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Equine gastro-intestinal system (Water absorption (At luminal pH most VFAs…
Equine gastro-intestinal system
Functional anatomy
Ascending colon (large) vastly modified into fermentation chamber
3 compartments separated by physiological valves
Ventral colon from dorsal colon - Narrow pelvic flexure
Dorsal colon from small colon - Narrowing transverse colon
Caecum separated from ventral - Caeco-colic valve
Transverse colon relatively short and norrows down to meet descending
Longitudinal muscle not continuous strand, arranged in taenial bands
Different number of taeniae in different parts
Shorter than LI itself resulting in sacculations
Enable mixing and delays transit time to allow fermentation
Fermentation
Vast majority of non-hydolysable CHO reaches LI, so extensive fermentation (cf moderate in ruminants)
Microbial products are VFAs, CH4, CO2
Some VFAs and ALL microbial protein egested (rabbits/rats - coprophagy)
Metabolism of VFAs
Energy source, especially horse
Homestasis of colonic epithelium by regulating genes controlling proliferation/apoptosis/differentiation
Acetate - Used in liver, oxidised in other cells to generate ATP, major source of acetyl CoA for lipid synth
Propionate - Substrate for gluconeogenesis
Butryrate - Energy production + cellular homestasis
Absorption
VFAs Absorbed by SCFA/bicarb exchanger
Na+ - Sodium channels + Na+/H+ exchanger, enhanced by aldosterone
Cl- - Bicarb/hydroxyl exchange
Water - Extensive reabsorption in horses as lots of water reaches LI, osmotic pressure, hydrostatic pressure and
solvent drag
Comparative physiology
In ruminants and hCHO immediately fermented so none passes into SI
In horse potentially lots of hCHO can pass into LI if overload of SI capacity to digest/absorb
SI occurs before fermentation vessel
Carbohydrate fermentation
VFAs cause decline in pH of luminal content - Can alter microflora
Can alter microflora
Protein fermentation
Equine hind-gut more capable of absorbing amino acids/peptides
Hence less taken up by microbes ("stolen")
Microbial nitrogen requirements met by urea secreted from ileum/LI
Water absorption
At luminal pH most VFAs in ionic form - poorly absorbed
Local mechanism of converting ions to respective acids to enhance absorption
Bicarb secreted in exchange for Cl- maintain pH at suitable level
When VFas absorbed net absorption of NaCl - Solvent drag so water reabsorbed
VFA absorption
Cf to ruminants VFAs absorbed intact rather than metabolised as absorbed
As in ruminants proportion of propionic acid increased with more hCHO passing into LI (If fed in excess, amylolytic bacteria with proliferate too much)
Microbiology
Bacteria - Lower levels of hCHO so amylolytic population consequently lower so fermentation slower
Protozoa - Ciliates as in ruminant but different species, small number but bacteria larger, if removed very little difference to fermentation efficiency
Fungi - Probably similar to ruminants, split apart lignin
Fore-gut vs hind-gut fermentation (hind gut 70% efficient as foregut)
Microbes less efficient, lower degree of amylolytic fermentation
Hind-gut fermenters lose microbial protein
However gut transit time can be altered to consume more if on low quality forage
Motility
Large intestinal transit time (fermentation takes several days in hind gut fermenters)
In horses caecum separated from colon by caeco-colic valve. No retrograde flow from colon to caecum
Caecal contractions: Segmental contractions principal type, mass contractions occur every 3-5 mins
Colonic contractions: Main are segmental, peristaltic and anti-peristaltic. Anti-peristaltic contractions more prominent in proximal colon (no retrograde flow)
Anti-peristaltic contractions occur principally in distal part of ventral colon (HORSES), slow movement of chyme from ventral to dorsal colon. Only small particles can pass via pelvic flexure ensuring slow transit time
Equine colic
Clinical sign of abdominal pain
True colic = Gastro-intestinal pain
False colic = Other abdominal organ pain (e.g. bladder, kidney, uterus)
Colic results in highest levels of equine morbitity & mortality and therefore accurate diagnosis is essential to treat successfully
Most useful diagnostic procedure is rectal examination (can only reach caudal 1/3rd of abdomen)
Rectal exam [use pictures from powerpoint]