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: At normal [ Na ] o , decrease ( 0 . 675 mM ) or increase ( 3 . 6 mM ) of [ Ca ] o did not modify BF ; a reduction of ten times ( 0 . 135 mM of normal [ Ca ] o was effective to reduce BF by 40 + / - 13 % .
### Output Text:
At normal [ <span class="chemical">Na</span> ] o , decrease ( 0 . 675 mM ) or increase ( 3 . 6 mM ) of [ <span class="chemical">Ca</span> ] o did not modify BF ; a reduction of ten times ( 0 . 135 mM of normal [ <span class="chemical">Ca</span> ] o was effective to reduce BF by 40 + / - 13 % . 