Large Intestine Physiology Flashcards
where is 90% of water and most nutrients absorbed?
in the small intestine
what are the 4 primary functions of the large intestine? also list 2 additional functions
primary:
1. process indigestible food material (fiber)
2. absorb water and electrolytes
3. produce and absorb vitamins
4. form and propel feces for elimination
other:
1. secretion of mucus to lubricate and facilitate the passage of feces
2. secretion of alkaline fluid to neutralize irritating acids formed by bacterial action on fiber
compare the relative size of the large intestine in hindgut fermenters, ruminants, omnivores, and carnivores
hindgut fermenters: largest LI
omnivores and ruminants tied for second place
carnivores have smallest LI
describe the ileocecal valve/ileocolic junction normal position and why
normal constricted and valved closed to prevent the large amount of bacteria in the large intestine from entering the small intestine
list and describe 2 factors that increase ileal peristalsis and valve opening to large intestine
- stomach: gastroileal reflex; gastric distension due to increased chyme triggers gastroileal reflex and gastrin secretion which increases ileal motility and relaxation of valve
- ileum: increased content fluidity, distension, and chemical irritation (all from digestion and absorption in SI) lead to increased ileal motility and relaxation of valve
list and describe factors that decrease ileal peristalsis and lead to closure of the valve to the large intestine (2)
- ileogastric reflex: distension of ileum decreases gastric motility
- chemo and mechanoreceptors in cecum decrease motility and lead to valve closure when the cecum is full
how is the motility/mixing of the large intestine accomplished?
teniae coli (bands of longitudinal muscle that do not completely surround the inner circular muscle) contract the haustra (sacculation) in a motility pattern called haustration that digs into and rolls over LI contents to mix chyme and microbial enzymes
describe the propulsive mass movements of the large intestine; what is responsible for this departure from normal mixing motility?
accomplished by the gastrocolic reflex; causes the LI to contract as a unit, forming a restrictive ring at a distended point in the colon, forcing fecal material to move down to the colon to be excreted to make room for the food currently eating/digesting
what are the functions of the large intestine microbiota? (5)
- communicate with the host via: microbe-associated molecular patterns (MAMP), metabolites, and contributing to host defense
- vitamin and protein synthesis
- assist in digestion and absorption
- prevention of harmful bacteria by outcompeting bad guys for nutrients
- digestion: critical in hindgut fermenters!
describe large intestine microbial digestion (3)
- occurs in all species but is critical in hindgut fermenters
- degrades fibers into volatile fatty acids (acetic acid, proprionic acid, butyrate) that serve as an energy source for epithelial cells
- microbial production of vitamins: K, thiamin, riboflavin, B12
what are the 4 factors of colonic motility?
- sphincters (ileocecal, cecocolic, internal and external anal sphincters)
- segmentation
- peristalsis
- antiperistalsis
describe antiperistalsis (4)
- sending food backwards for further digestion
- controlled by pacemakers (interstitial cells of cajal)
- under complex control; not fully understood but interactions between ENS, chemoreceptors, and mechanoreceptors
- often occurs in places where sharp turns are present (transverse colon in carnivores and equine, pelvic flexure in equine, central flexure cow), so can lead to obstruction
describe LI motility around the ileocecal junction/valve
junction is normally constricted and valve is normally closed to prevent flow of LI bacteria into SI
describe the rectum at rest
usually devoid of contents in carnivores and omnivores until defecation reflex is initiated; tonic constriction of internal and external anal sphincters means complete closure of sphincters as sympathetic tone has more influence than parasympathetic tone at rest
what happens when the feed bolus/shit enters the rectum?
the defecation reflex/rectosphincteric reflex is initiated
describe the defecation/rectosphincteric reflex?(2)
- primarily under parasympathetic control, involuntarily relaxes internal anal sphincter and external sphincter unless house trained
- can be voluntarily controlled in house-trained animals via voluntary constriction of the external anal sphincter (choose to close and make reflex subside until can’t hold it anymore or choose to go)
what causes constipation; generally? (2)
- when bowel movements become less frequent, fecal passage is delayed in the colon
- this increases water absorption so stool becomes very hard and dry and difficult to pass
what are 4 causes of constipation in small animals?
- too little dietary fiber causes fibers to form a gel that holds water in the lumen
- lack of exercise, medication, neurologic disorders
- excessive self-grooming
- dehydration
how is water absorption accomplished in the large intestine?
osmosis
why does ingesta need to be isosmotic?
because water moves towards high osmotic pressure areas, so do not want to much absorbed or too little absorbed
why does lactose intolerance cause diarrhea in dogs?
there is a lactase insufficiency, so lactose cannot be broken down into glucose and galactose for absorption, leaving more sugar in the lumen and drawing water into the lumen, leading to diarrhea (watery stool)
what are the absorptive cells of the SI and LI called?
SI: enterocyte
LI: colonocyte
describe active electrolyte absorption and how it helps with glucose absorption too (2)
- sodium potassium ATPase pump; uses energy to ensure there is high sodium EC and low sodium IN, so sodium wants to go into the cell (down its concentration gradient)
- cells take advantage of this and attach glucose to incoming sodium, forcing glucose into the cell against glucose concentration gradient (this glucose part is later on, after glucose was able to diffuse freely into cell in proximal duodenum)
describe passive electrolyte absorption
- diffusion along concentration gradient
- trancellular: via ion channels, across plasma membrane into cell, or regulates by opening or closing channels in response to cues
OR - paracellular: through tight cell junctions (for water and inorganic molecules)
what are the 3 mechanisms of sodium absorption?
- sodium potassium ATPase pump
- sodium hydrogen exchanger: using carbonic anhydrase to make bicarb and H+, forcing out the IC H+, allowing luminal sodium in, chloride also follows sodium in (coupled sodium chloride transport)
- diffusion
what are 3 mechanisms of chloride absorption?
- couple sodium chloride transport via sodium hydrogen exchanger
- paracellular due to sodium gradients (Cl- RLY likes Na+)
- couple with bicarb without Na+, occurs due to decreased pH in lumen
describe absorption of bicarb (2)
- mostly follows pH and other electrolytes; works as a buffer by combining with HCl to make NaCl, CO2, and H2O to increase pH
- the remaining not used as a buffer is absorbed in the ileum via ion exchange
describe absorption of potassium
most food is high in K+, so lumen K+ is high and increases along the GI tract as a consequence of H2O being absorbed and leading to paracellular passive diffusion of K+
(potassium follows water! decreased water absorption can lead to relative hypokalemia)
