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: Hydroxytyrosol improved the cardiac disturbances enhanced by doxorubicin by significantly reducing the percentage of altered mitochondria and oxidative damage .
### Output Text:
<span class="chemical">Hydroxytyrosol</span> improved the cardiac disturbances enhanced by <span class="chemical">doxorubicin</span> by significantly reducing the percentage of altered mitochondria and oxidative damage . 