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II. Colour

The colour of normal urine is amber yellow. The principal pigment is urochrome (stercobilin), which is yellow. This is a compound formed in cleavage of hemoglobin. Small amounts of uroerythrin, coproporphyrin and uroporphyrin are usually present. Uroerythrin is red, which is possibly derived from melanins. Uroporphyrin and coproporphyrins are brownish-red Fe-free pigments arising from heme synthesis.

1. urine becomes colorless in polyuria; if due to the sp.gr. is high (1.025-1.060).

2. in oligouria, with concentrated urine the color is darkbrown.

3. Riboflavin and one of its metabolic products, uroflavin may be present and gives a greenish fluorescence to the urine.

4. Abnormally, excessive amounts of some of the normal pigments notably coproporphyrin and uroporphyrins may be excreted.

5. Among the chromogens that are not normal is homogentisic acid which occurs in inborn error of metabolism known as alkaptouria. Note:in this condition, when passed, urine is of normal colour but assumes a smoky or blackish hue on standing. The darkening begins in top, from exposure to atmospheric O2 and travels downwards.

6. The color may also be changed due to the appearance of free hemoglobin in haemoglobinuria, or due to bile pigments or due to melanins.

7. Foreign pigments are occasionally found after their administration.

8. Porphyrinuria occurs in porphyria, a hereditary disorder of heme metabolism.

9. Simple porphyrinuria seen in toxic and disease conditions and lead poisoning and chronic liver diseases.

III. Specific Gravity

Due to presence of various substances in solution, the sp.gr.of urine is higher than that of pure water. The sp.gr.is influenced by:

(i) the volume of water excreted, and

(ii) varies inversely with urine output.

Apparatus: specific gravity is measured with an instrument called urinometer. It consists of a weighted cylinder which floats in urine and a stem calibrated in degrees of sp.gr.

The depth to which the urinometer sinks depends on the density of the urine.

The scale usually reads from 1.000 to 1.060.

The instrument is usually calibrated for use at a temperature of 15C.

Note:warm and freshly voided urine should be allowed to cool to room temperature before the specific gravity is measured.

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