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: Special attention must be paid to cardiac patients who are often exposed to heparin multiple times during their course of treatment .
### Output Text:
Special attention must be paid to cardiac patients who are often exposed to <span class="chemical">heparin</span> multiple times during their course of treatment . 