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Digestive physiology
Digestion & absorption
GIT motility & circulation
Ruminant & Pre-ruminant digestion
Revision of digestive system in farm
animals
• http://www.powershow.com/view/1fb91N2JjM/Digestive_Physiology_of_Farm_Animal
s_powerpoint_ppt_presentation
Digestion and absorption
• These are related but separate
• Digestion: breakdown of complex molecules/nutrients into simple
molecules
• Absorption: process of transporting simple molecules across intestinal
epithelium
• Both are a result of biochemical events occurring in the gut
• Both are important for nutrient assimilation
• No absorption if food not digested
• Digestion fruitless if no nutrient absorption
• Various assimilation disturbances exist
• Caused by a variety of diseases
Digestion and absorption
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Some diseases affect digestion while others absorption
But signs often similar
Therapies might be different
Diagnosing cause is a challenge
Diarrhea occurs if there’s mismatch btwn secretion and
absorption
Microanatomy of SI
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SI mucosa has large surface area
Has epithelial cells with ‘leaky’ junctions btwn them
Facilates contact btwn SI mucosa & luminal contents
3 levels of surface convolutions serve to expand surface area
– Plicae circulares: large folds of mucosa, not present in all spp
– Villi: finger-like projections in epithelium, present in all spp, increase
SA 10-14 times
– Brush border: submicroscopic microvilli, further increase SA
• Base of villi has gland-like structures k.a. crypts of Lieberkuhn
Microanatomy of SI
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Villi & crypts covered with continuous layer of cellular
epithelium
Epithelial cells covering villi & crypts = enterocytes
Enterocytes have two distinct cell membranes
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Apical membrane covers cell surface facing lumen (apex)
Contains microvilli
Covered by jelly-like layer of glycoprotein k.a. glycocalyx
Enzymes & other proteins attached to MV & project into
glycocalyx
Basolateral membrane
Not in direct contact with ingesta
Absorbed nutrients exit enterocytes thru basolateral
membrane
Microanatomy of SI
• Attachments btwn adjacent enterocytes = tight junctions
• Not necessarily tight at molecular stand point
• Junctions form narrow band attachment btwn adjacent
enterocytes near apical end
• Allows free passage of water & some electrolytes
• Lateral space btwn lateral surfaces of enterocytes
• Distended & filled with ECF
• ECF separated from fluid in the intestinal lumen by tight
junctions
• & from blood by basement membrane capillaries
Microanatomy of SI
• Both tight junctions & capillary endothelia are permeable
barriers
• Allow free passage of water & small molecules
• Thus there’s relatively free flow of water & most electrolytes
btwn SI lumen fluid & ECF in lateral space & blood
Intestinal surface microenvironment
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Made up of glycocalyx, mucus & unstirred water layer
Goblet cells interspaced btwn enterocytes secrete mucus
Mucus covers mucosa
At brush border surface mucus blends with glycocalyx
Form viscous coating that trap molecules near apical
membrane
• Near intestinal surface in the unstirred water layer
• These form diffusion barrier thru which nutrient pass thru
before entering enterocytes
Digestion
• Involves physical & chemical breakdown
• Physical breakdown results in
– Reduction in feed particle size
– Allows food flow thru GIT
– Increases surface area exposed to enzymes
• Begins with mastication
• Completed by grinding action of distal stomach
• Physical action in stomach aided by chemical actions –
enzymes (pepsin) & HCl
• Chemical action breaks connective tissue
Digestion
• Physical breakdown complete when feed leaves stomach
• Chemical digestion accomplished thru hydrolysis
• Hydrolysis = splitting chemical bond by insertion of water
molecule
• Such bonds are
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Glycosidic linkages in CHO
Peptide bonds in proteins
Ester bonds in fats
Phosphodiester bonds in nucleic acids
• Enzymes catalyze hydrolysis
Digestion
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Enzymes are of two general classes
Those that act within lumen of gut
Those that act at membrane surface of epithelia
Lumen enzymes
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Originate from GI glands like salivary, gastric & pancreatic glands
Thoroughly mixed with ingesta
Actions thruout lumen of associated segments
Catalyse luminal phase of digestion
Results in incomplete nutrient hydrolysis
Results in short chain polymers from original macromolecules
Digestion
• Membrane surface enzymes
• Complete the hydrolytic process
• Enzymes chemically bound to surface epithelium of
SI
• Break short chain polymers to monomers
• Monomers absorbed across epithelia
• = membranous phase of digestion
• Followed by absorption
Summary
• Luminal phase
