Phys + Pharm of large intestine Flashcards
What are the 4 functions of the large intestine?
- To extract Na+ and water from the luminal contents.
- To make and store faeces.
- To move faeces towards the rectum.
- Secrete alkaline mucus
How is electrolyte and water transported across the intestinal epithelium in the jejunum vs the colon?
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
What are haustral contractions?
Longitudinal muscles - 3 bands - teniae coli
H. contractions (segmented) mix the contents of the proximal colon.
This facilitates water absorption.
What are mass movements?
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
What is the gastrocolic reflex and what does it trigger?
Mediated by gastrin + extrinsic autonomic n
Mass movements
moves bolus to rectum
triggers defecation reflex
Describe the steps in the defecation reflex.
- Contraction of rectum
- Relaxation of the internal anal sphincter.
- Initial contraction of the external anal sphincter.
- Increased peristaltic activity in the sigmoid colon.
- Relaxation of the external anal sphincter.
- Expulsion of faeces.
What are the steps in the valsalva manoeuvre? (voluntary defecation)
- Full inspiration, forced expiration against a closed glottis, diaphragm to move downwards.
- abdominal + thoracic muscles contract.
- Inc pressure in the abdomen forces faecal contents into the rectum
defaecation reflex is initiated
What gives the brown colour of faeces?
derivatives of bilirubin (stercobilin, urobilin)
What gives the odour of faeces?
products of bacterial action (include skatole, indole, mercaptans and hydrogen sulphide)
What is the function of intestinal bacteria?
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
What is the class of drugs used to treat constipation?
Purgatives:
Bulk + Osmotic laxatives; Faecal softners; Stimulant purgatives
What is the class of drugs used to treat diarrhoea?
Anti-diarrhoeal agents:
Oral rehydration therapy, Anti-infective and Anti-motility agents
What types on the bristol stool chart indicate defecation?
Type 1 and 2
What is the MoA of bulk laxatives and give examples.
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
What is the MoA of osmotic laxatives and give examples.
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
What is the MoA of faecal softners and give examples.
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
What is the MoA of stimulant purgatives and give examples.
Stimulate GI motility
Ex: Senna, Bisacodyl
What is a cathartic colon?
Chronic use of stimulant laxatives
dependency
fluid/electrolyte imbalance
bloating/pain/incomplete faecal evacuation
What are the types of diarrohea? (MOISt)
Motility
Osmotic
Inflammatory
Secretory (e.g. cholera)
What is the MoA of oral rehydration therapies and give examples.
Restores fluid and electrolye balance
Isotonic or hypotonic solution of glucose and NaCl.
What is the MoA of anti-motility agents and give examples.
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)
