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: In the cardiovascular system , NO is involved in the regulation of a variety of functions .
### Output Text:
In the cardiovascular system , <span class="chemical">NO</span> is involved in the regulation of a variety of functions . 