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: Leakage of mannitol into the brain parenchyma through an altered BBB and secondary reversal of osmotic gradient is considered the major cause of rebound .
### Output Text:
Leakage of <span class="chemical">mannitol</span> into the brain parenchyma through an altered BBB and secondary reversal of osmotic gradient is considered the major cause of rebound . 