Testing the urine

Urine is produced by the kidneys. The volume of urine produced depends on fluid intake and diet. A typical urinary volume is between 0.8 and 2.6 L per day.

The kidneys maintain electrolyte balance by excreting excess electrolytes into the urine. The waste-products of nitrogen metabolism are removed by the excretion of urea. In order to maintain homeostasis the kidneys must produce a minimum volume of urine per day. This volume is determined by the concentrating power of the kidneys and the amount of solute to be excreted. An average Western diet requires the excretion about 800 mosmol of solute daily. The maximum concentration of solute in the urine is around 1200 mosmols therefore there is a minimum daily urine volume of approximately 650 ml.

Polyuria, oliguria and anuria are description of the volume of urine produced:

• polyuria:
  • is the production of an excessive volume of urine per day
  • often accompanied by nocturia and frequency
  • should be distinguished from urinary frequency
    
    
• oliguria is the production of less than 300 ml of urine per day
  
  
• anuria exists when there is no urinary output

With urinary tract infections the urine may become cloudy and smell fishy. Certain foods (eg. beetroot) and drugs (eg sulphasalazine) may colour the urine. In porphyria the urine turns red/browm on standing.