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: We conclude that PPA increases blood pressure by increasing systemic vascular resistance and cardiac output , and that propranolol antagonizes this increase by reversing the effect of PPA on cardiac output .
### Output Text:
We conclude that <span class="chemical">PPA</span> increases blood pressure by increasing systemic vascular resistance and cardiac output , and that <span class="chemical">propranolol</span> antagonizes this increase by reversing the effect of <span class="chemical">PPA</span> on cardiac output . 