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: Reversal by phenylephrine of the beneficial effects of intravenous nitroglycerin in patients with acute myocardial infarction .
### Output Text:
Reversal by <span class="chemical">phenylephrine</span> of the beneficial effects of intravenous <span class="chemical">nitroglycerin</span> in patients with acute myocardial infarction . 