Water Balance in the GI tract Flashcards
What type of process is absoption of water and what is it driven by
PAssive process
By the transport of solutes (particularly Na+) from the lumen of the intestines to the bloodstream
How much water enters the GI tract per day
9.3L
How much water enters the small intestine per day
8.3L
How much water enters the large intestine
1L
How much of the water that reaches the large intestine is absorbed
90%
What does faeces contain
100ml of water
50ml of cellulose, bilirubin and bacteria
What defines diarrhoea
Loss of fluid and solutes form the GI tract in excess of 500ml per day
What largely drives the absorption f water in the GI tract
Reabsorption of Na+
What is intestinal fluid movement always coupled to?
Solute movement
How may water move
Via transcellular or paracellular routes
What does the reabsorption of Na+ provide
A local osmotic force for reabsorption of water
What are the major mechanisms of postprandial Na+ absorption in the jejunum
Na+/ glucose and Na+/ amino acid co-transport
Where does Na+/H+ exchange take place
The apical and basolateral membranes in the jejunum
What stimulates the exchange at the apical membrane in the jejunum
Alkaline environment of the lumen due to the presence of bicarbonate from the pancreas
What are the primary mechanisms of Na+ absorption in the interdigestive period
Na+/H+ and Cl-/HCO3- exchange in parallel
What regulates NaCl+ absorption
Intracellular cAMP, cGMP and Ca2+ which all reduce NaCl absorption
A reduction in what is a cause of diarrhoea
NaCl
What type of cells mediate electrogenic Na+ absorption in the distal colon
Epithelial Na+ channels (ENaC)
What can cause an increase in the the Na+ absorption in the distal colon
Aldosterone
What are the 3 actions of aldosterone
opens ENaC
Inserts more ENaC into membrane from intraceullular vesicle pool
Increases synthesis of ENaC and Na+/K+ ATPase
How can Cl- be absorbed
Passively via transcellular or paracellular routes
What causes the driving force in the lumen of the small intestine
The electrogenic transport of Na+ (Na+/glucose and Na/ amino acid)
What causes the driving force in the lumen of the large intestine
Electrogenic movement of Na+ through ENaC
At what rate do cellular mechanisms of Cl- secretion occur
At a basal rate often overshadowed by a higher rate of absorption
Where does cellular mechanisms of Cl secretion occur
Crypt cells
What drives the inward movement of Na+, K+ and Cl- and via what?
Low intracellular Na+
via NKCC1
What does the increasing concentration of Cl- provide
an electrochemical gradient for Cl- to exit the cell via CFTR on the apical membrane
What provides a voltage depend secretion of Na+ through paracellular pathway
Lumen negative potential
What is the overall effect of CFTR
Secretory diarrhoea
In normal circumstances, why is there little secreiton of Cl-
The apical CFTR is either closed or not present
What activates CFTR in order for secretion to occur
Bacterial enterotoxins
Hormones and neurotransmitters
Immune cells products
Some laxatives
Acitvation of CFTR occurs indirectly as a result of what
Of the generation of second messengers such as cAMP
CGMP
Ca2+
The Cl- conductance mediated by CFTR result from what 2 things
opening of channels at the apical membrane
insertion of channels from intracellular vesicles into the membrane
What are 5 causes of diarrhoea
Infectious agents - viruses/ bacteria chronic disease toxins drugs psuchological factors
What can diarrhoea result in
dehydration
metabolic acidosis
hypokalaemia
What are 3 treatment option of severe acute diarrhoea
Maintenance of fluid and electrolyte balance
Use of anti-infective agents
Use of non-antimicrobial antidiarrhoeal agents
What are the 3 main causes of diarrhoea
Impaired absorption of NaCl Excessive secretion Non-absorbable or poorly absorbable solutes in intestinal lumen Hypermotility Excessive secretion
What type of drugs have anti-diarrhoeal activity
Morphine- like (opiates)
What are some of the actions of opiates on the alimentary tract
Inhibition of enteric neurones Decreased peristalsis Increased fluid absorption Construction of pyloric, ileocaecal and anal sphincters Increased tone of the large intestine
What are 3 major opiates used in diarrhoea
Codeine
Diphenoxylate
Loperamide
