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: Protective effect of a specific platelet - activating factor antagonist , BN 52021 , on bupivacaine - induced cardiovascular impairments in rats .
### Output Text:
Protective effect of a specific platelet - activating factor antagonist , <span class="chemical">BN 52021</span> , on <span class="chemical">bupivacaine</span> - induced cardiovascular impairments in rats . 