Physiology and pharmacology of the large intestine

Question 1

What is the length and diameter of the large intestine

Answer 1

1.7 m long, 6 cm diameter

Question 2

What makes up the large intestine

Answer 2

Caecum 
Appendix 
Ascending colon 
Transverse colon 
descending colon 
Sigmoid colon 
Rectum 
Anal canal 
Anus

Question 3

what are haustra

Answer 3

sac like pouches caused by the activity of the taeniae coli

Question 4

What are the taeniae coli

Answer 4

Strands of longitudinal smooth muscle in caecum and colon

Question 5

Where in the large intestine is smooth muscle thickened

Answer 5

At the internal anal sphincter which is surrounded by the skeletal muscle of the external sphincter

Question 6

What is the function of the appendix

Answer 6

no specialised function in humans

Question 7

What material does the caecum receive

Answer 7

indigestible residues
unabsorbed biliary components
unabsorbed fluids

Question 8

What reflex permits entre into the caecum

Answer 8

The gastroilial reflex which is in respone to gastrin and CCK allowing passage of material through the ileocaecal valve

Question 9

How does the ileocaecal valve act

Answer 9

Maintains a positive resting pressure
relaxes in response to duodenal distension
contracts in response to distension of the ascending colon
controlled by vagus nerve, sympathetic nerves, enteric neurons and hormones

Question 10

What connects the caecum and appendix

Answer 10

The appendiceal orifice which if blocked by faecalith (hard faeces) can cause appendicitis

Question 11

What are the primary functions of the colon

Answer 11

Absorption of Na+, Cl-, and H2O 
Absorption of short chain fatty acids 
Secretion of K+, HCO3-, and mucus 
Reservoir storing colonic contents 
Periodic elimination of faeces

Question 12

What happens to carbohydrates not absorbed by the small intestine

Answer 12

Carbs not absorbed by the small intestine are fermented by colonic flora into short chain fatty acids

Question 13

What are faeces composed of and how much are excreted per day

Answer 13

150g per day
composed of H2O (100g)
solid material (50g)
cellulose, bacteria, bilirubin, salt

Question 14

What structures increase absorption of the large intestine

Answer 14

Colonic folds
Crypts
Microvili

Question 15

What are colonocytes

Answer 15

surface epithelial cells which mediate electrolyte absorption

Question 16

Which cells mediate ion secretion

Answer 16

Crypt cells

Question 17

What do goblet cells secrete

Answer 17

Mucus containing glycosominoglycans - hydrated to form a slippery surface gel
trefoil proteins involved in host defence

Question 18

What substance enhances K+ secretion and Na+ absorption

Answer 18

Aldosterone

Question 19

K+ is maintained in secretory diarrhoea true/false

Answer 19

false, there is significant loss of K+ in the faeces

Question 20

What is haustration

Answer 20

non-propulsive segmentation

Question 21

What is the name for mas movement through the large intestine

Answer 21

peristaltic propulsive propulsive movements

Question 22

What is defaecation

Answer 22

periodic egestion

Question 23

What causes haustra

Answer 23

alternating contraction of circular muscle generated by slow wave activity

Question 24

What is the purpose of haustra

Answer 24

mixes contents and allows time for fluid and electrolyte reabsorption

Question 25

How many times a day does mass movement occur

Answer 25

One to three times a day, typically triggered by a meal via the gastrocolic response involving gastrin and extrinsic nerve plexus

Question 26

Describe how defaecation occurs

Answer 26

Mass movement - the rectum fills with faecal matter Rectal stretch receptors send afferents to the spinal cord and brain brain causes urge to defaecate Spinal cord activated parasympathetic efferents which cause contraction of the sigmoid colon and rectum and relaxation of internal anal spinchter If skeletal muscle externam anal spinchter is relaxed defaecation occurs and if not it is delayed

Question 27

What role do commensal bacteria in the large intestine play

Answer 27

increase intestinal immunity by competing with pathogenic microbes promote motility and help maintain mucosal integrity synthesise vitamin K2 and free fatty acids from carbohydrates Activate certain drugs

Question 28

Where does intestinal gas arise from

Answer 28

``` Swallowed air (eructation 'burped up') Bacteria in the colon which attacks indigestible carbohydrates ```

Question 29

What is constipation

Answer 29

presence of hard dried faeces in the colon

Question 30

What causes constipation

Answer 30

``` ignoring or supressing the urge to defaecate decreased colonic motility Obstruction of faecal movement Paralytic ileus following abdominal surgery Impairment of motility/defaecation reflex Hischprung disease (absence of secretion from enteric nervous system) ```

Question 31

What drugs treat constipation

Answer 31

Laxatives | purgatives

Question 32

What are purgatives

Answer 32

drugs that induce purging or cleaning of the bowels by promoting evacuation

Question 33

How do laxatives work

Answer 33

Increase peristalsis and/or soften faeces

Question 34

What are medically sound uses of laxatives/purgatives

Answer 34

when 'straining' is potentially damaging to health (angina) or when defecation is painful (haemorrhoids) to clear bowels before surgery or endoscopy To treat drug induced constipation or in bedridden or elderly patients

Question 35

What are four types of laxative

Answer 35

Bulk laxatives Stimulant purgatives Osmotic laxatives Faecal softners

Question 36

How do bulk laxatives work

Answer 36

``` They are indigestible polysaccharide polymers which improve stool consistency slow acting increase bulk and peristalsis retain H20 METHYLCELLULOSE ```

Question 37

How do stimulant purgatives work

Answer 37

``` increase H20 and electrolyte secretion increase peristalsis faecal softening rapidly acting BISACODYL ```

Question 38

How do osmotic laxatives work

Answer 38

``` Rapidly acting retain H20 increase bulk increase peristalsis MAGBESIUM SULPHATE ```

Question 39

How to faecal softeners work

Answer 39

Detergent like action increase H20 and electrolyte secretion increase peristalsis faecal softening

Question 40

What drugs are used to treat IBS and IBD

Answer 40

Glucocorticoids for acute attacks | Aminosalicylates (more for UC) for maintenance