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: The mean respiratory rate and oxygen saturations in the two groups were similar .
### Output Text:
The mean respiratory rate and <span class="chemical">oxygen</span> saturations in the two groups were similar . 