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: After 3 days of combined treatment , a marked elevation in plasma and tissue lithium levels accompanied a reduction in water intake .
### Output Text:
After 3 days of combined treatment , a marked elevation in plasma and tissue <span class="chemical">lithium</span> levels accompanied a reduction in water intake . 