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: The norepinephrine content was significantly increased while epinephrine remained unchanged .
### Output Text:
The <span class="chemical">norepinephrine</span> content was significantly increased while <span class="chemical">epinephrine</span> remained unchanged . 