The medulla part of the brain stem reacts quickly to increased levels of CO2 in the blood and stimulates a response from the ...

A response from the amigdala. The reason for this is that when the medulla starts signalling brain regions that are in upper parts, especially the amigdala, this then triggers the fear response (amigdala is responsible for this) which in turn alerts us on a conscious level that we should leave this room for example and go to somewhere where more oxygen is present.