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 K ( Na ) value for Na + returned to control value .
### Output Text:
The <span class="chemical">K</span> ( <span class="chemical">Na</span> ) value for <span class="chemical">Na</span> + returned to control value . 