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Specific Gravity of Urine
Urine specific gravity (USG) and osmolality are measures of the solute concentration in urine and are used to assess the ability of the renal tubules to concentrate or dilute the glomerular filtrate.
The diagram and notes below detail how the kidney concentrates urine.
| Schematic representation of the passive urinary concentrating mechanism |
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The thin ascending limb in the inner medulla and the thick
ascending limb in the outer medulla and first part of the distal
tubule are permeable to NaCl but impermeable to water (as indicated
by the thickened lining).
- In the water-impermeable thick ascending
limb, absorption of NaCl via the NaK2Cl carrier renders the
tubular fluid dilute and the outer medullary interstitium
hyperosmotic. Urea is poorly absorbed and is retained in the
tubular fluid.
- Water is resorbed down the osmotic
gradient established in the outer medulla in the last part
of the distal tubule and collecting ducts (the latter under
the influence of ADH).
- In the inner medulla, water and urea
are absorbed (under the action of ADH) from the collecting
duct. Urea contributes substantially to the medullary interstitial
osmotic gradient.
- The high concentration of urea in the
medullary interstitium osmotically extracts water from the
solute-impermeable descending limb, thus concentrating NaCl
in the descending-limb fluid.
- When this NaCl-rich fluid enters the
NaCl-permeable (water-impermeable) thin ascending limb, NaCl
is absorbed passively along its concentration gradient, producing
a relatively dilute renal tubule fluid.
The dilute renal tubular fluid will then become concentrated
when ADH stimulates water absorption in the collecting tubule
(step 3). The degree to which this water will be absorbed
(and how concentrated the urine will be) depends on the ability
of the tubule to create and maintain a hypertonic medullary
interstitium (for more information on this, see the countercurrent
exchange mechanism).
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Urine specific gravity is a measurement of the density of urine compared to pure water. For routine clinical purposes, USG is
determined using a refractometer (refractive index generally correlates well with USG). The USG is influenced by the number of molecules in urine, as well as their molecular weight and size, therefore it only approximates solute concentration. It is also affected by temperature, with urine density decreasing with increasing temperatures. The presence of large amounts of protein and glucose will alter the USG and should be considered when interpreting USG results. The following substances will incrase the USG by 0.001 units.
| Substance | Value |
| NaCl | 0.15 g |
| Urea | 0.36 g |
| Glucose | 0.27 g |
| Albumin | 0.4 g |
Urine osmolality is directly related to the number of particles in solution and is unaffected by molecular weight and size. Osmolality can be measured by freezing point depression (technique used at Cornell University) and changes in vapor pressure. Urine osmolality can be approximated from the USG, by multiplying the last 2 digits of the USG by 36.
Knowledge of urinary solute concentration is essential for proper
interpretation of BUN and serum creatinine, which are indicators of
glomerular filtration rate. The USG is very useful for identifying the
cause of azotemia.
The interpretation of several urine chemical parameters also is influenced
by the specific gravity of the specimen. Why does
this make sense? In addition, urinary constituents (erythrocytes,
leukocytes and casts) can lyse in dilute urine (USG < 1.008), affecting
interpretation of the urine sediment results.
| Species | Possible range | Usual range | "Adequate" |
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| Canine | 1.001-1.065 | 1.015-1.045 | >1.030 |
| Feline | 1.001-1.080 | 1.035-1.060 | >1.035 |
| Large Animals | 1.001-1.040 | 1.015-1.030 | >1.025 |
© Cornell University
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