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: Experimental hypertension created by the removal of NO , however , involves mechanisms in addition to decreased arterial vasodilator activity .
### Output Text:
Experimental hypertension created by the removal of <span class="chemical">NO</span> , however , involves mechanisms in addition to decreased arterial vasodilator activity . 