Large Intestine Physiology

Question 1

where is 90% of water and most nutrients absorbed?

Answer 1

in the small intestine

Question 2

what are the 4 primary functions of the large intestine? also list 2 additional functions

Answer 2

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

Question 3

compare the relative size of the large intestine in hindgut fermenters, ruminants, omnivores, and carnivores

Answer 3

hindgut fermenters: largest LI
omnivores and ruminants tied for second place
carnivores have smallest LI

Question 4

describe the ileocecal valve/ileocolic junction normal position and why

Answer 4

normal constricted and valved closed to prevent the large amount of bacteria in the large intestine from entering the small intestine

Question 5

list and describe 2 factors that increase ileal peristalsis and valve opening to large intestine

Answer 5

1. stomach: gastroileal reflex; gastric distension due to increased chyme triggers gastroileal reflex and gastrin secretion which increases ileal motility and relaxation of valve
2. ileum: increased content fluidity, distension, and chemical irritation (all from digestion and absorption in SI) lead to increased ileal motility and relaxation of valve

Question 6

list and describe factors that decrease ileal peristalsis and lead to closure of the valve to the large intestine (2)

Answer 6

1. ileogastric reflex: distension of ileum decreases gastric motility
2. chemo and mechanoreceptors in cecum decrease motility and lead to valve closure when the cecum is full

Question 7

how is the motility/mixing of the large intestine accomplished?

Answer 7

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

Question 8

describe the propulsive mass movements of the large intestine; what is responsible for this departure from normal mixing motility?

Answer 8

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

Question 9

what are the functions of the large intestine microbiota? (5)

Answer 9

1. communicate with the host via: microbe-associated molecular patterns (MAMP), metabolites, and contributing to host defense
2. vitamin and protein synthesis
3. assist in digestion and absorption
4. prevention of harmful bacteria by outcompeting bad guys for nutrients
5. digestion: critical in hindgut fermenters!

Question 10

describe large intestine microbial digestion (3)

Answer 10

1. occurs in all species but is critical in hindgut fermenters
2. degrades fibers into volatile fatty acids (acetic acid, proprionic acid, butyrate) that serve as an energy source for epithelial cells
3. microbial production of vitamins: K, thiamin, riboflavin, B12

Question 11

what are the 4 factors of colonic motility?

Answer 11

1. sphincters (ileocecal, cecocolic, internal and external anal sphincters)
2. segmentation
3. peristalsis
4. antiperistalsis

Question 12

describe antiperistalsis (4)

Answer 12

1. sending food backwards for further digestion
2. controlled by pacemakers (interstitial cells of cajal)
3. under complex control; not fully understood but interactions between ENS, chemoreceptors, and mechanoreceptors
4. 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

Question 13

describe LI motility around the ileocecal junction/valve

Answer 13

junction is normally constricted and valve is normally closed to prevent flow of LI bacteria into SI

Question 14

describe the rectum at rest

Answer 14

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

Question 15

what happens when the feed bolus/shit enters the rectum?

Answer 15

the defecation reflex/rectosphincteric reflex is initiated

Question 16

describe the defecation/rectosphincteric reflex?(2)

Answer 16

1. primarily under parasympathetic control, involuntarily relaxes internal anal sphincter and external sphincter unless house trained
2. 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)

Question 17

what causes constipation; generally? (2)

Answer 17

1. when bowel movements become less frequent, fecal passage is delayed in the colon
2. this increases water absorption so stool becomes very hard and dry and difficult to pass

Question 18

what are 4 causes of constipation in small animals?

Answer 18

1. too little dietary fiber causes fibers to form a gel that holds water in the lumen
2. lack of exercise, medication, neurologic disorders
3. excessive self-grooming
4. dehydration

Question 19

how is water absorption accomplished in the large intestine?

Answer 19

osmosis

Question 20

why does ingesta need to be isosmotic?

Answer 20

because water moves towards high osmotic pressure areas, so do not want to much absorbed or too little absorbed

Question 21

why does lactose intolerance cause diarrhea in dogs?

Answer 21

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)

Question 22

what are the absorptive cells of the SI and LI called?

Answer 22

SI: enterocyte
LI: colonocyte

Question 23

describe active electrolyte absorption and how it helps with glucose absorption too (2)

Answer 23

1. 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)
2. 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)

Question 24

describe passive electrolyte absorption

Answer 24

1. diffusion along concentration gradient
2. trancellular: via ion channels, across plasma membrane into cell, or regulates by opening or closing channels in response to cues
   OR
3. paracellular: through tight cell junctions (for water and inorganic molecules)

Question 25

what are the 3 mechanisms of sodium absorption?

Answer 25

1. sodium potassium ATPase pump
2. 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)
3. diffusion

Question 26

what are 3 mechanisms of chloride absorption?

Answer 26

1. couple sodium chloride transport via sodium hydrogen exchanger
2. paracellular due to sodium gradients (Cl- RLY likes Na+)
3. couple with bicarb without Na+, occurs due to decreased pH in lumen

Question 27

describe absorption of bicarb (2)

Answer 27

1. mostly follows pH and other electrolytes; works as a buffer by combining with HCl to make NaCl, CO2, and H2O to increase pH
2. the remaining not used as a buffer is absorbed in the ileum via ion exchange

Question 28

describe absorption of potassium

Answer 28

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)