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: Conversely , down - regulation of alphaENaC , betaENaC and gammaENaC mRNA expression on day 3 occurred in the presence of high aldosterone concentrations , and was followed by a return of sodium excretion to control values .
### Output Text:
Conversely , down - regulation of alphaENaC , betaENaC and gammaENaC mRNA expression on day 3 occurred in the presence of high <span class="chemical">aldosterone</span> concentrations , and was followed by a return of <span class="chemical">sodium</span> excretion to control values . 