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: RELEVANCE TO CLINICAL PRACTICE : When administering subcutaneous heparin injections , it is important to extend the duration of the injection .
### Output Text:
RELEVANCE TO CLINICAL PRACTICE : When administering subcutaneous <span class="chemical">heparin</span> injections , it is important to extend the duration of the injection . 