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: The opioid antagonist naloxone has been shown to block or reverse the hypotensive actions of captopril .
### Output Text:
The opioid antagonist <span class="chemical">naloxone</span> has been shown to block or reverse the hypotensive actions of <span class="chemical">captopril</span> . 