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: Previous reports of these phenomena as well as recent studies of the effect of heparin are reviewed .
### Output Text:
Previous reports of these phenomena as well as recent studies of the effect of <span class="chemical">heparin</span> are reviewed . 