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: Rebound hypertensive after sodium nitroprusside prevented by saralasin in rats .
### Output Text:
Rebound hypertensive after <span class="chemical">sodium nitroprusside</span> prevented by <span class="chemical">saralasin</span> in rats . 