– Large polymeric molecules (starch & protein)
– Enzymes active in gut lumen (from salivary, gastric &
pancreatic glands)
• Membranous phase
– Small polymer molecules (polysaccharides, peptides)
– Enzymes active at surface of gut (synthesized in
enterocytes & attached to apical membrane
– Result in monomeric molecules for absorption
(monosaccharides, amino acids)
Regulation of GI function
• Diverse & specialized processes take place in different
sections
• Fundamental consistency in anatomy of the GIT
• Composed of 4 basic layers/tunics
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Tunica serosa
Tunica muscularis
Tunica submucosa
Tunica mucosa
• Mucosa most variable in structure & function
Regulation of GI function
Regulation of GI function
Read more
• http://www.vivo.colostate.edu/hbooks/pathp
hys/digestion/index.html
Quiz
1. Surface area of the small intestine is at three
levels of surface convolutions :
i. _______ circulares: Large _______ , not
present in all species
ii. _______ : finger-like epithelial projections
Increase area by 10-14 fold
iii. _______ border composed of _______
2. _______ of Lieberkühn are highly mitotic,
secretory cells at _______ of _______
3. Mucus blends into the gelatinous _______ ,
forming a viscous molecule-trapping layer
_______ : A layer of flowing water adhering to
the intestinal epithelium
Quiz answered
1. Surface area of the small intestine is at three levels of
surface convolutions :
i. Plicae circulares: Large folds, not present in all species
ii. Villi: finger-like epithelial projections
Increase area by 10-14 fold
iii. Brush border composed of microvilli
2. Crypts of Lieberkühn are highly mitotic, secretory cells at
base of villi
3. Mucus blends into the gelatinous glycocalyx, forming a
viscous molecule-trapping layer
4. Unstirred water: A layer of flowing water adhering to the
intestinal epithelium
Small intestines
Small intestines
Small intestines
Regulation of the GI function
• Robust & complex mechanisms for control &
communication
• Involves nervous & endocrine systems - inbuilt GIT
versions
• Many DS diseases are associated with dysfunction of
the relationship
• Regulation of GI function achieved thru
– Enteric/Intrinsic NS
– Enteric/intrinsic ES
– Motility of GI
Enteric/Intrinsic nervous system
• Influences motility, ion transport & GI blood
flow
• Control partly emanates from CNS
• Local NS k.a. enteric or intrinsic NS
• ENS has immense complexity and magnitude
• Has as many neurons as the spinal cord
• Principal components
– 2 neuron networks/plexuses embedded in GI wall
• Extend from esophagus to anus
Intrinsic NS
• Intrinsic motor nerves innervate
– Vascular muscle
– Gut muscle
– Glands in gut wall
• Gut smooth muscle innervation different from skeletal
muscle
• No direct synaptic junction btwn GI nerve & muscle
fibres
• Axons end in vesicular structures (varicosities)
• Contain neuroregulatory transmitter substances
Intrinsic NS
• Secreted by nerves in response to action
potential
• Affect actions of nearby muscles or glandular
cells
• Either inhibitory or stimulatory
• Gut also receives extrinsic innervation from
ANS
• Parasympathetic & sympathetic NS form link
• Link btwn intrinsic NS &CNS
Enteric nervous system
• Myenteric plexus
– Located btwn longitudinal & circular layers of muscle in the
tunica muscularis
– Exerts control primarily over GI motility
• Submucusa plexus
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Buried in submucosa
Senses env within lumen
Regulate GI blood flow
Control epithelial cell function
Sparse and missing in some sections where these functions are
minimal such as the esophagus
• There are also minor plexuses beneath serosa
Enteric nervous system
The arrangement of the enteric
plexuses, depicted for the small
intestine. A: appearance in separated
layers. The myenteric plexus, consisting
of numerous ganglia and connecting
nerve fibre bundles, lies between the
longitudinal and circular muscle layers. A
second ganglionated plexus is in the
submucosa. These plexuses provide
nerve fibre plexuses in the muscle, in the
mucosa and around arterioles. B: The
enteric plexuses shown in a cross
section of the intestine
Enteric nervous system
• Enteric plexuses have three types of neurons –
sensory, motor & interneurons
• Sensory neurons
– Receive info (sensory input) from receptors in
mucosa & muscle
– Different types identified
– Respond to mechanical, thermal, osmotic and
chemical stimuli
Enteric nervous system
• Mechanoreceptors (muscular layers)
• Monitor distension
• Chemoreceptors (mucosa)
– Monitor chemical conditions in lumen
• Thermal
– Monitor temperature
• Osmotic
– Monitor ion concentration
• Baroreceptors
– Monitor pressure
Intrinsic NS
• Motor neurons
– Control motility & secretion, possibly absorption
– Have large # of effector cells – smooth muscle,
secretory cells, GI endocrine cells
• Interneurons
– Integrate msg from sensory neurons & provide it
to motor neurons
Enteric nervous system
• Uses various neurotransmitters
• Acetylcholine is the major one
– Generally neurons using Ach are excitatory.
