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: It begins when heparin exposure stimulates the formation of heparin - platelet factor 4 antibodies , which in turn triggers the release of procoagulant platelet particles .
### Output Text:
It begins when <span class="chemical">heparin</span> exposure stimulates the formation of <span class="chemical">heparin</span> - platelet factor 4 antibodies , which in turn triggers the release of procoagulant platelet particles . 