Free water clearance and renal concentrating ability

5 Free water clearance and renal concentrating ability 

One of the most important functions of the normal kidney is its ability to respond to changes in the state of hydration by excreting dilute or concentrated urine. Appropriate water excretion or retention provides a very sensitive measure of general renal integrity. 

The simplest measure of the ability of the kidney to conserve water is provided by osmolality of urine excreted by a water-deprived subject. The osmolality of plasma changes relatively little and averages close to 300 mOsm analysis of the osmolar concentration of urine thus leads directly to the (U/P) osmol ratio. Simple and convenient osometers are available for such determinations. Human urine may range from osmolalities below 100 mOsm to a maximum of perhaps 1800, that is, to a urine sixfold more concentrated than plasma. The rat, in contrast, can readily concentrate its urine nine- to tenfold. 

A more quantifafive measure of urine concentration ability is the clearance of free wafer (Cnp), derived in furn from fhe osmolal clearance, (UV/P) osmol. The laffer in effect represents the volume that would be required to excrete total urinary solute in isosmotic solution, that is, at a concentration of 300 mOsm. If filtrate were excreted as such, the osmolal clearance would equal the GFR. If the urine is more dilute than the filtrate, the dilution may be visualized as that volume of filtrated from which solute was removed without reabsorption of wafer this volume is defined as the free water clearance (Cnp), or free wafer excretion, and is given below, where V stands as usual for the urine volume excreted per unit time  

Several intrarenal factors are involved in the production of concentrated urine, including especially the attaiiunent and maintenance of high interstitial osmolality in the medulla as a result of solute filtration and reabsorption, and of the countercurrent concentrating mechanism, which depends in turn on normal medullary blood flow. Urinary concentrating ability can therefore not be associated with a specific aspect of renal function but serves rather as reflection of its general integrity, and can thus prove useful for screening purposes. 

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