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: Side effects of yohimbine included excessive sweating , increased anxiety , and a wound - up feeling in some patients .
### Output Text:
Side effects of <span class="chemical">yohimbine</span> included excessive sweating , increased anxiety , and a wound - up feeling in some patients . 