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 resulting hypotension can be produced by a decrease in systemic vascular resistance or cardiac output through a decrease in venous return .
### Output Text:
The resulting hypotension can be produced by a decrease in systemic vascular resistance or cardiac output through a decrease in venous return . 