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: Total tissue catecholamine content of the kidney on a molar basis was unchanged .
### Output Text:
Total tissue <span class="chemical">catecholamine</span> content of the kidney on a molar basis was unchanged . 