– Stimulate:
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smooth muscle contraction,
increases in intestinal secretions,
Release of enteric hormones and
Dilation of blood vessels
• Norepinephrine also used extensively
– Derives from extrinsic sympathetic neurons
– Effect is almost always inhibitory
Enteric nervous system
• Functions autonomously
• Note: digestion requires communication links with
CNS
• Parasympathetic & sympathetic fibers connect CNS &
ENS
• Or connect CNS directly to GIT
• Thru this gut provides sensory info to CNS & CNS can
affect GI function
• Signals from outside DS relayed to DS e.g. sight of
appealing food stimulates stomach secretions
Extrinsic Innervation
• Most of GIT receives parasympathetic innervation
from vagus nerve
• Part of terminal colon innervation thru pelvic nerve
• Preganglionic fibers synapse on cell bodies of
intrinsic system
• Extrinsic sympathetic fibres enter gut thru post
ganglionic fibres
• Sympathetic fibres synapse on neurons of INS
• Others have direct effect on GI muscles & glands
Enteric/Intrinsic endocrine system
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Hormones from many endocrine glands affect DS fxn
Most profound effect from enteric hormones
GIT is largest endocrine organ in the body
Three hormones best studied are gastrin, CCK & secretin
EES is diffuse with single hormone-secreting cells scattered in
mucosa
E.g. stomach G cells are scattered among epithelial cells
Hormones synthesized within cells & secreted into blood
Hormones secreted in response to fairly specific stimuli
Endocrinocytes respond to changes in env within DS lumen
Their apical border is in contact with lumen & continuosly
‘taste’ luminal env
Enteric/Intrinsic endocrine system
• Illustration of control: SI
– Ingesta from stomach = acidic
– SI is basic
– Presence of acidic ingesta stimulates secretin
secretion
– Secretin stimulates basic pancreatic secretions
that neutralize SI lumen contents
• ENS & EES interrelated
Enteric/Intrinsic endocrine system
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Extensive # & variety of endocrine cells
Usually grouped together into gland
GI endocrine cells thruout gut epithelium
Broad base & narrow apex
Narrow apex exposed to lumen
Sample luminal contents
Secretory granules in base
Storage form of hormones
Endocrine cell
Enteric/Intrinsic endocrine system
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Anatomy allows sensing lumen changes
Respond by releasing hormones
Hormone released into mucosal area
Absorbed in blood stream
Not secreted into gut lumen
Each E cell has characteristic distribution
E.g. gastrin producing cells mostly in distal stomach
Cholecystokinin in SI esp proximal region
Major GI hormones
Hormone
Site of
production
Action
Release
stimulus
Gastrin
Distal
stomach
1o: stimulates acid secretion
from stomach glands
2o: gastric motility; stomach
epithelium growth
Protein in
stomach; high
gastric pH;
vagal
stimulation
Secretin
Duodenum
1o: bicarbonate secretion from Acid in
pancreas
duodenum
2o: biliary bicarbonate sec
Cholecystoki
nin (CCK)
Duodenum - 1o: enzyme secretion from
ileum
pancreas
2o: inhibits gastric emptying
Proteins &
fats in SI
Major GI hormones
Hormone
Site of
production
Action
Release
stimulus
Gastric
Duodenum +
inhibitory
upper
peptide (GIP) jejunum
1o: inhibits gastric motility
& secretory activity
2o: stimulates insulin
secretion if sufficient glc
present
CHO & fats in
SI
Motilin
1o: regulates motility
patterns btwn meals
2o: may regulate tone of
lower esophageal
sphincter
Acetylcholine
Duodenum +
jejunum
GI motility & circulation
GI motility
• Muscle contractions & motility are integral parts of
digestive fxn
• There are two fundamental patterns of motility
• Propulsion principally thru peristalsis
• Mixing thru segmentation contractions esp in SI
• These facilitate digestion and transportation of
ingesta
GI motility: peristalsis
GI motility: mixing
GI motility
• Continuous contractions occur
• The faster the contractions the faster the ingesta
movements
• More contractions when feeding than at rest
• Amplitude of contractions vary
GI motility - parasympathetic
• Acetylcholine
• Very specific control
• Increase GIT activity by
– Promoting peristalsis
– Relaxing sphincters
– Increase rate of glandular secretion
• Mouth & stomach
• Small & large intestine are largely controlled
by local factors
GI motility - sympathetic
• Norepinephrine
