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 role of vasopressin as a pressor agent to the hypertensive process was examined .
### Output Text:
The role of <span class="chemical">vasopressin</span> as a pressor agent to the hypertensive process was examined . 