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### Input Text: Peripheral venous blood samples were withdrawn from the femoral vein four times ( once every 5 min ) before ischemia ( 0 time ) and 5 , 15 , and 30 min after ischemia .
### Output Text:
Peripheral venous blood samples were withdrawn from the femoral vein four times ( once every 5 min ) before ischemia ( 0 time ) and 5 , 15 , and 30 min after ischemia . 