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 area under the curve was 537 + / - 149 ng / ml x hours , volume of distribution ( Vd ) 3504 + / - 644 l / m2 , and total clearance ( ClT ) was 204 + 39 . 3 l / hour / m2 .
### Output Text:
The area under the curve was 537 + / - 149 ng / ml x hours , volume of distribution ( Vd ) 3504 + / - 644 l / m2 , and total clearance ( ClT ) was 204 + 39 . 3 l / hour / m2 . 