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: Postoperatively , ERPF returned to preoperative values , whereas GFR was higher than preoperative values .
### Output Text:
Postoperatively , ERPF returned to preoperative values , whereas GFR was higher than preoperative values . 