Explain the word "Permeability" in reference to nephron

[0192837465]

Can somebody please explain, in detail, what the word "permeability" means?
Specifically, when one says that the collecting ducts in the nephron are "impermeable" to water without the presence of ADH (vasopressin). Please use examples that relate to the kidney or nephron.

Thank you for your help!

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[phospho]

Can somebody please explain, in detail, what the word "permeability" means?
Specifically, when one says that the collecting ducts in the nephron are "impermeable" to water without the presence of ADH (vasopressin). Please use examples that relate to the kidney or nephron.

Thank you for your help!

http://books.google.com/books?hl=en...ts=LeOsaiGRQj&sig=FpKPA5igkZr3DVLT212hTujUky0

go to page 89

this is definitely not on the MCAT though

 

[Vihsadas]

Can somebody please explain, in detail, what the word "permeability" means?
Specifically, when one says that the collecting ducts in the nephron are "impermeable" to water without the presence of ADH (vasopressin). Please use examples that relate to the kidney or nephron.

Thank you for your help!

It means exactly that. The water will flow along the collecting duct part of the kidney tubule toward the ureter because it cannot "permeate" through the wall of the collecting duct toward the interstitial space (and therefore the plasma). If the hormone ADH is circulating in the bloodstream, then small channels called 'aquaporins' (Aquaporins not MCAT relevant) will insert into the collecting duct membrane and allow water to flow through the wall of the collecting duct toward the plasma instead of being collected in the bladder.

When the wall of any part of the kidney tubule allows any ion, fluid, or protein to pass across it (be reabsorbed into the plasma/interstitial space), it is said to be 'permeable' to that ion, fluid or plasma.

 

[0192837465]

Thank you for the help!

 

[DrMattOglesby]

I think just as important as the flow of ducts is the permeability of the glomerulus itself.
the glomerulus is the capillary bed bunched inside the glomerular capsule.
in order for blood to to be filtered by the kidneys, it must seep through a "filtration membrane."
this filtration membrane consists of fenestrated capillaries (window-like holes in the simple squamos endothelium) that allow for most formed elements to permeate OUT of the blood vessel. Due to the size of the holes in the fenestrated capillaries--larger proteins and erythrocytes will not be able to find their way through this filtration membrane.

while this sounds pertinent, i dont think it will appear on the MCAT:
--the rest of the filtration membrane is composed of a basal lamina and the visceral layer of the glomerular capsule containing modified simple squamos ET called podocytes.

Once the plasma and smaller particles have pushed themselves into the lumen of the gomerular capsule, they are officially called "filtrate." This filtrate will continue through the ducts of the nephron and be subjected to the other two major functions of the nephron (the first was filtration), whicha are; reasborption and secretion.
Reabsorption occurs in the Proximal Convuluted Tubule and this is where we see things like water, amino acids, glucose, sodium and other organic acids pulled back into the ECF.
Secretion occurs at both the PCT and Distal Convuluted Tubule but most books concentrate on it just at the DCT. Items secreted include excess sodium and protons.

one more point about permeabilities though; the descending limb of the loop of Henle is permeable to water, which allows water to diffuse out due to a higher concentration of solutes deeper in the medulla which offer an osmotic pressure to pull the water out.
the ascending limb is permeable to NaCl, yet this is actively pumped out into the intertubular space (it will help to pull water out of the descending limb).

that about wraps it up for permeabilities...
just to have a more complete briefing on the nephron though--we also might see that the DCT convulutes its way next to the glomerular capsule to form the juxtagomerular apparatus (consisting of macula densa and granular cells). These will detect sodium levels and blood pressure and then we can start playing with hormones like renin and angiotensin II.

i love making pee...