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: As heparin remains one of the most frequently used medications today with potential for HIT with every heparin exposure , a close vigilance of platelet counts must be practiced whenever heparin is initiated .
### Output Text:
As <span class="chemical">heparin</span> remains one of the most frequently used medications today with potential for HIT with every <span class="chemical">heparin</span> exposure , a close vigilance of platelet counts must be practiced whenever <span class="chemical">heparin</span> is initiated . 