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: Central cardiovascular effects of AVP and ANP in normotensive and spontaneously hypertensive rats .
### Output Text:
Central cardiovascular effects of <span class="chemical">AVP</span> and ANP in normotensive and spontaneously hypertensive rats . 