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: We recently showed elevated reactive oxygen species ( ROS ) , reduced urinary excretion of NO metabolites ( NOx ) , and increased NO sequestration as nitrotyrosine in various tissues in rats with lead - induced hypertension .
### Output Text:
We recently showed elevated reactive <span class="chemical">oxygen</span> species ( ROS ) , reduced urinary excretion of <span class="chemical">NO</span> metabolites ( NOx ) , and increased <span class="chemical">NO</span> sequestration as <span class="chemical">nitrotyrosine</span> in various tissues in rats with <span class="chemical">lead</span> - induced hypertension . 