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: BACKGROUND : Although different methods to prevent bruising and pain following the subcutaneous injection of heparin have been widely studied and described , the effect of injection duration on the occurrence of bruising and pain is little documented .
### Output Text:
BACKGROUND : Although different methods to prevent bruising and pain following the subcutaneous injection of <span class="chemical">heparin</span> have been widely studied and described , the effect of injection duration on the occurrence of bruising and pain is little documented . 