It is also affected by temperature, with urine density decreasing with increasing temperatures. 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. For routine clinical purposes, USG isĭetermined using a refractometer (refractive index generally correlates well with USG). Urine specific gravity is a measurement of the density of urine compared to pure water. 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.Īdapted from: Schrier: Renal and Electrolyte Disorders. This high concentration of urea osmotically extracts water from the solute-impermeable descending limb, thus concentrating NaCl in the descending-limb fluid.ĥ. In the inner medulla, water and urea are absorbed (under the action of ADH) from the collecting duct, contributing substantially to the medullary interstitial osmotic gradient.Ĥ.

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).ģ. Urea is poorly absorbed and is retained in the tubular fluid.Ģ. 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. 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).ġ. Schematic representation of the passive urinary concentrating mechanism The diagram and notes below detail how the kidney concentrates 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. Osmolality is normally used for more detailed analysis, but USG remains popular for its convenience.Specific Gravity of Urine Specific Gravity of Urine decreased concentration of solutes in urine) may be associated with renal failure, pyelonephritis, diabetes insipidus, acute tubular necrosis, interstitial nephritis, and excessive fluid intake (e.g., psychogenic polydipsia). Hypovolemic patients usually have a specific gravity >1.015.ĭecreased specific gravity (hyposthenuria, i.e. In neonates, normal urine specific gravity is 1.003. A specific gravity greater than 1.035 is consistent with frank dehydration. increased concentration of solutes in the urine) may be associated with dehydration, diarrhea, emesis, excessive sweating, urinary tract/bladder infection, glucosuria, renal artery stenosis, hepatorenal syndrome, decreased blood flow to the kidney (especially as a result of heart failure), and an excess of antidiuretic hormone caused by the syndrome of inappropriate antidiuretic hormone secretion. Increases in specific gravity (hypersthenuria, i.e. In adult humans, normal specific gravity values range from 1.010 to 1.030.Īdults generally have a specific gravity in the range of 1.010 to 1.030.

The concentration of the excreted molecules determines the urine's specific gravity. The body excretes some of these waste molecules via urination, and the role of the kidney is to concentrate the urine, such that waste molecules can be excreted with minimal loss of water and nutrients. One of the main roles of the kidneys in humans and other mammals is to aid in the clearance of various water-soluble molecules, including toxins, toxicants, and metabolic waste.

For background on kidney function, see Renal function.

Urine specific gravity range