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 extent of hypotension and changes in brain tissue oxygenation ( PbtO ( 2 ) ) and in cerebral blood flow were studied in a separate group of animals .
### Output Text:
The extent of hypotension and changes in brain tissue oxygenation ( PbtO ( 2 ) ) and in cerebral blood flow were studied in a separate group of animals . 