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: We hypothesized that epithelial Na channel ( ENaC ) subunit dysregulation may be responsible for the increased sodium retention .
### Output Text:
We hypothesized that epithelial <span class="chemical">Na</span> channel ( ENaC ) subunit dysregulation may be responsible for the increased <span class="chemical">sodium</span> retention . 