"Urine" - what is it, definition of the term
Renal excretory fluid is a clear‑to‑yellow aqueous solution chiefly composed of water, urea, electrolytes, and metabolic waste products, generated by the kidneys to eliminate nitrogenous compounds and regulate fluid‑electrolyte balance; in rodents such as rats and mice, this fluid also functions as a medium for chemical communication, territorial marking, and pathogen transmission.
Detailed information
The urinary fluid of rodents consists primarily of water, urea, creatinine, electrolytes, and trace metabolites. In laboratory rats, average daily excretion ranges from 10 to 20 mL per kilogram of body weight; mice produce 5 to 10 mL per kilogram. Concentrations of urea typically fall between 150 and 300 mmol L⁻¹, while creatinine levels are 30–70 µmol L⁻¹. Electrolyte composition includes sodium (40–80 mmol L⁻¹), potassium (30–60 mmol L⁻¹), chloride (50–100 mmol L⁻¹), and calcium (2–5 mmol L⁻¹).
Physiological parameters display species‑specific ranges:
- pH: 6.0–7.5 in rats; 6.2–7.2 in mice
- Specific gravity: 1.020–1.040 (rat); 1.015–1.035 (mouse)
- Osmolality: 300–800 mOsm kg⁻¹ (rat); 250–700 mOsm kg⁻¹ (mouse)
Collection techniques influence sample integrity. Metabolic cages provide uncontaminated samples for 24‑hour collections; bladder puncture yields acute specimens but introduces hematuria risk. Immediate cooling to 4 °C preserves metabolite stability; prolonged storage requires freezing at –80 °C.
Analytical applications exploit these fluids for toxicology, endocrinology, and disease modeling. Quantification of corticosterone metabolites monitors stress responses; measurement of proteinuria detects renal dysfunction; metabolomic profiling reveals alterations in metabolic pathways associated with genetic modifications.
Pathological changes manifest as deviations from baseline values. Elevated urea and creatinine indicate impaired glomerular filtration; reduced specific gravity suggests polyuria; acidic shifts in pH correlate with metabolic acidosis. In experimental models of diabetes, rodents exhibit persistent hyperglycosuria, increased urine volume, and lowered osmolar concentration.
Standardization of sampling times (e.g., morning versus night) reduces circadian variability. Normalizing analyte concentrations to creatinine concentration corrects for fluctuations in fluid output, enabling comparative studies across individuals and experimental groups.