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: Parturients with normal volume status , heart valves and pulmonary vasculature most often respond to this hypotension with a compensatory increase in heart rate and stroke volume .
### Output Text:
Parturients with normal volume status , heart valves and pulmonary vasculature most often respond to this hypotension with a compensatory increase in heart rate and stroke volume . 