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: Spinal or suprabulbar structures could be responsible for the gradual recovery of the hypertension in the lesioned rats .
### Output Text:
Spinal or suprabulbar structures could be responsible for the gradual recovery of the hypertension in the lesioned rats . 