In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The kinetics of urinary sodium excretion and the appearance of proteinuria were comparable with those reported previously .
### Output Text:
The kinetics of urinary <span class="chemical">sodium</span> excretion and the appearance of proteinuria were comparable with those reported previously . 