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: Results indicate that subpopulations of mu receptors may mediate selective behavioral and cardiorespiratory responses to morphine .
### Output Text:
Results indicate that subpopulations of mu receptors may mediate selective behavioral and cardiorespiratory responses to <span class="chemical">morphine</span> . 