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: During hypotension produced by SNP similar decreases were observed in mean pulmonary arterial pressure ( p less than 0 . 01 ) , central venous pressure ( p less than 0 . 001 ) , left ventricular end - diastolic pressure ( p less than 0 . 01 ) , total peripheral resistance ( p less than 0 . 001 ) , rate pressure product ( p less than 0 . 001 ) , and oxygen content difference between arterial and mixed venous blood ( p less than 0 . 05 ) , while heart rate ( p less than 0 . 001 ) and cardiac output ( p less than 0 . 05 ) were increased .
### Output Text:
During hypotension produced by <span class="chemical">SNP</span> similar decreases were observed in mean pulmonary arterial pressure ( p less than 0 . 01 ) , central venous pressure ( p less than 0 . 001 ) , left ventricular end - diastolic pressure ( p less than 0 . 01 ) , total peripheral resistance ( p less than 0 . 001 ) , rate pressure product ( p less than 0 . 001 ) , and <span class="chemical">oxygen</span> content difference between arterial and mixed venous blood ( p less than 0 . 05 ) , while heart rate ( p less than 0 . 001 ) and cardiac output ( p less than 0 . 05 ) were increased . 