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: METHODS : Two interventions were studied that have been shown previously to improve blood pressure with calcium channel blocker overdose .
### Output Text:
METHODS : Two interventions were studied that have been shown previously to improve blood pressure with <span class="chemical">calcium</span> channel blocker overdose . 