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Intestines 2/ (LARGE INTESTINE) (Fat digestion and absorption…
Intestines 2/ (LARGE INTESTINE)
Fat digestion and absorption
Carnivore/omnivore diets high in fat, herbivore diets low (but herbivores are able to absorb fat)
Main source of fat in diets = Tri-glycerides of 16 or 18 C atoms
Lipase breaks down tri-glycerides into mono-glycerides and FFAs
Most lipase in pancreatic juice
Fat digestion also requires bile salts (secreted by liver)
Bile salts
Lipase water soluble/fats are water insoluble. Digestion must occur at water/fat interface
Bile salts emulsify fat
Bile has water soluble and lipid soluble component
Mixing contractions of SI break fat into smaller droplets, bile keeps them as droplets
Micelles
Emulsified fat droplets too large to fit between microvilli
Bile salts aggregate together forming micelles
Micelles absorb mono-glycerides and FFAs
Micelles small enough to enter space between microvilli
Cellular metabolism of fat
Endoplasmic reticulum re-esterifies FFAs/mono-glycerides to form tri-glycerides
Coalesce with cholesterol & phospholipids to form chylomicrons
Chylomicrons packaged in golgi apparatus, transferred to basolateral membrane then into ECF via exocytosis
Fat soluble vitamins absorbed in same way
Too large for capillaries so transported into lymphatic vessels (BYPASS LIVER)
Absorption of water
Para-cellular - Across tight junctions between epithelial cells
Trans-cellular - Across cell membrane via transporter proteins
Most water is re-absorption of digestive juice (~80%) in small intestine (carnivores/omni ~90%, ruminants ~70%)
Horse - Most absorption occurs in large intestine
Absorption of minerals
Most absorbed irrespective of requirements
Na+ via many transporters
K+ and Cl- via diffusion
Bicarb by bicarb/chloride antiporter
Absorption of iron
Regulated according to requirements
Secondary active transport, couples with H+ via divalent metal transporter 1 (DMT-1)
When iron low transferrin is unsaturated, Fe2+ transferred to blood and binds to transferrin
When iron high, transferrin saturated. Fe2+ remains in cell bound to apoferritin to form ferritin
Fe3+ poorly absorbed, reduced to Fe2+ by vitamin C or brush border enzyme (ferri-reductase)
Absorption of calcium
Absorbed according to requirements
Passive active transport
When calcium low, active transport stimulated by calcitrol
Ca2+ pumped out of cell by Ca2+ ATPase
Intro to large intestine
Functions: Absorption, fermentation
Glands in mucosa: Secrete mucous, bicarb to neutralise VFAs, no digestive enzymes
Functional anatomy (Large intestine)
Caecum
Colon:
Ascending/transverse/descending
Ascending modified in different species
Rectum
Histology (large intestine)
4 layers (same as rest of GI tract):
Mucosa
Submucosa
Muscularis
Serosa
Colonocytes (absorptive) - No villi, only crypts, microvilli are less dense than in SI
Goblet cells - More than SI, predominate in crypts
Fermentation
Microbial fermentation of VFAs (as energy source especially in horse)
VFAs
Acetate: Used in liver, oxidised in other cells to generate ATP, major source of acetyl CoA for lipid synth
Propionate: Substrate for gluconeogenesis
Butyrate: Energy production, cellular homeostasis
Absorption
VFAs absorbed by SCFA/bicarb exchanger
Na+ absorbed by sodium channels & Na+/H+ exchanger
Cl- - Absorbed by bicarb/hydroxyl exhcnage
Water - Osmotic pressure, hydrostatic pressure, solvent drag (extensive in horses)
Diarrhoea
Diarrhoea = Water lost in faeces through increased secretion and/or decreased absorption
Nutritional (microbial imbalance): Overload of gut capacity causes osmotic flow of water into gut lumen
Infections (e.g. E.Coli, salmonella, cholera): Bacterial enterotoxins bind to cell membrane stimulating cAMP in glands
Stress (strong activation of parasymp system)
Treatment
IV fluids (fluids, ions to replace, bicarb for metabolic acidosis)
Oral rehydration therapy (solution with NaCl & glucose, causes more efficient water absorption)
Regulation of motility
Intrinsic
Pacemaker cells, slow oscillations
Pacemaker region in centre of colon sense waves in both directions (peristaltic and anti-peristaltic contractions)
Pacemaker region at distal end of colon sends waves aborally - peristaltic contractions
Extrinsic
Stronger neuronal influence than rest of gut
Transit of food faster after a meal
Constipation
Abnormal accumulation of food material in gut, usually in colon
Common in dogs that have eaten bones or dry food
Prostate/anal glands
Megacolon (contractions weakened)
Treated by:
Oral fluids to soften faecal material
Paraffin oils to lubricate
Drugs to strengthen contractions
Defaecation
Rectum usually empty
Mass movement of colon transfers faeces to rectum
Pressure sensitive cells stimulate defaecation reflex
Outer sphincter under conscious control in carnivores/omnivores but not horses/ruminants
Can consciously retain anal spgincter closed until urgency to defaecate deminishes
Inner sphincter relaxes
Contraction of abdominal muscles helps evacuation process