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: Specific risk factors have not been identified ; therefore , all patients receiving heparin should be monitored .
### Output Text:
Specific risk factors have not been identified ; therefore , all patients receiving <span class="chemical">heparin</span> should be monitored . 