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 clinical effects of heparin are meritorious , but side effects do exist .
### Output Text:
The clinical effects of <span class="chemical">heparin</span> are meritorious , but side effects do exist . 