How to search through entire CodeSystem

Hello. I’ve been testing with your terminology server for a couple of days now.

I need to write an HTML form for creating Observations, so I need a source to extract the ‘code’ element, and the resource indicates that the ValueSet includes all the codes in LOINC’s CodeSystem.

My question is: how can I query the entire codesystem, for example using a “$expand?filter=Heart Rate”-like query. I’ve seen I can do this with all the ValueSets available, but I cannot find the ValueSet that contains the entire code system, and my attempts to search through the CodeSystem directly have been futile.

Any help will be appreciated.