External anal sphincter (skeletal muscle)
Internal anal sphincter (smooth muscle)
Anal canal
Fig. 14-26, p.657
140
Large intestine haustra contractions
outer longitudinal smooth-muscle layer consists
of 3 separate, longitudinal bands of muscle
taeniae coli the longer, underlying circular
smooth-muscle (than the outer longitudinal
layer) are gathered into pouches or sacs called
haustra movements of large intestine are slow
-gt appropriate for its absorptive and storage
functions primary method of motility for colon
is haustral contractions
141
Large intestine haustra contractions
initiated by autonomous rhythmicity of smooth
muscle cells, similar to small intestinal
segmentation, but 30 mins between contractions
(controlled by intrinsic plexuses) slowly mix
to expose colonic contents to absorptive mucosa,
and allow more time for microbial
digestion contents delivered to colon consist
of indigested food, unabsorbed biliary
components, and remaining fluid after H2O and
salt extraction, extreta or feces remain -gt
primary function of large intestine is to store
them
142
Large intestine mass movements
general after meals, a marked increase in
motility takes place -gt large segments of colon
contract simultaneously mass movements drive
colonic contents into the distal portion of the
large intestine -gt storage until
defecation when food enters stomach, mass
movements occur by gastrocolic reflex -gt
mediated by gastrin and extrinsic autonomic
nerves -gt colonic contents into
rectum gastroileal reflex also moves the
remaining small-intestine contents into the
large intestine -gt so a general GI tract
response when new food enters
143
Large intestine defecation
mass movements move fecal materials into rectum
-gt distension of rectum -gt stimulate stretch
receptors in rectal wall -gt defecation reflex
-gt internal anal sphincter (smooth muscle)
relax rectum and sigmoid colon
contract external anal sphincter (skeletal
muscle) voluntary defecation is assisted by
increase in intra-abdominal pressure
contraction of abdominal muscles and
forcible expiration
144
Large intestine secretion and absorption
large intestine does not secrete any digestive
enzymes, colonic buffers consists of alkaline
mucus solution large intestine lacks the
adequate luminal surface area, and specialized
transport mechanisms (for glucose or amino
acids) Na is actively absorbed in large
intestine, Cl follows passively (electrical
gradient), then H2O follows osmotically fecal
materials (100g H2O and 50g solids) include
undigested/unabsorbed food, bacteria, bilirubin