Renal physiology - tricky question

When compared with antidiuresis, the effect of drinking 1 L pure water can result in which of the following findings?

 A. Clearance H2O > 0.0 mL/min
 B. Clearance urea < Clearance urea for antidiuresis control
 C. Filtered load Na+ > filtered load Na+ for antidiuresis control
 D. Glomerular filtration rate (GFR) > 125 mL/min
 E. Urine osmolarity > Plasma osmolarity




The correct answer is A)

 Clearance is the flow rate at which a substance is removed from the blood. In the kidney, it represents a balance between glomerular filtration/secretion and reabsorption. Several mechanisms are in place to promote antidiuresis (i.e., to promote the reabsorption of water from the tubular fluid to prevent dehydration; urine concentration). The water clearance rate (CH2O) is usually negative, as more water is reabsorbed than is filtered (i.e., urine concentration) during any given period of time. As a result, the body normally produces a highly concentrated, low-volume urine. Drinking large quantities of pure water raises the value of CH2O considerably to a positive number. This reflects water diuresis where there is a reduced rate of water reabsorption accompanied by the production of a large volume of dilute urine. This protects body fluids against dilution, and thus prevents electrolyte imbalances and their attendant complications.

 B) Urea is passively processed by the kidney. With increased urine flow in water diuresis, more urea is washed out, causing its clearance to be increased.

 C) The filtered sodium load (GFR × PNa+) is unaffected by water loading.

 D) GFR remains remarkably stable regardless of water loading.

 E) Uosm falls precipitously during water diuresis, producing a large-volume, low-concentration urine.