Flashcards in Water Balance in the GI tract Deck (59)
What is a passive process driven by the transport of solutes (particularly Na+) from the lumen of the intestines to the bloodstream?
Absorption of water
How much water enters the tract per day?
How much water is absorbed by the small intestine?
How much water enters the large intestine, and of that what percentage is absorbed?
1 litre, 90% absorbed
What has 100ml in faeces and what has 50ml, along with, bilirubin and bacteria?
What is defined as a loss of fluid and solutes from the GI tract in excess of 500ml per day?
What is intestinal fluid movement always coupled to?
What two ways may water move?
What two prinicple mechanisms of Na absorption occurs throughout the small intestine and is most omportant in the post-prandial period (also occurs in the colon in the new born)?
What prinicple mechanism of Na movement occurs in the duodenum and jejunum and is stimulated by HCO3?
Where does Na+/H+ exchange occur?
In duodenum and jejunum
What stimulates Na+/H+ exchange?
Where and at what time does the parallel Na+/H+ and Cl-/HCO3 exchange occur?
In the ileum and colon, most important in the interdigestive period
What principle mechanism for Na exchange occurs in the colon (distal) and is regulated by aldosterone?
Epithelial Na+ channels (ENaC)
What to mechanisms of postprandial Na+ absorption are examples of secondary active transport and are electrogenic?
Na+/glucose and Na+/amino acid co-transport
Collectively, what does the overall transport of Na+ generate?
A transpepithelial potential (Vte) in which the lumen is negative
Due to the transepithelial potential in which the lumen is negative, what does this drive?
Parallel absorption of Cl-
Where does Na+/H+ exchange in the jejunum occur at?
Both the apical and baseolateral membranes (via different isoforms of the exchanger)
During Na+/H+ exchange in the jejunum, what two things contribute to transepithelial movement of Na+ and the regulation of intracellular pH?
NHE2 and NHE3
What is exchange at the apical membrane, in the jejunum stimulated by?
The alkaline environment of the lumen due to presence of bicarbonate from the pancreas
In the jejunum, what is absent?
A parallel Cl--HCO3 exchanger
What is the primary mechanism of Na+ absorption in the interdigestive period?
Na+/H+ and Cl-/HCO3- exchange in parallel
In Na+/H+ and Cl-/HCO3-, what is absorption like?
In Na+/H+ and Cl-/HCO3-, what is it regulated by?
Intracellular cAMP, cGMP and Ca2+, all of which reduce NaCl absorption
What is reduction in NaCl absorption a cause of?
Explain how secretory diarrhoea is caused due to infection with E. coli?
Enterotoxin from which activates adenylate cyclase and increases intracellular cAMP, reducing NaCl absorption
What mediates electrogenic Na+ absorption in the distal colon?
Epithelial Na+ channels (ENaC)
What is ENaC increased by?
What are the three actions of aldosterone on ENaC?
1. opens ENaC
2. Inserts more ENaC into membrane from intracellular vesicle pool
3. Increases synthesis of ENaC and Na+/K+-ATPase