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: The rate - limiting constituent of collecting duct sodium transport is the epithelial sodium channel ( ENaC ) .
### Output Text:
The rate - limiting constituent of collecting duct <span class="chemical">sodium</span> transport is the epithelial <span class="chemical">sodium</span> channel ( ENaC ) . 