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: It decreased by 16 + / - 3 % by lowering [ Na ] o to 78 mM ( LNa ) , 23 + / - 2 % by lowering simultaneously [ Na ] o to 78 mM and [ Ca ] o to 0 . 675 mM ( LNa + LCa ) and 31 + / - 5 % by lowering [ Na ] o to 78 mM plus increasing [ Ca ] o to 3 . 6 mM ( LNa + HCa ) .
### Output Text:
It decreased by 16 + / - 3 % by lowering [ <span class="chemical">Na</span> ] o to 78 mM ( LNa ) , 23 + / - 2 % by lowering simultaneously [ <span class="chemical">Na</span> ] o to 78 mM and [ <span class="chemical">Ca</span> ] o to 0 . 675 mM ( LNa + LCa ) and 31 + / - 5 % by lowering [ <span class="chemical">Na</span> ] o to 78 mM plus increasing [ <span class="chemical">Ca</span> ] o to 3 . 6 mM ( LNa + HCa ) . 