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: Mean circulatory filling pressure , an in vivo measure of venomotor tone , was not elevated in LNNA hypertension at 1 or 2 weeks after LNNA .
### Output Text:
Mean circulatory filling pressure , an in vivo measure of venomotor tone , was not elevated in <span class="chemical">LNNA</span> hypertension at 1 or 2 weeks after <span class="chemical">LNNA</span> . 