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 influence of L - dopa on the renal excretion of potassium was studied in 3 patients with hypokalemia and in 5 normokalemic patients by determination of renal plasma flow , glomerular filtration rate , plasma concentration of potassium and sodium as well as urinary excretion of potassium , sodium and aldosterone .
### Output Text:
The influence of <span class="chemical">L - dopa</span> on the renal excretion of <span class="chemical">potassium</span> was studied in 3 patients with hypokalemia and in 5 normokalemic patients by determination of renal plasma flow , glomerular filtration rate , plasma concentration of <span class="chemical">potassium</span> and <span class="chemical">sodium</span> as well as urinary excretion of <span class="chemical">potassium</span> , <span class="chemical">sodium</span> and <span class="chemical">aldosterone</span> . 