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: BACKGROUND : Heparin - induced thrombocytopenia presents 5 to 12 days after heparin exposure , with or without arterial or venous thromboemboli .
### Output Text:
BACKGROUND : <span class="chemical">Heparin</span> - induced thrombocytopenia presents 5 to 12 days after <span class="chemical">heparin</span> exposure , with or without arterial or venous thromboemboli . 