Asthmatics breathe on average about twice as much air as they should. This cools and dries the airways making them sensitive and causing the mucus glands to produce more mucus to moisten them.
At the same time they lose carbon dioxide from the blood stream which makes smooth muscle wrapped around the airways to tighten and the Mast cells to release histamine which causes inflammation and narrows the airways further.
If you hyperventilate for a period of time the brain gets used to the lower levels of carbon dioxide and makes you continue to hyperventilate to keep the level low.
The very basis of the Buteyko Method is breathing through your nose continuously. This makes it a lot harder to hyperventilate.
Just as the respiratory centre slowly came to accept lower levels of carbon dioxide as normal, it can slowly be retrained to accept higher levels. By practicing the Buteyko techniques, the respiratory centre is reconditioned to accept the levels healthy people should have.
Breathing rates slow down considerably. Less allergens are now inhaled and the cooling and dehydration of the airways previously experienced, is lessened. Irritation of the sensitive airways becomes greatly reduced.
Mucus and histamine production decreases and carbon dioxide levels rise. There is less inflammation and airways are not constricted, resulting in free, easy breathing.