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: This study demonstrates that hydroxytyrosol protect rat heart damage provoked by doxorubicin decreasing oxidative damage and mitochondrial alterations .
### Output Text:
This study demonstrates that <span class="chemical">hydroxytyrosol</span> protect rat heart damage provoked by <span class="chemical">doxorubicin</span> decreasing oxidative damage and mitochondrial alterations . 