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Heme catabolism starts with the oxidative cleavage of the porphyrin between rings A and B by heme oxygenase, which produces biliverdin, a green linear tetrapyrrole. The reddish-orange bilirubin is subsequently created by reducing the central methenyl bridge of biliverdin. Heme degradation can be seen as a visual expression in the fading hues of a bruise as it heals.

In aqueous solutions, the extremely lipophilic bilirubin is insoluble. Bile secretes bilirubin derivatives, which are mostly further broken down by bacterial enzymes in the large intestine. Urine gets its distinctive colour because some of the urobilinogen that results from this process is reabsorbed and carried via the circulation to the kidney, where it is changed into yellow urobilin and expelled. However, the majority of urobilinogen is microbially transformed into the dominant colour of faeces, the intensely red-brown stercobilin.