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: It seems that adenosine A1 and A2 receptors might be involved in this reaction .
### Output Text:
It seems that <span class="chemical">adenosine</span> A1 and A2 receptors might be involved in this reaction . 