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: Increasing recognition of the importance of calcium in the pathogenesis of cardiovascular disease has stimulated research into the use of calcium channel blocking agents for treatment of a variety of cardiovascular diseases .
### Output Text:
Increasing recognition of the importance of <span class="chemical">calcium</span> in the pathogenesis of cardiovascular disease has stimulated research into the use of <span class="chemical">calcium channel blocking agents</span> for treatment of a variety of cardiovascular diseases . 