large intestine physiology Flashcards
what are tenia coli
three longitudinal smooth muscle bands around the large intestine
what are haustra
small pouches generated by sacculation, due mainly to tenia coli shorter length
what is SCFA production - fermentative digestion
non-enzymatic digestion by microbes which produce short-chain (volatile) fatty acids (SCFA or VFA). similar to bovine rumen
what are the 3 VFA
acetate, propionate, butyrate
what do VFA do
ferment hydrolysable nutrients that escape digestion/absorption in SI and indigestible complex molecules that reach LI intact
how much energy do horses obtain from SCFA produced in the ceum
cencum and ventral colon
30%
up to 70% horses energy from LI
what is butyrate
- respiratory fuel for colonic epithelium
- downregulates inflammatory genes
- maintains epithelial homeostasis
what is propionate
used by liver as precursor for gluconeogenesis
what is acetate
used by peripheral tissues and precursor for lipogenesis
small amounts of lactic acid is…
poorly absorbed and not a major nutrient
increased amounts of lactic acid…
intraluminal acidosis and increase in intraluminal osmolality - disease
proteins produced by microorganisms are …
not usable b/c amino acids and proteins are not absorbed from large intestine
nitrogen excretion
- synthesis of microbial protein consumes non-protein nitrogen
- traps nitrogen in the lumen of the LI
what makes up the microbiome
bacteria, protozoa, fungi, archaea, viruses, parasites
high diversity and species richness is associated with
health
LI is a relatively ____ environment compared to the upper GI tract
stable
what does a hay diet produce
a more stable microbiome
clostridia
major SCFA producer
lachnospiraceae and ruminococcaecae
- butyrate - critical for mucosal health and depleted in disease
- turns lactate into acetate and propionate
streptococcus
amylolytic, lactate producer
lactobacillus
lactate producer
what does the ratio of VFA:lactic acid depend on
diet
* grain = more propionic and lactic acid - decreases intraluminal pH
* grain decreases ruminococcus and fibrobacter, clostrodoales, lachnospiraceae
what is toxic to LI microbiota
fat
* high oil, increases proteobacteria
what happens when there is a decrease (<50%) firmicutes
disease (colic)
what happens when there is an overabundance in proteobacteria (upper GI)
diease (colic)
what are the functions of the LI
- supply nutrients, water, electrolytes, VFAs, and vitamins
- maintain ideal environment for microbiome
- protect mucosa
- maintainlow viscosity of digesta and lubricate mucosa
- excrete substances such as nitrogen and potassium
- occurs via crypts and surface epithelium
what is the LI a reservoir of
water and electrolytes
what percent of a horses body weight do they secrete water and electrolytes into their digestive tract daily
30%
what are the primary electrolytes that are secreted
Cl-, Na+, HCO3-, PO43-
affected in diarrhea
water secretion is ____ driven by osmotic gradient generated by solute secretion
passive
what is electrolyte secretion driven by
electrocgenic Cl- secretion
how is Na+ and K+ secreted into lumen
paracellular pathways
what are the electrolytes that are primarily absorbed
Na+,K+, Cl-, P
how is Na+ actively absorbed
by electrochemical gradient generated by Na+/K+ ATPase
what is electroneutral Na+ absorption
- Na+ exchanged for another cation (H+)
- coupled with Cl- and VFA absorption in exchange for HCO3-
what is electrogenic Na+ absorption
- sodium absorbed against electrochemical gradient
- uncoupled entry of Na through Na channels - epithelial sodium channel
what is electrogenic Na+ absorption regulated and inhibited by
aldosterone and diuretics that close the sodium channels
how is K+ acitvely absorbed
in exhange for H+ via H+, K+ ATPase
how is Cl- passively absorbed
with cations (Na+) via paracellular pathways and by Cl- HCO3- echange through transcellular pathway
how is P diffused
paracellular pathways
____ movement of water occurs ____ along hydrostatic, oncotic, and osmotic gradients
bidirectional; passively
hydrostatic gradient
interstitial - capillary pressure
oncotic gradient
interstitial - capillary pressure (macromolecule conc.)
