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: Human corticotropin - releasing hormone ( hCRH ) and thyrotropin - releasing hormone ( TRH ) are known to stimulate ventilation after i . v . administration in humans .
### Output Text:
Human <span class="chemical">corticotropin</span> - releasing hormone ( hCRH ) and <span class="chemical">thyrotropin</span> - releasing hormone ( TRH ) are known to stimulate ventilation after i . v . administration in humans . 