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: If the platelet count falls to less than 100 , 000 / mm3 , while the patient is receiving heparin , platelet aggregation testing , using the patient ' s plasma , is indicated .
### Output Text:
If the platelet count falls to less than 100 , 000 / mm3 , while the patient is receiving <span class="chemical">heparin</span> , platelet aggregation testing , using the patient ' s plasma , is indicated . 