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: Recent evidence indicates that oxidative stress and mitochondrial dysfunction are key factors in the pathogenic process .
### Output Text:
Recent evidence indicates that oxidative stress and mitochondrial dysfunction are key factors in the pathogenic process . 