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: These side effects are relatively rare in a given individual , but given the extremely widespread use of heparin , some are quite common , particularly HITT and osteoporosis .
### Output Text:
These side effects are relatively rare in a given individual , but given the extremely widespread use of <span class="chemical">heparin</span> , some are quite common , particularly HITT and osteoporosis . 