Which portion of the nephron responsible for fluid absorption

[aktorque]

A 54-year-old missionary traveling alone in Southern Asia loses his bag of supplies. After 24 hours without food or water, his urine osmolarity is 1100 mOsm/L. The fluid filtered by the glomerulus is primarily reabsorbed by which portion of the nephron?


A. Proximal tubule
B. Loop of Henle
C. Distal tubule
D. Proximal collecting duct
E. Distal collecting duct

[donofitaly]

A. Proximal tubule

(This is a UW question).

[doctorsmonsters]

I think it would the site for action of ADH which I don't exactly remember but proximal collecting duct rings some bells so my answer would be D.

[doctorsmonsters]

Originally Posted by donofitaly

A. Proximal tubule

(This is a UW question).

oh ok, can you describe how???

[donofitaly]

Originally Posted by doctorsmonsters

oh ok, can you describe how???

Proximal Tubule absorbs 66% of the fluid, be it in any condition from normal hydration to severe dehydration.

[FrontalBossing]

Originally Posted by doctorsmonsters

oh ok, can you describe how???

Regardless of what is going on, whether a person is running a marathon, playing guitar hero, hasn't eaten in 4 days, has been living on coffee and m&m's while studying step1, it doesn't change the fact that 65% percent of re-absorption of water occurs at the proximal tubule.

It's a common question used to trip people up.

[doctorsmonsters]

Now I'll never forget that. Thank you guys.

[aktorque]

Correct ans is A) Proximal tubule

This patient is suffering from dehydration. His concentrated urine reflects that his kidneys are functioning properly to conserve water. More than 60% of the fluid filtered in the glomerulus is reabsorbed in the proximal tubules by default (FrontalBossing and donofitaly mentioned above) , regardless of the patient’s hydration status. Fluid in the proximal tubule is reabsorbed is osmotically with solutes; no concentration or dilution of urine occurs in this segment.

In the dehydrated state, plasma osmolarity increases, stimulating osmoreceptors in the anterior hypothalamus. In response, the anterior hypothalamus produces antidiuretic hormone (ADH) in the supraoptic nuclei, which is then delivered via nerve axons to the posterior pituitary for release into the circulation. ADH increases the water permeability of the collecting ducts to cause water conservation and production of a maximally concentrated urine. The kidney is capable of concentrating the urine to a maximum osmolarity of 1200 mOsm/L. Still, without eventual fluid replenishment, dehydration will ultimately progress to death because even at its maximum efficiency the kidney must produce approximately 0.5 L of urine per day to eliminate metabolic waste products.