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: All of the patients ' plasma blood urea nitrogen ( BUN ) and creatinine levels were measured on the second and seventh day after the administration of intravenous contrast material .
### Output Text:
All of the patients ' plasma <span class="chemical">blood urea nitrogen</span> ( <span class="chemical">BUN</span> ) and <span class="chemical">creatinine</span> levels were measured on the second and seventh day after the administration of intravenous <span class="chemical">contrast</span> material . 