• Mass discharge (all or nothing)
• Decrease GIT activity by
– Decreasing peristalsis
– Contracting sphincters
– Can totally inhibit movement
Gastric motility
• Fundus/corpus
– Storage, volume adaptation (receptive relaxation)
– Muscle cells partially contracted at resting membrane
potential
• Hyperpolarization leads to relaxation and increased volume
– Relaxation under parasympathetic control
• Swallowing, esophageal & stomach distension
• Antrum – pumping action,
– contractions every 3-5 min, vagal stimulation
• Pyloric sphincter mixes by blocking passage,
coordinated flow (2-10 ml/min)
Gastric secretion & motility
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Under neural & humoral mechanisms
Neural: local autonomic reflexes
Involve cholinergic neurons & impulses
From CNS thru vagus nerve
Humoral: hormones e.g. gastrin, CCK
Vagal stimulation increases gastrin secretion
Some vagal fibers direct stimulation of fundus cells
Increase acid & pepsin secretion
Gastric motility & emptying
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Entry of feed in stomach
Organ relaxes by reflex
Process of receptive relaxation
Triggered by pharynx & esophagus movement
Peristaltic contractions follow
Mix & squirt feed into duodenum
Controlled rate
Peristaltic wave, most marked in distal stomach
When well developed occur 3/min
Gastric motility & emptying
• Rate of emptying into duodenum depend on
feed type
• CHO rich feed leaves stomach in few hrs
• Protein –rich feed leaves slowly
• Slowest emptying with meal containing fat
SI motility
• Coordinated contractions facilitate digestion and absorption
• Chyme mixed with digestive enzymes from pancreas and bile salts
from biliary system
• Nutrient molecules in lumen constantly dispersed,
• Allows epithelial contact to complete enzymatic digestion and
absorption
• Chyme moves down
– making way for next load
– eliminating indigestible/ toxic substances
• Two cycle states, each with distinctive patterns of motility (mixing &
peristalsis)
– Interdigestive
– Feed state
Migrating motor complex (MMC)
• Interdigestive period (between meals)
– lumen largely devoid of contents
– housekeeping contractions propagate from the
stomach through the entire small intestine,
sweeping it clear of debris.
– Complex pattern of motility
– the cause of "growling".
Migrating motor complex (MMC)
• Fasting motility in stomach & SI
• Phase I – inactive 30-60 min
• Phase II – irregular 20 – 40 min
– ½ segmentary ½ peristaltic
• Phase III – intense contractions
– ¾ peristaltic
– 10-20 min
• Phase IV – spiking activity subsides
The interdigestive motility pattern
– 0-5 min
(migrating motor complex) is
organised in cycles of activity and
quiescence
Migrating motor complex
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Cycle last 80-120 min
Develops 4-5 hrs after meal
Starts in stomach and moves to ileum
Starts over when reaches terminal ileum (one
part always active)
• May be only motility pattern for non-meal
feeders (cattle)
Migrating motor complex functions
• House keeping
–Sweep debris and bacteria down GIT
–Bacterial overgrowth is likely without
phase III
• Muscle tone – helps keep muscle
functional
Feed state motility (FSM)
• Following a meal (Chyme presence)
– segmentation contractions: chop, mix and roll the
chyme
– peristalsis slowly propels it toward the large intestine
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Duodenum contracts 2 -3 times per antral wave
Moves ingesta down
Segmentary rhythmic mixing
Peristaltic moving of ingesta – move 4-8 inches
per wave (mixing inevitable)
Feed state motility (FSM)
• Continuous irregular motility (phase II of
MMC)
• ½ segmentary & ½ peristaltic
• Simultaneous activity through stomach & SI
• Begins with eating
• Last 4 to 5 hours after a meal
SI motility regulation
• Controlled predominantly by excitatory and inhibitory
signals from enteric nervous system
• Modulated by
– inputs from ANS & CNS,
– gastrointestinal hormones
• Coordinated movement from stomach down
• Feedback inhibition optimize digestion & absorption
• Duodenal & jejunal receptors sense caloric chyme
content
• Increased content inhibit stomach contractions
Large intestine
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Extended retention of ingesta (1-2d)
Proximal (storage fermentation)
Distal (elimination of fecal waste)
Haustrations
– Not rigid or stationary
– Reduce transit rate
– Increase retention time
