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: These include augmented endothelin - 1 ( ET - 1 ) release , increased sympathetic nervous system activity , and elevated tissue oxidative stress .
### Output Text:
These include augmented endothelin - 1 ( ET - 1 ) release , increased sympathetic nervous system activity , and elevated tissue oxidative stress . 