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: While there appear to be beta 2 - receptors in the human heart , their physiologic or pathologic roles remain to be defined .
### Output Text:
While there appear to be beta 2 - receptors in the human heart , their physiologic or pathologic roles remain to be defined . 