Large Intestine

Question 1

Describe the types of large intestine motility

Answer 1

Segmentation contractions - majority in proximal colon, mixes chyme

Peristaltic contractions - move chyme slowly along colon

Mass movement - peristaltic wave that occurs 1-3x/day, moves colonic contents long distances

Question 2

Describe movement in the distal colon

Answer 2

Fecal material semi-solid and moves slowly b/c most water absorption takes place in the proximal colon

Segmentation contractions prominent, but cause little movement

Mass movement propel most fecal matter into rectum

Question 3

Describe movement in the rectum/anal canal

Answer 3

Frequency of segmental contractions greater in rectum than sigmoid colon causing retrograde movement of fecal matter

Accounts for suppository retention

Question 4

What is the rectosphincteric reflex?

Answer 4

Fecal material forced into rectum causes the internal anal sphincter to relax

If defacation doesn’t occur, internal anal sphincter contracts and rectum relaxes to accommodate fecal matter

Question 5

Describe the make up of feces

Answer 5

Contains inorganic material, undigested plant fibers, bacteria and water

composition relatively unaffected by diet variation

Question 6

What is Hirschsprung’s disease?

Answer 6

Caused by absence of enteric nervous system from segment of colon

Results in constriction and loss of coordinated movement of involved segment

Causes severe constipation, treated by surgically removing aganglionic segment

Question 7

What is constipation?

Answer 7

Increased time of passage/transit of material through colon

Causes increased storage capacity of cecum/ascending/transverse colon

Decreased propulsive capacity of descending colon and sigmoid

Question 8

What are the physiological functions of colonic flora?

Answer 8

Digest carbs in large intestine

Form secondary bile acids and deconjugate bile acids

Generate short-chain fatty acids such as acetate, propionate, and butyrate which are absorbed by colon

Question 9

What are the sources of gas?

Answer 9

Swallowed air - removed by eructation

Gas formed by bacterial action in ileum and colon

Diffusion of gas from bloodstream

Question 10

What is the composition of gas?

Answer 10

CO2, H2, and N2

CO2 and H2 readily absorbed through mucosa

1/3 of adults produce methane due to genetic factors

Question 11

What are the four mechanisms of Na transport in the small intestine?

Answer 11

Restricted diffusion through channels

Na-glucose/Na-AA cotransport (duodenum/jejunum)

Na-Cl cotransport (ileum)

Na-H exchange (duodenum/jejunum)

Question 12

How is Na transported across the basolateral membrane?

Answer 12

Na-K pump

Question 13

Describe Cl transport

Answer 13

Passive diffusion via paracellular route

Cotransport with Na/K

Exchange with HCO3

Question 14

What is the mechanism of intestinal secretion?

Answer 14

Crypt cells bring Cl into the cell via Na-Cl cotransport

Cl channels in the apical membrane activated by increase in cAMP or Ca levels (mutated in CF) - closed in resting conditions

Cl diffuses into the lumen

Na follows via paracellular pathways, water follows

Question 15

What is osmotic diarrhea?

Answer 15

Accumulation within small intestine of nonreabsorbable solutes

Excess solute attracts water from intestinal wall

Question 16

What is secretory diarrhea?

Answer 16

Excess stimulation of secretory cells in crypts

Cl channels always open, resulting in fluid accumulation in the lumen

Inhibition of Na absorption is another cause

Question 17

What is inflammatory/infectious diarrhea?

Answer 17

Bacterial/viral infection causes loss of enterocyte that leads to inability to reabsorb

E.g. Salmonella, Campylobacter, C. diff

Question 18

What is the result of cholera or E. coli infection?

Answer 18

Secretory diarrhea

Toxin constitutively activates Cl channel

Enterotoxins secreted by E. coli activate guanylin receptors in intestine

Question 19

What is oral rehydration therapy?

Answer 19

Oral administration of Na and glucose

Activates Na-glucose contransport mechanism, stimulating the reabsorption of Cl and H2O