Comparative mechanics of mammalian respiratory system

Compliances (C) of lung, thorax and respiratory system as well as resistances (R) of the respiratory system (lower airways + lung + chest wall) and of upper airway (primarily laryngeal and nasal) were measured in 5 species of mammals, spanning a 1000-fold range of body weights (mouse to dog), immediately following sacrifice with an overdose of sodium pentobarbital. The results indicate that compliance is proportional to BW1.0, while respiratory resistance and upper airway resistance have exponents of -0.819 and -0.702, respectively. The reciprocal of the time constant, tau -1 = (RC)-1 is proportional to BW-0.298 for respiratory resistance alone and BW-0.326 when upper airway resistance is included. Since breathing frequency varies as BW-0.28, these results indicate a proportional relationship between breathing frequency and passive emptying time. This suggests that passive respiratory mechanics play a major role in determining TE and therefore TTOT for animals during quiet breathing. Changes in volume history were found not to affect the slope of the relationships between compliance and body weight.