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: Group II showed a better perioperative renal function as determined by serum blood urea nitrogen and serum creatinine levels .
### Output Text:
Group II showed a better perioperative renal function as determined by serum blood <span class="chemical">urea nitrogen</span> and serum <span class="chemical">creatinine</span> levels . 