Physiology + Pharmacology of the large intestine

Question 1

What makes up the large intestine?

Answer 1

Caecum and appendix
Colon
- ascending
- transverse
- descending
- sigmoid
Rectum
Anal canal and anus

Question 2

What is the longitudinal smooth muscle layer in caecum and colon divided into?

Answer 2

Three strands. - the taeniae coli

Question 3

Where is smooth muscle thickened?

Answer 3

internal anal sphincter - surrounded by the skeletal muscle of the external anal sphincter

Question 4

What forms the haustra?

Answer 4

Activity of the taeniae coli and circular muscle layers in colon
- moves due to contraction

Question 5

What is entry of substances into the caecum permitted by?

Answer 5

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

Question 6

What is the role of the ileocaecal valve?

Answer 6

maintaining a positive resting pressure
relaxing in response to distension of the duodenum
contracting in response to distension of the ascending colon
being under the control the vagus nerve, sympathetic nerves, enteric neurones and hormonal signals

Question 7

How is the appendix attached to the distal caecum?

Answer 7

the appendiceal orifice

Question 8

What may block the the appendiceal orifice to cause appendicitis?

Answer 8

faecalith- a hard mass of fecal matter

Question 9

Name the functions of the colon?

Answer 9

Absorption: (net) of Na+, Cl- and H2O to condense ileocaecal material to solid, or semi-solid, stool

Absorption: of short chain fatty acids:
Carbohydrate not absorbed by the small intestine is fermented by colonic flora to short chain fatty acids

Secretion: (net) of K+, HCO3- and mucus

Reservoir: Storage of colonic contents

Periodic elimination of faeces

Question 10

Where is mucus produced?

Answer 10

goblet cells in the large intestine

Question 11

How is the surface area of the colon increased?

Answer 11

colonic folds, crypts and microvilli

Question 12

What mediates electrolyte absorption which, by osmosis, drives absorption of H2O?

Answer 12

surface epithelial cells - colonocytes

Question 13

What do crypt cells mediate?

Answer 13

ion secretion

Question 14

What do goblet cells secrete?

Answer 14

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

Question 15

What are Na+ absorption and K+ secretion are enhanced by?

Answer 15

aldosterone

Question 16

Describe Haustration?

Answer 16

haustra are saccules caused by alternating contraction of the circular muscle – similar to segmentation in function, but much lower frequency (minutes) – contributes to long transit time
occurs in proximal colon – causes oral movement (reverse)
generated by slow wave activity
mixes content – allows time for fluid and electrolyte reabsorption

Question 17

What is mass movement?

Answer 17

simultaneous contraction of large sections (about 20 cm) of the circular muscle of the ascending and transverse colon (haustra disappear)
- drives faeces into distal regions

Question 18

What triggers mass movement?

Answer 18

triggered by a meal (often breakfast) via the gastrocolic response involving:
gastrin
extrinsic nerve plexuses

Question 19

Where does mass movement occur and what does it trigger?

Answer 19

distal colon

defaecation reflex in response to stretch

Question 20

What does passive rectal distension trigger?

Answer 20

smooth muslce of the internal anal spinchter - retrosphincteric reflex

Question 21

What happens if defaecation is not desired?

Answer 21

Contraction of skeletal muscle of external anal sphincter

Question 22

Where do intestinal gases arise from?

Answer 22

swallowed air (most ‘burped up’ – eructation), some enters small intestine but is either absorbed, or passed to the colon 
bacteria in the colon which attack forms of carbohydrate that are indigestible to humans
gas that is not absorbed in the large intestine is 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

Question 23

What is the role of commensals in the colon?

Answer 23

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. used in treatment of IBD)

Question 24

What is Hirschprung disease?

Answer 24

colon that is devoid of neurons as enteric system fails to grow – normally treated by surgical resection, causes constipation

Question 25

What is purgation?

Answer 25

the emptying of not just the large intestine but also the small intestine

Question 26

When should laxatives and purgatives NOT be used?

Answer 26

When there is an obstruction of the bowel

Question 27

What is the role of laxatives?

Answer 27

increase peristalsis and/or soften faeces causing, or assisting, evacuation

Question 28

When should purgatives and laxatives be used?

Answer 28

when ‘straining’ is potentially damaging to health (e.g. patients with angina), or when defaecation is painful (e.g. haemorrhoids) predisposing to constipation
to clear the bowel before surgery, or endoscopy
to treat drug-induced constipation, or constipation in bedridden, or elderly patients