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: Nephropathy caused by Chinese herbs has previously been reported , usually involving the use of aristolochic acids .
### Output Text:
Nephropathy caused by <span class="chemical">Chinese herbs</span> has previously been reported , usually involving the use of <span class="chemical">aristolochic acids</span> . 