osmotic gradients
intestinal lumen, cytoplasm, interstitial space, capillaries
* electrolytes (Na+), VFAs
* affected by absorption, secretion, and intraluminal fermentation
what is the main mechanism of absorption of SCFA
transcellular diffusion of protonated SCFAs
what is required in the absorption of SCFA
high H+
SCFA are absorbed in echange for
HCO3-
what are vesicles and what are they dependent on
- monocarboxylate transporter protein
- pH, time, concentrate
what is the order of VFA/SCFA of absorption
acetate>propionate>butyrate
what is indirect regulation of LI secretion and absorption
motility, capillary hydrostatic pressure
what is direct regulation of LI secretion and absorption
direct control of epithelial ceells - autocrine, paracrine, endocrine, immunologic, neurocrine mechanisms
what do regulatory cells function as
sensors or connected to other cells with capacity to respond to stimuli (low systemic blood pressue, hyperkalemia)
what is sympathetic stimuli of LI regulation
pro-absorptive
what is parasympathetic stimuli of LI regulation
anti-absorptive/pro-secretory
what are Na and water regulatory mechanisms
- renin-angiotensin-aldosterone - decreases secretion/increases absorption NaCl and water
- catecholamines & aldosterone - increases water absorption
- angiotensin II - increases electroneutral Na+ absorption
- aldosterone - increases electrogenic Na+ absorption via increase expression of Na+ channels and N/K pump and increases electroneutral Na+ absorption
- vasopressin - increases water absorption
- atrial natriuretic peptide released by heart in response to atrial wall stretching secondary to hypervolemia - decreases Na+ and water absorption
what are the interstitial cells of Cajal
- electrical pacemaker cells of GI tract in submucosa - spontaneous electrical and mechanical activity of smooth muscle
- bipolar cells or spindle shaped
- don’t form their own network
- generate slow waves - smooth muscle cells - depolarizaiton - Ca ion entry - phastic contractions - peristalsis and segmentation contractions
what are the functions of motility
- mixing digesta
- contact intestinal contents w/ mucosa
- adequate time for fermention and absorption
- transit for defecation and empyting colon
- transit slower than SI
digesta moves from ileum through the ____ into the cecum
ileocecal valve (one-way valve)
horses eating roughage contents from ileum to ____
cecum is 90% water
what is partially filled with watery digesta
cecum
what is the cecal base filled with
has at pressure slightly negative to relative atomospheric
cecal contents empty into right ventral colon through
cecocolic orifice
how is digesta moved through the cecocolic orifice
- muscular contractions form a contriction at cecal base cranial to ileocecal papilla - cranial and caudal compartments & elevation of ventral cecal base and opening for cecocolic orifice
- contraction of cranial comp. of cecal base moves digesta and gas from cecum - right ventral colon
- large amounts of digesta move bacl to cecal body (from base)
what prevents reflux of digesta
sigmoid configuration of cecolic juction
where are fecal balls formed
decending colon
* secretion increases mucus and this decreases resistance to passage
what are the functions of mucus produced by goblet cells
- physical barrier against acids, microbial products, food particles, food-associated toxins, first-line defense against microorganims
- lubricates luminal contents; facilitates digesta transit by minimizing friction with mucosa (descending colon)
- maintains intestinal homeostasis; immunology
what is the structure of mucus in the LI
- branched glycoproteins; interact with external environment via hydrophilic nature influence viscosity
- two mucus layer in colon: inner and outer
what is the structure of the inner layer of mucus
- secreted by surface goblet cells
- anchored to surface goblet cells/adhered to mucosal epithelium
- stratified and organized as a filter to separate epithelium from microorganisms - no bacteria
- endogenous protease activity and slow expansion to 2-3x volume increases the pore size and allows bacteria to penetrate forming outer later
what happens if there is no mucus
bacteria come in contact with epitheloum and cause inflammation and can lead to cancer
what is the structure of the outer layer of mucus
- loosely adhered
- harbors bacteria
- bacterial flora and cytokines regulate penetrability
