Phys + Pharm of large intestine

Question 1

What are the 4 functions of the large intestine?

Answer 1

1. To extract Na+ and water from the luminal contents.
2. To make and store faeces.
3. To move faeces towards the rectum.
4. Secrete alkaline mucus

Question 2

How is electrolyte and water transported across the intestinal epithelium in the jejunum vs the colon?

Answer 2

Jejunum: Na+ enters on coupled transporters, leaves cell via Na+/K+ ATPase. Water - paracellular route
Colon: Na+ enters on Na+ channel, leaves cell via Na+/K+ ATPase. Water - transcellular route

Question 3

What are haustral contractions?

Answer 3

Longitudinal muscles - 3 bands - teniae coli
H. contractions (segmented) mix the contents of the proximal colon.
This facilitates water absorption.

Question 4

What are mass movements?

Answer 4

Propel the luminal contents from the beginning of the transverse colon to the sigmoid colon.
Series of modified peristaltic events.
1-3 times a day

Question 5

What is the gastrocolic reflex and what does it trigger?

Answer 5

Mediated by gastrin + extrinsic autonomic n
Mass movements
moves bolus to rectum
triggers defecation reflex

Question 6

Describe the steps in the defecation reflex.

Answer 6

1. Contraction of rectum
2. Relaxation of the internal anal sphincter.
3. Initial contraction of the external anal sphincter.
4. Increased peristaltic activity in the sigmoid colon.
5. Relaxation of the external anal sphincter.
6. Expulsion of faeces.

Question 7

What are the steps in the valsalva manoeuvre? (voluntary defecation)

Answer 7

1. Full inspiration, forced expiration against a closed glottis, diaphragm to move downwards.
2. abdominal + thoracic muscles contract.
3. Inc pressure in the abdomen forces faecal contents into the rectum
   defaecation reflex is initiated

Question 8

What gives the brown colour of faeces?

Answer 8

derivatives of bilirubin (stercobilin, urobilin)

Question 9

What gives the odour of faeces?

Answer 9

products of bacterial action (include skatole, indole, mercaptans and hydrogen sulphide)

Question 10

What is the function of intestinal bacteria?

Answer 10

immunity against common bacteria
converts bilirubin to urobilinogens 
forms secondary bile acids
degrades digestive enzymes
digests mucus
synthesizes certain vitamins (e.g. vitamin K)
metabolizes undigested polysaccharides

Question 11

What is the class of drugs used to treat constipation?

Answer 11

Purgatives:

Bulk + Osmotic laxatives; Faecal softners; Stimulant purgatives

Question 12

What is the class of drugs used to treat diarrhoea?

Answer 12

Anti-diarrhoeal agents:

Oral rehydration therapy, Anti-infective and Anti-motility agents

Question 13

What types on the bristol stool chart indicate defecation?

Answer 13

Type 1 and 2

Question 14

What is the MoA of bulk laxatives and give examples.

Answer 14

Retain water in the GI lumen, softening and increasing faecal bulk and promote increased motility.
Ex: methylcellulose + ispaghula husk
Good 1st choice in IBS and constip

Question 15

What is the MoA of osmotic laxatives and give examples.

Answer 15

Promote move. of fluid by osmosis across the wall into the GI lumen; accelerates small int. transit and results in an abnormally large volume of fluid entering colon.
Distension leads to purgation.
Ex: Mag. suplh + Mag. Hydroxide (saline purgatives)
Macrogols
Lactulose

Question 16

What is the MoA of faecal softners and give examples.

Answer 16

Anionic surfactants: lower surface tension at oil-water interface, allowing water or fats to enter the stool.
Faecal matter is softened.
Ex: Docusate, arachis oil

Question 17

What is the MoA of stimulant purgatives and give examples.

Answer 17

Stimulate GI motility

Ex: Senna, Bisacodyl

Question 18

What is a cathartic colon?

Answer 18

Chronic use of stimulant laxatives
dependency
fluid/electrolyte imbalance
bloating/pain/incomplete faecal evacuation

Question 19

What are the types of diarrohea? (MOISt)

Answer 19

Motility
Osmotic
Inflammatory
Secretory (e.g. cholera)

Question 20

What is the MoA of oral rehydration therapies and give examples.

Answer 20

Restores fluid and electrolye balance

Isotonic or hypotonic solution of glucose and NaCl.

Question 21

What is the MoA of anti-motility agents and give examples.

Answer 21

Act on m-opioid receptors in the myenteric plexus.
Increases tone and rhythmic contractions of colon, but diminishes propulsive activity.
Pyloric, ileocaecal and anal sphincters are contracted.
Ex: loperamide (imodium)