Physiology and Pharmacology of Fluid Balance and Motility in the GI Tract Flashcards
Describe the absorption of fluid in the GI tract
Typically, roughly 9 litres of fluid enters the GI tract per day. Of this 9 litres:
Roughly 8 litres is absorbed by the small intestine
The remaining 1 litre is passed to the large intestine of which 90% is absorbed
100ml is excreted as faeces
Give a definition for diarrhoea
When the loss of fluids and solutes from the GI tract in the form of faeces exceeds 500ml per day
Give some infectious causes of diarrhoea
Bacterial (campylobacter)
Viral (norovirus)
Parasitic
Give some non-infectious causes of diarrhoea
IBD/IBS Anxiety/depression Hyperthyroidism Drug-induced (Mg salts, cytotoxic drugs, beta blockers) Broad spectrum antimicrobials
What does diarrhoea cause?
Dehydration
Metabolic acidosis
Hypokalaemia
How is severe acute diarrhoea treated?
Maintaining fluid and electrolyte balance
Use of antimicrobials
Use of antimotilities and spasmolytic agents
How do oral rehydration salts work?
Oral rehydration salts contain sodium ions and glucose. This increases absorption of Na+ and glucose by SGLT1, which leads to increased absorption of water
Describe the role of antimicrobials in treating diarrhoea
Antimicrobials have a limited role in managing diarrhoea as most diarrhoea is viral in origin and bacterial diarrhoea resolves quickly
However, in C.diff colitis, metronidazole (IV/oral) and vancomycin (oral) are used
In what cases are antimotilities used?
Exhausting/idiopathic cases
Non-infective diarrhoea
Mild traveller’s diarrhoea
How do anti-motility agents stop diarrhoea?
Inhibit enteric neurones Decrease peristalsis Increase fluid absorption Constrict pyloric, ileocaecal and anal sphincters Increase tone of large intestine
What are the main anti-motility agents used to treat diarrhoea?
Opiates - loperamide, diphenoxylate, codeine and racecadotril
What is constipation?
The infrequent production of hard stools requiring straining to pass
What can cause constipation?
Ignoring/suppressing the urge to defecate
Decreased colonic motility (improper diet, drugs etc.)
Lack of exercise
Impairment of motility/defecation reflex
What is the difference between laxatives and purgatives?
Laxatives produce a milder action causing the passing of soft but formed stools
Purgatives produce a stronger action causing more fluid evacuation
What do laxatives and purgatives cause?
Retained water
Increased peristalsis
Increased water and electrolyte secretion
Softened faeces
