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: Reactive oxygen species particularly affect the cardiac myocytes because these cells seem to have a relatively poor antioxidant defense system .
### Output Text:
Reactive <span class="chemical">oxygen</span> species particularly affect the cardiac myocytes because these cells seem to have a relatively poor antioxidant defense system . 