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: Removal of the carotid sinuses caused an elevation blood pressure and heart rate and abolished the negative chronotropic effect of norepinephrine .
### Output Text:
Removal of the carotid sinuses caused an elevation blood pressure and heart rate and abolished the negative chronotropic effect of <span class="chemical">norepinephrine</span> . 