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: The role of the renin - - angiotensin system in the maintenance of blood pressure during halothane anesthesia and sodium nitroprusside ( SNP ) - induced hypotension was evaluated .
### Output Text:
The role of the renin - - <span class="chemical">angiotensin</span> system in the maintenance of blood pressure during <span class="chemical">halothane</span> anesthesia and <span class="chemical">sodium nitroprusside</span> ( <span class="chemical">SNP</span> ) - induced hypotension was evaluated . 