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: Consequently , lithium pretreatment caused a fall in filtration fraction and an increase in fractional Li excretion .
### Output Text:
Consequently , <span class="chemical">lithium</span> pretreatment caused a fall in filtration fraction and an increase in fractional <span class="chemical">Li</span> excretion . 