MODULE SUPPLEMENT: RENAL SYSTEM
Concentrating and Diluting Functions; Water Homeostasis
What
would be the impact of the changes discussed so far on the
capacity of the aging kidney to handle fluid
balance?
Changes cause several functional
deficits:
-
 nephrons   solute load per nephron an obligatory diuresis
-
rate of blood flow through the medulla a "wash out" of the hypertonicity
concentrating capacity
-
responsiveness to ADH which capacity to retain fluid
-
aldosterone
sodium preservation and subsequent
water preservation
- functional nephrons
capacity to deal with a fluid
challenge
- functional nephrons
transport maximum
capacity
- functional nephrons
with GFR renal threshold for glucose
glucose appears in the urine only after higher blood levels
are achieved than in the young adult
Overall, the kidney becomes less able to dilute or concentrate urine and there is a decreased ability to 'cope' optimally with either dehydration or a water load.
These
changes, with the decrease in maximum urinary
concentrating ability, while real, shouldn't be clinically
significant UNLESS water loss is severe
OR thirst perception is not intact.
Unfortunately, thirst perception seems to be
blunted in older persons which enhances their vulnerability
to dehydration, especially in light of decreased body fluid
(McKenna & Thompson, 1998).
Changes in thirst may be related to alterations in
endogenous opiods. Some data suggest that drinking
response (in young men) is mediated by opiods and that
this system is blunted or absent in elderly persons (Silver
& Morley, 1992). NOTE that persons with Alzheimer's
disease may be at even increased risk for
dehydration/altered thirst defense because Vasopressin
(which would enhance water preservation/retention) release
appears to be normally under control of cholinergic control
and cholinergic fibers are especially impacted in
Alzheimer's disease.
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