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: Our study was designed to determine the effects of combined nitroglycerin and phenylephrine therapy .
### Output Text:
Our study was designed to determine the effects of combined <span class="chemical">nitroglycerin</span> and <span class="chemical">phenylephrine</span> therapy . 