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: Heart rate ( HR ) , MAP , stroke volume ( SV ) , cardiac output ( CO ) , and frontal lobe oxygenation ( S ( c ) O ( 2 ) ) were registered .
### Output Text:
Heart rate ( HR ) , MAP , stroke volume ( SV ) , cardiac output ( CO ) , and frontal lobe oxygenation ( S ( c ) O ( 2 ) ) were registered . 