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: These in vivo markers of kidney dysfunction and their correction by metformin were complemented by in vitro studies of mitochondrial function .
### Output Text:
These in vivo markers of kidney dysfunction and their correction by <span class="chemical">metformin</span> were complemented by in vitro studies of mitochondrial function . 