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: Adverse effect of the calcium channel blocker nitrendipine on nephrosclerosis in rats with renovascular hypertension .
### Output Text:
Adverse effect of the <span class="chemical">calcium</span> channel blocker <span class="chemical">nitrendipine</span> on nephrosclerosis in rats with renovascular hypertension . 