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: Interactive effects of variations in [ Na ] o and [ Ca ] o on rat atrial spontaneous frequency .
### Output Text:
Interactive effects of variations in [ <span class="chemical">Na</span> ] o and [ <span class="chemical">Ca</span> ] o on rat atrial spontaneous frequency . 