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 common factor relating thromboembolism and thrombocytopenia is heparin - induced platelet aggregation .
### Output Text:
The common factor relating thromboembolism and thrombocytopenia is <span class="chemical">heparin</span> - induced platelet aggregation . 