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: Though this fall would be associated with a decrease in wall tension , and , therefore , of myocardial oxygen consumption , it may not be of sufficient magnitude to prevent a net increase in myocardial oxygen consumption .
### Output Text:
Though this fall would be associated with a decrease in wall tension , and , therefore , of myocardial <span class="chemical">oxygen</span> consumption , it may not be of sufficient magnitude to prevent a net increase in myocardial <span class="chemical">oxygen</span> consumption . 