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: Blood pressure , heart rate , respiratory rate and oxygen saturation were monitored over 1 hour .
### Output Text:
Blood pressure , heart rate , respiratory rate and <span class="chemical">oxygen</span> saturation were monitored over 1 hour . 