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: The hemodynamics of oxytocin and other vasoactive agents during neuraxial anesthesia for cesarean delivery : findings in six cases .
### Output Text:
The hemodynamics of <span class="chemical">oxytocin</span> and other vasoactive agents during neuraxial anesthesia for cesarean delivery : findings in six cases . 