Mammalian tracheal dimensions: scaling and physiology.

Mammalian tracheal dimensions vary with body mass in a way which suggests that expiratory flow limitation, inertance, and resistance are dominant constraints on tracheal design, rather than dead space or minimum work or effort. The fraction of total resistance attributable to the trachea appears to be about the same regardless of body size. Tracheal inertance appears to vary with body mass to the -1/2 power, so that inertial pressures, like elastic and resistive pressures, are invariant with body size and natural frequency, like respiratory frequency, varies with body mass to the -1/4 power. Other dominant constraints on tracheal design may include clearance functions and specialized habitat or behavior.