Download Digestion and Absorption

Digestion and Absorption
Energy and nutrient acquisition for life activities.
No photosynthetic ability -in higher forms
Over evolution development of the digestive system can be
observed
Simple devices for filteration of particles in water
Locomotion to acquire food.
Development of a complex system for digestion
Development of a system for absorption
Development of metabolic processes to manage
digestion of food
absorption of nutrients.
Associated Organs:
Sensory organs - sight, smell, taste and feel.
Salivary glands - moisten food
Pancreas - digestive function, metabolic
regulation
Liver - digestive and metabolic function,
Gall bladder - digestive function
Extrinsic and Intrinsic Nervous system - Control
of digestion
Organs of digestion:
Mouth - acquisition of food - particle size reduction
Esophagus - transfer of bolus
Rumen complex - fermentative digestion - three
chambers
Stomach/ Abomasum - acid digestion reduction in
particle size
Small intestine - enzymatic digestion & absorption
Cecum - fermentative digestion
Large intestine - Water re-absorption and stool
formation
Structure.
Organized for regulated passage: Muscular organ
Vascular - to facilitate nutrient absorption
Enervated - to maintain control
the Gastrointestinal tract is organised in four layers.
1. Mucosa : epithelium - columnar, glandular, squamous cells,
muscularis mucosa smooth muscles.
2. Submucosa - connective tissue with blood vessels, lacteals, nerves,
neurons.
3. Muscularis externa - two layers of muscles circular and
longitudinal. -smooth muscle
4. Adventitious coat - loose epithelium
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Structure - smooth muscle
Spindle shaped cells Bundles - 75-200
Central nucleus Mitochondria - energy transactions for contraction
Vesicles - calcium required to participate in contraction
cells are linked - gap junctions
Protein of contraction - actin and myosin.
Organized in two layers - Longitudinal and circular
Structure - vasculature
Many arteries provide blood to the GIT
Rate of blood flow is approximately 1.5 l/min
increased by action of hormones and local reflexes
Blood from GIT is collected by venules to veins to the Portal vein
taken to Liver
Collected from liver to Inferior vena cava - to heart.
Lymph - serous fluid
propelled by organ rhythm Lymph ducts to the Thoracic duct to
Inferior venacava
Pass through lymph nodes for filteration.
Structure - Enervation
Controlled by two nervous systems - extrinsic & intrinsic
Extrinsic - Somatic and Autonomic
Somatic - control of voluntary/ striated muscles ,eg. In mouth and
anus
Autonomic - Sympathetic and Parasympathetic -Coordination of
smooth muscle activity by reflexes - not under voluntary control
Organs - brain, spinal chord neurons with long fibres
Sympathetic nervous system - transmitter is Adrenalin - relaxing
effect
Para sympathetic system - Acetyl choline - change cell membrane
permeability to allow water uptake and become tense
Principal nerve is the Vagus, but there are 12 pairs of cranial nerves
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Structure - Enervation
Electrical conductivity - cell to cell
Longitudinal to circular muscles
Electrical conductivity influenced by chemical constituents.
Rhythmic contraction activity in the GIT- unidirectional.
Functions of the enteric nervous system
1. Contraction of smooth muscles - create peristaltic wave
2. Release secretions from cells in the GIT
3. Release hormones from endocrine cells
4. Control patterns of blood flow
5. Control activities of the immune cells.
Intrinsic nervous system - Meisner and Aurobach Systems.
Local control
Complete initiated motion
Influenced by substances in extra-cellular fluid.
Which set off receptors in GIT.
Regulatory peptides
Control of digestive function - motility, intestinal absorption, cell
growth, secretion of digestive enzymes, electrolytes, water, other
hormones, and mucins
Gastrin - acid secretion - G cells
Cholecystokinin - CCK - pancreatic enzyme secretion
Secretin - bicarbonate secretion
Gastric inhibitory peptide Somatostatin - paracrine action
Vasoactive intestinal peptide - nerurocrine
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GIT hormones
GIT hormones
Gastrin - Synthesized and secreted by G cells of antrum
Stimulates - acid secretion in fundic cells, pepsinogen secretion,
gastric motility and trophic action on GIT cells
stimulated by vagal stimulation,
ingestion of food, distension, fat
GIP - inhibits gastric secretion and stimulates intestinal secretions
and insulin secretion
CCK - secreted by I- cells of intestine into blood stream
Stimulates - enzyme secretion from pancreas, contraction of the gall
bladder augments secretin action
Stimulated by protein and fat in duodenum
Motilin - Contraction of the smooth muscle
Secretin - secreted by S cells if the intestine. Target cell pancreatic
acinar cells for release of bicabonate and water.
Substance P - neuropeptide increases blood flow to the GIT,
Inhibits acid secretions, intestinal motility and pancreatic enzyme
secretion.
