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: Hypotension in response to oxytocin was associated with a decrease in systemic vascular resistance and a compensatory increase in stroke volume , heart rate and cardiac output .
### Output Text:
Hypotension in response to <span class="chemical">oxytocin</span> was associated with a decrease in systemic vascular resistance and a compensatory increase in stroke volume , heart rate and cardiac output . 