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: Severe gastric ulcers accompanied with increased ulcerogenic factors , including gastric acid back - diffusion , histamine release , LPO generation and luminal hemoglobin content were also observed in these rats .
### Output Text:
Severe gastric ulcers accompanied with increased ulcerogenic factors , including gastric acid back - diffusion , <span class="chemical">histamine</span> release , LPO generation and <span class="chemical">luminal</span> hemoglobin content were also observed in these rats . 