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: METHODS : The alteration in the NO pathway was assessed by measuring nitrite levels in serum / urine and by evaluating the changes in vascular reactivity of the isolated perfused rat kidney ( IPRK ) system .
### Output Text:
METHODS : The alteration in the <span class="chemical">NO</span> pathway was assessed by measuring <span class="chemical">nitrite</span> levels in serum / urine and by evaluating the changes in vascular reactivity of the isolated perfused rat kidney ( IPRK ) system . 