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: Thus , in borderline hypertensive men , exaggerated responses to caffeine were : selective for diastolic BP , consistent with greater vasoconstriction , replicated in 2 protocols , and representative of nearly all borderline hypertensives .
### Output Text:
Thus , in borderline hypertensive men , exaggerated responses to <span class="chemical">caffeine</span> were : selective for diastolic BP , consistent with greater vasoconstriction , replicated in 2 protocols , and representative of nearly all borderline hypertensives . 