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: Cerebral blood flow and cerebral metabolic rate for oxygen were measured during isoflurane - induced hypotension in 10 patients subjected to craniotomy for clipping of a cerebral aneurysm .
### Output Text:
Cerebral blood flow and cerebral metabolic rate for <span class="chemical">oxygen</span> were measured during <span class="chemical">isoflurane</span> - induced hypotension in 10 patients subjected to craniotomy for clipping of a cerebral aneurysm . 