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 aim of this study is to examine the role of gastric acid back - diffusion , mast cell histamine release , lipid peroxide ( LPO ) generation and mucosal microvascular permeability in modulating gastric hemorrhage and ulcer in rats with atherosclerosis induced by coadministration of vitamin D2 and cholesterol .
### Output Text:
The aim of this study is to examine the role of gastric acid back - diffusion , mast cell <span class="chemical">histamine</span> release , lipid peroxide ( LPO ) generation and mucosal microvascular permeability in modulating gastric hemorrhage and ulcer in rats with atherosclerosis induced by coadministration of <span class="chemical">vitamin D2</span> and <span class="chemical">cholesterol</span> . 