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: Gastric acid back - diffusion , mucosal LPO generation , histamine concentration , microvascular permeability , luminal hemoglobin content and ulcer areas were determined .
### Output Text:
Gastric acid back - diffusion , mucosal LPO generation , <span class="chemical">histamine</span> concentration , microvascular permeability , <span class="chemical">luminal</span> hemoglobin content and ulcer areas were determined . 