534 



VERTEBRATE LIFE AND ORGANIZATION 



Brain 



Sternum 



Position of Rit3 

 during inspiration 



expiration 



Position of Rib 4 

 during inspiration 



expiration 



Diaphragm during expiration 



inspiration 



Position of abdominal 

 muscles during inspiration 



Spinal 

 cord 



l^exp: 



iration 



Neuron circuit pTom 

 inspiratory to 

 expiratory center 



Inhibiting neuron 

 from lung 



Lund 



Neuron from 

 inspiratory center 



Diaphragm 



'Abdominal muscles 



Neuron From 

 expiratory center 



Figure 26.10. Mechanics and control of breathing. A, The elevation of the ribs 

 and depression of the diaphragm during inspiration increases the size of the chest 

 cavity, indicated by tlie black area. B, A diagram of the nervous mechanism for 

 controlling the rhythm of breathing. See text for explanation. 



intrapulmonary pressure falls to about 3 mm. of mercury below at- 

 mospheric pressure, and air passes into the lungs until intrapulmonary 

 and atmospheric pressures are the same. During normal expiration, the 

 size of the thorax is decreased, intrapulmonary pressure is raised to 

 about 3 mm. of mercury above atmospheric pressure, and air is driven 

 out of the lungs until equilibrium is again reached. During inspiration, 

 the thorax is enlarged by the contraction of the dome-shaped diaphragm 

 and the external intercostal muscles. The diaphragm pushes the ab- 

 dominal viscera posteriorly and increases the length of the chest cavity 

 (Fig. 26.10 A); the external intercostals raise the sternal ends of the 

 ribs and expand the dorsoventral diameter of the chest. During expira- 

 tion, the relaxation of these muscles and the contraction of antagonistic 

 muscles decrease the size of the thorax. Contraction of abdominal 

 muscles forces the abdominal viscera against the diaphragm and pushes 

 it forward; internal intercostals pvdl the sternal ends of the ribs pos- 

 teriorly. The elastic recoil of the lungs, which are stretched during 

 inspiration, is also important in expelling air. 



The lungs of an adult man can hold about 6 liters of air, but in 

 quiet breathing they contain only about half this amount, of which 0.5 

 liter is exchanged in any one cycle of inspiration and expiration. This 

 half liter of tidal air is mixed with the 2.5 liters of air already in the 

 lungs. Vigorous respiratory movements can lower and raise the intra- 

 pulmonary pressure 60 mm. of mercury below and above atmospheric 

 pressure, and under these conditions 4 to 5 liters of air can be exchanged. 

 There is always, however, at least a liter of residual air left in the lungs 

 to mix with the tidal air, for the strongest respiratory movements cannot 



