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: Systemic anticoagulation is unsafe and regional citrate anticoagulation in the absence of a functional liver carries the risk of citrate toxicity .
### Output Text:
Systemic anticoagulation is unsafe and regional <span class="chemical">citrate</span> anticoagulation in the absence of a functional liver carries the risk of <span class="chemical">citrate</span> toxicity . 