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: This demonstrates the participation of the renin - - angiotensin system in antagonizing the combined hypotensive effects of halothane and SNP .
### Output Text:
This demonstrates the participation of the renin - - <span class="chemical">angiotensin</span> system in antagonizing the combined hypotensive effects of <span class="chemical">halothane</span> and <span class="chemical">SNP</span> . 