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: Furthermore , the pressor action of vasopressin appears to be important in the development of this model of hypertension , since the enhanced pressor responsiveness to the hormone was observed in the initial stage of hypertension .
### Output Text:
Furthermore , the pressor action of <span class="chemical">vasopressin</span> appears to be important in the development of this model of hypertension , since the enhanced pressor responsiveness to the hormone was observed in the initial stage of hypertension . 