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: Laboratory testing has revealed a falling platelet count , increased resistance to heparin , and aggregation of platelets by the patient ' s plasma when heparin is added .
### Output Text:
Laboratory testing has revealed a falling platelet count , increased resistance to <span class="chemical">heparin</span> , and aggregation of platelets by the patient ' s plasma when <span class="chemical">heparin</span> is added . 