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: Mean arterial pressure was decreased to 50 mm Hg for 30 minutes either by hemorrhage ( HEM , n = 7 ) or by continuous infusion of sodium nitroprusside ( SNP , n = 7 ) .
### Output Text:
Mean arterial pressure was decreased to 50 mm Hg for 30 minutes either by hemorrhage ( HEM , n = 7 ) or by continuous infusion of <span class="chemical">sodium nitroprusside</span> ( <span class="chemical">SNP</span> , n = 7 ) . 