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: Low - molecular - weight heparin for the treatment of patients with mechanical heart valves .
### Output Text:
<span class="chemical">Low - molecular - weight heparin</span> for the treatment of patients with mechanical heart valves . 