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: Myocardial O2 consumption and O2 availability were directly dependent on the coronary perfusion .
### Output Text:
Myocardial <span class="chemical">O2</span> consumption and <span class="chemical">O2</span> availability were directly dependent on the coronary perfusion . 