Somatostatin - by D cells- distributed at many locations. Inhibitory
in function paracrine in effect
VIP - released from nerves - stimulates GIT bicarbonate secretions
and relaxes sphincters
Mouth and esophagus
Mouth - oral cavity entry way to the digestive tract
Pharynx- orderly passage of the food
Esophagus - passage way to distal GIT - Lower esophageal sphincter
Chewing - tearing & grinding action by teeth, facilitated by the
tongue and by mixing in of saliva
Saliva - many triggers
Functions - moisten food, dissolve & disperse sapid substances
Oral cavity lubrication, enzymatic digestion - amylase, lipase
Composition 99% water and ions (Na, K, HCO3, NH3) <.5%
protein, mucins
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Rumen
Mouth and esophagus
Three sets of salivary glands -sublingual, parotid, submaxillary
control - involuntary and voluntary control.
Swallowing - reflex - under neuronal control
Can be voluntary
Vomiting - not reverse peristalsis, brought on by increasing pressure
by action of the diaphragm
Ruminants - suborder Ruminantia - animal families - Giraffidae,
Cervidae, Antilocapridae, Bovidae and Camelidae eg. giraffes, deer,
antelope, cattle and camels.
Chew their cud,
Complex of four chambers Reticulum - recepticle - connected to rumen and omasum by reticulo
orifice - participates in regurgitation and passage of liquid to omasum
Rumen - fermentation chamber, muscle pillars
Omasum - water absorption - may have nutrient absorption ability
Abomasum - comparable to the stomach in non-ruminants
papilae
Reticulu
m
Rumen
stratified squamus epithelium organized as
different structures in the three
forestomachs of ruminants
Omasum
leaves
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Table 1. Relative number and mass of microbial organisms in the rumen
Number
Net mass
Percent of total
Group
per mL
(mg/100 ml)
microbial mass
Small Bacteria
1 x1010
1600
60-90
Selenomonads
1 x 108
300
Oscillospira flagellates
1 x 106
25
Entodinia
3 x 105
300
Dasytricha+Diplodinia
3 x 104
300
Isotricha+epidinia
1 x 104
1100
Fungi
1 x104
-
Ciliated protozoa
10-40
5-10
Nitrogenous feed fermentation in the rumen
Table 2. Effect of dilution rate on fermentation products in a continuous fermentor with
mixed rumen population and using glucose as an energy source.
Dilution (turnover) rate (h)
Product
0.02
0.06
0.12
Acetatea
1.18
1.11
1.13
Propionatea
0.16
0.22
0.26
Butyratea
0.23
0.18
0.15
a
1.04
Methane
1.67
1.34
a
5.59
5.19
5.03
YATPb
7.50
11.6
16.70
ATP
Yglucosec
42.40
60.20
83.90
Nitrogen in cells (%)
9.90
-
12.00
Microbial crude protein/mole glucose (g)
26.1
-
61.20
Adapted from Van Soest (1994) Microbes in the Gut. In 2nd Edition Nutritional Ecology
of the Ruminant. Comstock Publishing of Cornell University Press Ithaca and London. p
– 253-280
a
Moles produced per mole glucose fermented
b
VFA production during fermentation of carbohydrates
c
g cells/mole ATP
g cells/mole glucose
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Abomasum/Stomach
The stomach
11% of GIT in mature animals
Two parts:
Fundus and antrum
Invaginated inner lining - gastric pits
Three muscle layers - circular, longitudinal and
oblique
distal stomach - circular muscles thicken
into pyloric sphincter
Gastric pits
Gastric pits
Numerous types of cells produce variety of
products
1. Mucoid cells - mucous
2. Oxyintic/parietal cells - acid, intrinsic factor
3. Chief/peptic cells - enzymes -pepsinogen,
renin
4.
Enterochromafin cells - histamine
Cells in antral region G - gastrin and Dsomatstatin
Hersey and Sachs 1998 Gactric acid secretion,
Physiol Rev 75:155-189.
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Stomach emptying
Food entry into abomasum - continuous
meal eating animals - exponential emptying
related to volume
movement includes - propulsion - distal
Grinding - demunition of food
retropulsion - retention of digesta
Factors affecting emptying:
pH, osmolarity, fat, fatty acid, ionization, fatty
acid unsaturation, Tryptophan.
Liver and its function
Liver : excretory, digestive, metabolic and
immunity.
Urea synthesis from ammonia for excretion,
Detoxification of toxins and RBC pigments for
excretion
Bile salts synthesis - aid in fat digestion
Synthesis of Proteins, glycogen, glucose and
cholesterol
Haemopoietic for embryo
Storage of minerals and vitamins.
Cells of the liver:
Endothelial cells - line sinusoids - filter
Kupffer cells - macrophage like function
Adipocytes - fat synthesis and vitamin storage
pit cells - endocrine function regulatory role
hepatocytes - principal cell - protein synthesis and bile
salt synthesis by conjugation of bile acid with amino acid.
