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: Acute blood pressure elevations with caffeine in men with borderline systemic hypertension .
### Output Text:
Acute blood pressure elevations with <span class="chemical">caffeine</span> in men with borderline systemic hypertension . 