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 results suggested that ammonia blocks calcium channels .
### Output Text:
These results suggested that <span class="chemical">ammonia</span> blocks <span class="chemical">calcium</span> channels . 