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 events are consistent with a severe reaction to calcium chelation by sodium citrate anticoagulant resulting in symptomatic systemic hypocalcemia .
### Output Text:
The events are consistent with a severe reaction to <span class="chemical">calcium</span> chelation by <span class="chemical">sodium citrate</span> anticoagulant resulting in symptomatic systemic hypocalcemia . 