Stored in gall bladder
Bile - surfactant for fat digestion.
Enterohepatic circulation Gall bladder - intestine deconjugation in L intestine - absorbed and returned to
liver.
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Pancreatic Function
Exocrine function: digestive enzymes
Enzymes proteins synthesized by cells in the ascinus.
Delivered to the small intestine as inactive zymogen
granules.
Endocrine function: Protein hormones secreted into blood.
Synthesis by islet cells
Stages in Protein synthesis and secretion: Synthesis,
Segregation, Intracellular transport, Concentration,
Intracellular storage, Discharge.
Triggers: CCK, neural input and insulin
Pancreatic enzymes
Proteases Trypsinogens - endopeptidase, activated by entrokinase of instestine,
autocatalysed and catalyses other proteases - large peptides
Chymotrypsimnogens - large peptides
Proelastease - connective tissue digestion - large peptides
Pro-carboxypeptidase - small peptides
Pancreatic amylase - starch - disaccharides
TG - lipase - hydrolyses fatty acids from the top and bottom of TG
Pro-co-lipase - activator for lipase
Cholesterol esterase - hydrolyses cholesterol esters
Phospholipase A2 - fatty acid from phospholipids
Intestines
Cattle -
Sheep/goats
SI Cecum LI
capacity
SI
Cecum
LI
capacity
46 m 1m
10m
26m
0.6m
6m
252 L
356 L
29 L
44L
DNAase and RNAase - digestion of DNA and RNA
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Structure of the small intestine
Sections of the small intestine:
duodenal - short
jejunal - extended and highly folded
ileal - distal portion - under control of ileo cecal
sphincter
Crypt - regeneration centre of the cells on the villi
Villi - finger like projections.
Villus -centre have muscle , blood capillaries,
lacteals,
Villi and Crypt
Figure 44. Microscopic appearance of normal duodenal villi in the
pig as viewed from the lumen.(From and with permission of
Hornich et al., 1973 and The American College of Veterinary
Pathologists, Inc.).
Villi: monolayer of cells
origin from bottom of crypt and migrate to top and
are extruded from the tip.
Differentiate into many cell types
goblet cells - mucous
endocrine cells - hormones
tuft cells - chemoreceptors
paneth cells - secretory at base of crypt - Ig - G&A
and secretory component
Columnar cells - enterocytes
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Endocrine cells
EC SIKMo DLN-
Seretonin, Substance P
Secretin
Cholecystokinin - Intact protein and fat
Gastric inhibitory peptide
Motilin
Somatostatin
Glucagon -like - peptide
Neurotensin
Enterocyte
• Principal cell type - organized in a single layer
• Attached to a basement membrane
• Participate in membrane bound enzymatic
hydrolysis
• Tight junctions
• Microvilli - glycocalyx - trap susbtrate
• IMP - scharidases - sucrase, maltase, lactase,
transport proteins eg SGLT1, CaBP, B12+IF
receptor
• Basal membrane - Na/K ATPase, GLUT2
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Small intestinal digestion
Luminal digestion: Pancreatic enzymes
proteases, - protein to peptides amino acids
amylase - starch to limit dextrins and maltose
lipase and co-lipase - lipids to fatty acids,
cholesterol and glycerol,
nucleases - recovery of phosphorus
Membrane bound digestion: disacharidases hexoses
Cellular digestion: peptides to amino acids
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Absorption
Peptides and Amino acids
Glucose: Carrier mediated absorption SGLT1
Saturable kinetics,
Graded substrate specificity
P-D gradient in expression
Inducible by substrate
Large protein with 7 trans membrane spans
Released by GLUT2 at BLM
Fascilated transport - GLUT5 - fructose at mucosal
and released by GLUT2 at BLM not energy
requiring
AA - poorly absorbed , energy requiring
Peptides - transport across mucosal membrane into
enterocyte depends upon:
chain length
dipeptide > tripeptide>>tetrapeptide
3 types of transporters
Na dependant - primary active
Na independent - secondary active
H+ [] gradient - tertiary active
Peptides hydrolysed intracellularly and released
into blood by electro-chemical gradient.
Factors controlling peptide
transport and hydrolysis
Peptides
Di P
Tri P
Tetra P
LL Di P
DD Di P
DL, LD P
LD P
DL P
transport
+++
++
+
+++
poor
+++
hydrolysis
+++
++
+
+++
++
+
++
Configuration of AA at COOH end of peptide is
important for transport
Transporters expressed in P-D gradient.
All nutrients in blood transported to liver by portal
vein.
Blood flows through the liver to hepatic veinules
and collected into hepatic vein to inferior vena
cava to heart for general circulation
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Un-absorbed components of digesta enters Large
intestine
Water re-absorption
Fermentative digestion of digestible substrate
if ileal digesta - high residual starch
fermented by rumen type bacteria
consumption - excretion = gross measure of
nutrient extraction
Formation of feces and evacuation
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