Urine is coloured yellow by the pigments urochrome and uroerythrin, but it darkens on standing due to the oxidation of urobilinogen to urobilin.

Abnormal constituents of urine include

• Glucose
• Ketones
• Bilirubin
• Erythrocytes
• Large numbers of leucocytes and
• Casts.

Despite the concentrating ability of the kidney, a minimum of 500 ml/day is required to eliminate the urea and other electrolytes.