Renal Physiology Question

[dara678]

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Ok, for all those physiology people out there -- HELP ...

I understand that the glomerular filtration rate has to decrease in cases of dehydration and kidney stones, in order to maintain homeostasis. What I don't understand is why, when the colloid osmotic pressure in the blood plasma increases (due to dehydration, for instance) the osmotic pressure in the nephron INCREASES and the fluid pressure in the Bowman's capsule increases. (I mean, I understand that they have to in order to reduce the overall glomerular filtration rate, but I don't understand why.) If anyone knows, please help ...

 

[greenwave34]

osmotic pressure is not the only thing changing/determining filtrate! Remember that filtrate/resorbtion depends on passive as well as active transport mechanisms. Hormonal regulation is the KEY!

ADH and aldosterone act independently on the same part of the nephron to produce the same result: maintained mean arterial pressure.

Aldoserone acts on the collecting duct to increase Na+ reabsorbtion and K+ secretion (which in turn increases H20 reabsorbtion, because water always follows sodium) and ADH acts on the collecting duct by increasing the permiability of water (thus more reabsorbtion back into the blood stream).

Also, within the glomerular capsule, the filtrate passing into the nephron might actually increase due to higher osmotic pressure. BUT, nearly all of that filtrate will be returned to the blood via ADH/Aldosterone regulation.


Hope that helps. Let me know if you need more ideas.

GW

 

[babinski bob]

One of the main tenets of renal physiology is that GFR must be preserved at all costs. However, in cases of severe dehydration, renal perfusion is ultimately compromised to maintain blood pressure and cerebral perfusion which is most important.

In the case of dehydration, this means that the afferent arteriole constricts (to decrease renal flow and maintain systemic pressure). As a result, the hydrostatic pressure at Bowman's space drops and the osmotic pressure increases (due to the hypovolemia). The immediate result is decreased GFR. In an attempt to maintain GFR, the efferent arteriole constricts to INCREASE the perfusion pressure in Bowman's capsule. HOWEVER, the NET effect is still an overall REDUCTION in GFR since the hydrostatic pressure is still lower and the osmotic pressure is higher than in the case of a normal healthy individual.

Remember, the net filtration pressure is the sum of the hydrostatic pressure (governed by afferent and efferent constriction and dilation) and the osmotic pressure (governed by plasma protein concentration).