When you get off the plane, and are walking with everyone else, you will see airline or airport personnel asking who has an international connection (conexão internacional), and these passengers are directed off to a different corridor. I haven't done it (I've arrived for domestic connections). Just to be 100% sure, I'd check with the airline (United?), and make sure you don't need a Brazilian visa. I don't think you will, though if you miss your connection you may have a long stay in the secure area (be prepared to fly to EZE if that's the only feasible option). I think Americans need to fill out an online form in advance when flying into one of the two Buenos Aires airports, but I can't recall if it's EZE or AEP. A friend ran into that. There was no fee.

I assume your two flights were booked together. You'll need to show your full itinerary to make sure can board a flight to Brazil without a Brazilian visa. It would be at the airline check-in counter that you'd be asked to show such a visa if you were flying to just Brazil. I'm sure they've run into this before.