One of the basic terms with regards to urine excretion is diuresis – the final urine output
produced by the kidneys per time unit (usually 24 hours).
Final urine output in adults varies between 1500 – 2000 ml/24 hrs, which is 30 – 60 ml/hrs, and in children varies between 300 –
1500 ml/24 hrs.
Changes in the amount of urine output may be caused by excessive or insufficient fluid intake, as well as by other diseases, e.g. diabetes mellitus, insufficient kidney function etc.
We can distinguish:
• Osmotic diuresis – increased final urine output due to the presence of certain substances in the fluid filtered by the kidneys; these substances are water-binding, thus preventing water resorption;
• Water diuresis – increased final urine output due to reduced absorption of water in the kidneys; this phenomenon occurs upon suppressing the formation of an antidiuretic hormone. The urine is evaluated in terms of quantity, colour, odour and density. Urine pH is in the 4.5 – 8 range.
Changes related to the quantity of urine output include:
- Anuria – decrease in daily diuresis below 100 ml or complete discontinuance. In young children, it is defined by a urine output of 0–0.5 ml/kg/h and in adults by 0 - 50 ml/day. Anuria develops from the oliguria. The most common causes of anuria are acute renal failure, severe dehydration, shock etc.
- Oliguria is a condition, where the urine output does not exceed 500 ml in 24 hrs. This may be caused by renal failure, shock.
- Polyuria is a condition defined by excessive urine output, i.e. more than 150% of normal urine output per time unit, depending on the fluid intake (more than 2000 ml/24 hrs/l). The causes of polyuria include excessive fluid intake (polydipsia), excessive osmotic loading in diabetes, deficiency of antidiuretic hormone (diabetes insipidus neurohumoralis) or antidiuretic hormone receptor (diabetes insipidus renalis), chronic renal insufficiency, release of large swelling, congenital defects etc.
The colour of urine is determined by bile pigments. It depends on the amount of urine.
- Normally, urine has a bright yellow, amber colour. The pathological conditions involving the change of urine colour to a reddish-brown and in an increased level of urobilinogen, for example occur with a high fever.
- Brown coloured urine is due to the increased amount of bilirubin and urobilinogen, e.g. in liver and biliary tract diseases.
The smell of urine has a characteristic aroma. A urine collection bottle open for a long time will result in the decomposition of the urea to ammonia, causing a sharp pungent odour.
Urine density ranges from 1005 – 1030 kg/l. The larger the amount, the lower the specific weight. The exception is diabetes mellitus, where the more sugar then the larger the amount and the higher the specific weight.