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 and thyrotropin - releasing hormone modulate the hypercapnic ventilatory response in humans .
### Output Text:
Human <span class="chemical">corticotropin</span> - releasing hormone and <span class="chemical">thyrotropin</span> - releasing hormone modulate the hypercapnic ventilatory response in humans . 