In the human respiratory system, eupnea or eupnoea is normal, good, unlabored breathing, sometimes known as quiet breathing or resting respiratory rate. In eupnea, expiration employs only the elastic recoil of the lungs.
Eupnea is the natural breathing in all mammals, including humans. Eupnea does not require any volitional effort whatsoever, but occurs whenever a mammal is in a natural state of relaxation, i.e. when there is no clear and present danger in their environment. When a mammal perceives potential danger, eupnea stops, and a much more limited and labored form of breathing occurs.
Eupnea is an efficient and effective form of breathing, which maximizes air intake, and minimizes muscular effort.
During eupnea, neural output to respiratory muscles is highly regular, with rhythmic bursts of activity during inspiration only to the diaphragm and external intercostal muscles.
Etymology and pronunciation
The word eupnea uses combining forms of eu- + -pnea, from Greek eupnoia, from eu-, "well" + pnoia, "breath". See pronunciation information at dyspnea.