Urine changes in disease
Production and composition of urine depend on glomerular filtration, tubular reabsorption and tubular secretion.
Changes that can occur in the volume, appearance, constituents, and mass density (specific gravity) of
urine in disease are as follows:
Volume
What is the normal volume of urine in 24hrs?
The volume of urine excreted daily depends on fluid intake, diet, climate, and other physiological factors. It is usually between 1–2 litres per 24 hours.
What causes increase in urine volume?
An increase in the volume of urine is called
polyuria. It occurs in diabetes mellitus due to an increase in the osmolality of the filtrate preventing the normal reabsorption of water (osmotic diuresis).
Polyuria also occurs when the secretion of the antidiuretic hormone is reduced, e.g. in diabetes
What causes decrease in urine volume?
A decrease in the volume of urine excreted is called oliguria. It occurs when the renal blood flow and, or, glomerular filtration rate are reduced. One of the causes of a reduced renal blood flow is low
blood pressure (hypotension) caused for example by severe dehydration or cardiac failure. A fall in glomerular filtration rate occurs in acute glomerulonephritis (inflammation of the kidney glomeruli) and also in the early stages of acute tubular necrosis.
If severe, oliguria progresses to a complete cessation of urine flow, this is called anuria and is usually due to severe damage to the renal tubules (acute tubular necrosis). Acute tubular necrosis may
follow any of the conditions which cause severe hypotension or may be due to a direct toxic effect on
the tubules by drugs or following an incompatible
blood transfusion.
Interpretation of urine appearance
Normal freshly passed urine is clear and amber in colour. A dilute urine appears pale in colour and a concentrated one has a dark yellow appearance. The yellow colour is due to the pigments
urochrome, urobilin, and porphyrins.
When normal urine has been allowed to stand for some time, a white phosphate deposit may form
if the urine is alkaline (dissolved by adding a drop of acetic acid) or a pink uric acid deposit may form if the urine is highly acidic or concentrated (disappears
on warming). A ‘mucus’ cloud may also form if normal urine is left to stand.
Changes of urine appearance in difference condition
The appearance of urine may be altered in many
conditions including:
● Urinary tract infections in which the urine appears cloudy because it contains pus cells and bacteria.
● Urinary schistosomiasis in which the urine often appears red and cloudy because it contains blood (haematuria).
● Malaria haemoglobinuria (blackwater fever) and other conditions causing intravascular haemolysis in which the urine appears brown and cloudy
because it contains free haemoglobin (haemoglobinuria).
● Jaundice in which the urine may appear yellowbrown or green-brown because it contains bile pigments or increased amounts of urobilin (oxidized urobilinogen).
● Bancroftian filariasis in which the urine may appear milky-white because it contains chyle.
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