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: A study on the effect of the duration of subcutaneous heparin injection on bruising and pain .
### Output Text:
A study on the effect of the duration of subcutaneous <span class="chemical">heparin</span> injection on bruising and pain . 