- energy source for bacteria
- mucus viscosity increases toward the distal GI tract and determines the spatial distribution of the microbiome
what is regulated vesicle secretion
- fusion of single mucus vesicle membrane with plasma membrane
- regulated by typical vesicle exocytosis components
what is compound exocytosis
- all mucus granules fused together and empty the mucus as a single layer
- more dramtic and whole cell explodes
- critical in maintaining intestinal barrier
what do surface goblet cells have
rapid mucin synthesis and secretion with little storage
what are upper crypt goblet cells
- slower secretion
- mucus accumulates in granules
how is mucus production regulated
- endocytosis, autophagy, ROS generation, inflammasome assemly and activation
- regulated by intracellular Ca2+ and Ca2+ mobilizing agents (ACh and histamine)
what mobilizing agent does the colon only respond to
histamine
what does ACh target
crypt goblet cells by activating M3 muscarinic receptors leading to mobilization of Ca2+
what is the rectum
temporary storage of feces before defecation
what are the two main anal sphincters
- internal anal sphincter: controlled by parasympathetic fibers; involuntary
- external anal sphincter: skeletal muscle controlled by somatic nerve supply from inferior anal branch of pudendal nerve; voluntary
what does defecation involve
integrationof peristalsis on most colonic regions (gastrocolic reflex)
pre-defecation
increased propulsive peristaltic waves
sensation of defectory urge
increased propagative peristalsis
how does defecation occur
- feces contact receptors in upper anal canal - relax inner anal sphincter - defecatory urge
- rectal walls expand with feces - stretch receptor stimulation - defecatory urge
- defecatory urge - initial voluntary contraction of the external anal sphincter
what does rectal evacuation require
- correcting angle rectum
- contraction of abdominal muscles and diaphragm
- relaxation of pelvic wall muscles
- relaxation of external sphincter
- peristalsis then facilitate the movement of feces through the anal canal
what are the epidemiologies of management factos that affect LI function
horses
- increase hours in stall/decrease exposure to pasture
- recent travel within 24 hrs
- crib-biting/windsucking
- recent change in diet
- poor hay quality
- high concentrate diet
- recent change in exercise
- inadequate parasite management
- inadequate dental care
what does poor feed quality cause
impaction at pelvic flexure in left ventral colon
when do most impactions occur
in winter b/c decreased water intake and change in diet
what does meal feeding cause
increase in volumes of water secreted into the GI tract - hypovolemia - renin-angiotensisn-aldosterone - increase Na+ absorption - impaction
what do high concentrate meal feeding cause
- increase secretion of large fluid vol. into SI - transient hypovolemia - increase renin-angiotensin-aldosterone - increase colonic fluid absorption
- decrease SI transit time - increases fermentable soluble CHO in cecum/colon - increases renin-angiotensin-aldosterone - increases colonic fluid absorption
- dysbiosis
what happens in horses moving from pasture to a stall with contolled exercise
- decrease fecal output
- increase fecal dry matter content
- increase water consumption
- decrease colonic motility
what are laxatives
substances that promotes bowel movements
what is an example of a bulk-formin laxative
psyllium
what is an example of a lubricant laxative
mineral oil
what is an example of a hypersmotic and saline laxative
MgSO4
what is the main laxative that is mostly used
water and electrolytes
what is salmonella sp.
- faculative anaerobe
- intracellular pathogen
- ingested from fecal contaminated feed
- entrapped in esophageal mucus
- survive gastric acidity
- colonizes digestive tract
what do dead salmonella produce
endotoxin (LPS) causing colitis
where does salmonella invade and multiply
enterocytes
how does salmonella attach to enterocytes
fibrial appendages
what are clinical features of salmonella
- colic
- diarrhea
- systemic inflammatory response
- hypovolemic shock
- electrolyte disturbances
- hypoproteinemia/hypoalbuminemia
- pre-renal/renal (AKI) failure
- laminitis
treatments of salmonella
- IV fluid & electrolyte therapy
- colloidal support
- analgesia
- digital cryotherapy
- +/- antimicrobials
- antidiarrheal (absorbs bacterial toxins - biosponge)
