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: When comparing all lithium treated versus non - lithium - treated groups , lithium caused a reduction in glomerular filtration rate ( GFR ) without significant changes in effective renal plasma flow ( as determined by a marker secreted into the proximal tubules ) or lithium clearance .
### Output Text:
When comparing all <span class="chemical">lithium</span> treated versus non - <span class="chemical">lithium</span> - treated groups , <span class="chemical">lithium</span> caused a reduction in glomerular filtration rate ( GFR ) without significant changes in effective renal plasma flow ( as determined by a marker secreted into the proximal tubules ) or <span class="chemical">lithium</span> clearance . 