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: Preoperative assessment should focus on cardiovascular status and serum potassium level .
### Output Text:
Preoperative assessment should focus on cardiovascular status and serum <span class="chemical">potassium</span> level . 