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: BACKGROUND : Unfractionated heparin sodium ( UFH ) or low - molecular weight heparin ( LMWH ) is used in anticoagulant protocols at several institutions to prevent thrombosis after liver transplantation .
### Output Text:
BACKGROUND : <span class="chemical">Unfractionated heparin sodium</span> ( <span class="chemical">UFH</span> ) or <span class="chemical">low - molecular weight heparin</span> ( LMWH ) is used in anticoagulant protocols at several institutions to prevent thrombosis after liver transplantation . 