110 RESPIRATION 



secretions, such as the lymph or urine. In these it may 

 amount to as much as 70 mm. In the tissues it must be 

 higher than this, since the greater part of the COg is 

 washed away by the blood and excreted in the lungs. 

 We may safely say, therefore, that the passage of COg out 

 of the tissues, hke the entry of oxygen, is due to diffusion. 

 We have seen that the dissociation of haemoglobin is 

 facilitated by rise in the hydrogen ion concentration and 

 by rise in temperature. When the cell becomes active the 

 increased tension of CO2 and the rise in temperature which 

 result affect the blood in such a manner as to make it 

 more easily part with its oxygen. In other words, the 

 chemical and thermal effects of increased consumption of 

 oxygen cause an increased supply of oxygen. 



LUNG VENTILATION 



The ventilation of the lungs is effected by co-ordinated 

 muscular movements which cause a rhythmic alteration 

 in the capacity of the thoracic cavity. To this alteration 

 the lungs adapt themselves owing to the elasticity of the 

 lung tissue and to the potential vacuum of the pleural 

 cavity. 



The Muscular Mechanism 



From a respiratory point of view the chest can be 

 divided into two parts — an upper part, conical in shape, 

 corresponding externally to the upper five ribs and inter- 

 nally to the upper lobe of the lung, and a lower part, 

 almost cyhndrical in shape, corresponding externally to 

 the lower ribs and internally to the lower lobe. The 

 changes in capacity which these two parts undergo differ 

 both in kind and in the manner in which they are produced. 



In the lower part of the chest the principal muscle 

 involved is the diaphragm, which is aided in its action by 

 the abdominal and the lower intercostal and interchondral 

 muscles. The diaphragm is attached posteriorly to the 

 spine by the crura and arcuate hgaments, and anteriorly 



