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: Heparin , first used to prevent the clotting of blood in vitro , has been clinically used to treat thrombosis for more than 50 years .
### Output Text:
<span class="chemical">Heparin</span> , first used to prevent the clotting of blood in vitro , has been clinically used to treat thrombosis for more than 50 years . 