Physiology of Large Intestine

Question 1

what comprises the large intestine

Answer 1

caecum
appendix
colon
rectum
anal canal and anus

Question 2

name the different parts of the colon

Answer 2

ascending
transverse
descending
sigmoid

Question 3

structure of caecum and colon

Answer 3

longitudinal smooth muscle layer

divided into 3 strands - taeniae coli, but encircles the rectum and anal canal

Question 4

structure of internal sphincter and external sphincter

Answer 4

thickened smooth muscle surrounded by skeletal muscle of the external sphincter

Question 5

functions of caecum and appendix

Answer 5

no specialised functions

caecum receives material from terminal ileum

Question 6

how is material from terminal ileum permitted to the caecum

Answer 6

via the gastroileal reflex in response to gastrin and CCK through one-way ileocaecal valve

Question 7

what material does the caecum receive from the terminal ileum

Answer 7

indigestible residues
unabsorbed biliary components
unabsorbed fluid

Question 8

action of the ileocaecal valve - pressure

Answer 8

maintains positive resting pressure

Question 9

action of the ileocaecal valve - relaxation

Answer 9

relaxes in response to distension of duodenum

Question 10

action of the ileocaecal valve - contraction

Answer 10

contracts in response to distension of the ascending colon

Question 11

nervous control of ileocaecal valve

Answer 11

vagus nerve
sympathetic nerves
enteric neurones
hormonal signals

Question 12

structure of appendix

Answer 12

blind-ended tube with extensive lymphoid tissue connected to distal caecum via appendiceal orifice

Question 13

what causes appendicitis

Answer 13

appendiceal orifice may become obstructed by faecalith

Question 14

primary functions of the colon

Answer 14

absorption
secretion
reservoir
periodic elimination of faeces

Question 15

primary functions of the colon - absorption

Answer 15

absorbs;
sodium, chloride and water
short chain fatty acids

Question 16

why does the colon absorb sodium, chloride and water

Answer 16

to condense ileocaecal material to solid, or semi-solid, stool

Question 17

why does colon absorb short chain fatty acids

Answer 17

carbohydrate that is not absorbed by small intestine is fermented by colonic flora to short chain fatty acids

Question 18

primary functions of the colon - secretion

Answer 18

secretes potassium, bicarbonate and mucus

Question 19

primary functions of the colon - reservoir

Answer 19

stores colonic contents

Question 20

primary functions of ascending and transverse colon

Answer 20

fluid reabsorption

bacterial fermentation

Question 21

primary function of descending colon

Answer 21

final ‘drying’ (desiccation) and storage

Question 22

structure of mucosa of the colon

Answer 22

lacks villi

possesses colonic folds, crypts and microvilli - increasing surface area

Question 23

function of surface epithelial cells (colonocytes) on mucosa of colon

Answer 23

mediate electrolyte absorption which, by osmosis, drives absorption of water

Question 24

function of crypt cells on mucosa of colon

Answer 24

mediate ion secretion

Question 25

function of goblet cells on mucosa of colon

Answer 25

secrete;
copious mucus containing glycosaminoglycans – hydrated to form a slippery surface gel
trefoil proteins involved in host defence

Question 26

describe transepithelial movement of electrolytes

Answer 26

complex - involves numerous transporters and ion channels

Question 27

how is sodium absorption and potassium secretion enhacned

Answer 27

by aldosterone

Question 28

describe haustration

Answer 28

pattern of motility - non-propulsive segmentation

Question 29

action of haustration

Answer 29

haustra are saccules caused by alternating contraction of the circular muscle (similar to segmentation - lower in frequency)
contributes to long transit time

Question 30

how does haustra contribute to long transit time

Answer 30

disappear before and reappear after a mass movement
generated by slow wave activity
mixes content – allows time for fluid and electrolyte reabsorption

Question 31

describe peristaltic propulsive movements

Answer 31

patterns of motility - mass movement

Question 32

action of peristaltic propulsive movements

Answer 32

simultaneous contraction of large sections of circular muscle of the asending and transverse colon - driving faeces into distal regions

Question 33

how is peristaltic propulsive movements triggered

Answer 33

by a meal via the gastrocolic response involving;
gastrin
extrinsic nerve plexuses

Question 34

how is defaecation reflex triggered

Answer 34

mass movement in the distal colon propels faeces into rectum - triggering reflex in response to rectal stretch

Question 35

action of commensal bacteria in the large intestine

Answer 35

increase intestinal immunity by competition with pathogenic microbes
promote motility and help maintain mucosal integrity
synthesise vitamin K2 and free fatty acids (from carbohydrate) that are absorbed
activate some drugs (e.g. IBD)

Question 36

how do intestinal gases arise

Answer 36

swallowed air - (may be burped - eructation) enters small intestine and absorbed, or passed to the colon
bacteria in the colon which attack forms of carbohydrate that are indigestible to humans

Question 37

how is gas not absorbed by the large intestine expelled

Answer 37

expelled through the anus - selective expulsion requires abdominal contractions; internal and external sphincters are contracted to form an ‘exit’ too narrow for solid matter to escape