Sodium and Water Transport Flashcards
How is sodium reabsorbed in the proximal tubule?
Coupled with the secretion of H+ via the NHE3 antiporter
Additional sodium enters in symport with glucose, AA’s, and phosphate
Transport across the BL membrane consists mostly of NA-K pump, with some bicarbonate symport
How is chloride reabsorbed in the proximal tubule?
Transcellular - enters via antiport with organic base and leaves via Cl channels in the BL membrane
However, a significant amount is reabsorbed paracellularly
How is water reabsorbed in the proximal tubule?
Transcellularly and Paracellulary via aquaporins
Permeability to water is so great that it keeps pace with active sodium reabsorption
Describe glomerulotubular balance as it relates to sodium.
Changes in GFR result in a proportional change in the filtered load of Na
The PT reabsorbs a constant fraction of the filtered load of salt and water
However, the absolute quantity will change depending on the GFR
What is osmotic diuresis?
Increased urine flow that is due to extra amount of non-reabsorbed solute within the tubular lumen
E.g. mannitol and severe DM
What are the differences in reabsorption between the descending and ascending loop of Henle?
Descending - only reabsorbs water
Ascending - Only Sodium and Chloride
What is the Na-K-Cl symporter (NKCC)?
Major transporter in the thick ascending limb
Target for loop diuretics
How is K transported in the thick ascending limb?
NKCC
Channels in both membrances that recycle K from the cell interior to the lumen and interstitium
What is Bartter’s syndrome?
Charachterized by very large urinary losses of NACl, hypokalemia
Caused by mutations of genes encoding proteins that transport ions in the thick ascending limb
How are sodium and chloride transported in the distal tubule?
Apical membrane contains a Na-Cl symporter (NCC) and sodium channels
Sodium leaves the BL membrane via Na-K ATPase, chloride via channels
*NCC is the target for thiazide diuretics
What is Gitelman’s syndrome?
Charachterized by increased excretion of Na, MG, Cl, and K
Caused by mutations int he gene that codes for the NCC in the distal tubule
How are sodium, chloride and water absorbed in the collecting tubule?
Sodium and water are absorbed via channels
Chloride reabsorption here is passive and paracellular
What is the role of aldosterone and ADH in the collecting tubule?
Aldosterone stimulates sodium channels being added to the apical membrane
ADH stimulates aquaporins
What is Liddle’s syndrome?
Caused by mutations in the gene that codes for epithelial sodium channel in the principal cells of the collecting duct
Characterized by severe hypertension, low plasma renin activity, metabolic alkalosis due to hypokalemia and hypoaldosteronism
What is obligatory water loss?
Water loss that must happen to be able to dissolve solutes in the urine