Large intestines
• Motility
– Single haustra contractions
– Some sequential contractions
– Move orally in proximal colon – delayed transit
– Move backwards in distal colon – increased transit
– Few peristaltic contractions
Large intestines
• Mass movements
– Occurs several times/day, usually after meal
– Disappearance of haustral folds
– Contractions move caudally propelling digesta to
distal colon/rectum
– After movement haustral folds return
– Parasympathetic –increases motility
– Sympathetic – decreases motility
GI circulation
• Main arteries
– Celiac – stomach, spleen, part of pancreas
– Hepatic - 1/3 of blood flow
– Superior mesenteric – SI, part of pancreas of LI
– Inferior mesenteric – main supply to LI
• Veins
– Portal vein - Drains blood from GI
– PDV = portal drained viscera
– Hepatic vein drains nearly all splanchic blood
Intestine circulation
• Most perfused tissues in body
– Each villus with own vein and artery
– 50-60% blood to liver & SI
• PDV receives 30-35% of total cardiac output
• Increase in dietary intake increases flow to PDV
– Increased cardiac output
– %PDV remains relatively constant
• 75% to mucosa & submucosa – very metabolically
active
• 25% to muscularis & serosa
Counter-current mechanism
• Allows water/solute to pass from arteriole to
venule without passage through capillary bed
• Arteriole <20 µm from venule
• Vasculature has limited permeability
– Na+ cannot pass without transporter
– H2O & O2 can pass
Hunger contractions
• Stomach musculature rarely inactive
• After emptying, mild peristaltic contractions
begin
• Gradual intensity increase
• More intense, mildly painful
• Associated with hunger sensation
• Interaction with feeding & satiety centres in
brain
Vomiting
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Visceral reflex
Starts with salivation & nausea sensation
Glottis closes
Prevents vomitus aspiration into trachea
Contraction abdominal wall muscles
Increase intra-abdominal pressure
Chest in fixed position
Esophagus & gastric cardiac sphincter relax
Reverse peristalsis begins
Vomiting
• Gastric contents ejected
• Vomiting centre in medulla oblongata
regulates
• Stimulated by
– upper GIT mucosa irritation
– Chemoreceptors stimulated by circulating
chemical agents
– Emotionally charged stimuli e.g. smells, sight
Absorption
• Substances pass thru lumen of GIT into circulation
• Different ways
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Diffusion
Non-ionic diffusion
Facilitated diffusion
Solvent drag
Active transport
Endocytosis
Absorption
• Movement of products of digestion across intestinal mucosa
into vascular system for distribution
• Need to understand
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Processes of diffusion
Differences in composition of ICF & ECF
Electrical polarity across membranes
Na+, K+ ATPase pump
Selective ion channels
• Uses specialized nutrient transport systems
• Exist in apical & basolateral membranes
• Involve specific proteins embedded in membranes
Absorption
• Proteins provide transport pathway
• Interact with specific organic nutrients & inorganic ions to
effect their transport across membranes
• Transport mechanisms classified as
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active,
secondary active,
tertiary active and
passive
Absorption
• Active transport
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Involves direct consumption of metabolic energy – ATP
ATP expended to move ions/molecules against electrical/chemical gradient
Na+,K+-ATPase pump important transport pathway
Lies in basolateral membrane
• Secondary & tertiary active tranport
– Utilize transcellular Na+ ion electrochemical gradient as source of energy
• Passive transport
– Occurs either thru ion channels in cell membranes
– Or directly thru the tight junctions
Water & Na+ Absorption
• Na+ actively absorbed by SI to absorb water
from lumen
• Water movement follows Na+ via osmotic
gradient
• Water moves out of arteriole to venule due to
Na+ gradient created from venule to arteriole
• Increased Na+ conc in arteriole pulls water
from lumen
Summary
• Digestion & absorption depend on mechanisms that:
– Propel feed thru GIT
– Soften feed
– Mix feed with bile & digestive enzymes
• Mechanisms involve
– Dependence on intrinsic properties of intestinal muscle
– Operation of visceral reflexes
– Action of hormones
• GI Motility and circulation designed to suit and facilitate
both digestion and absorption