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 morbidity and mortality associated with heparin - induced thrombosis remain high despite numerous empirical therapies .
### Output Text:
The morbidity and mortality associated with <span class="chemical">heparin</span> - induced thrombosis remain high despite numerous empirical therapies . 