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: Clinical manifestations of this disorder include hemorrhage or , more frequently , thromboembolic events in patients receiving heparin .
### Output Text:
Clinical manifestations of this disorder include hemorrhage or , more frequently , thromboembolic events in patients receiving <span class="chemical">heparin</span> . 