538 THE RESPIRATORY EXCHANGE 



globin, for they obtain the oxygen they need from the gas which 

 is in solution in the plasma. 



The amount of carbon monoxide absorbed by the blood depends 

 upon the relative affinities of oxygen and carbon monoxide for 

 haemoglobin and the relative pressures of the two gases in the 

 arterial blood. The affinity of the gas is about 140 times that 

 of oxygen. When an animal or man no longer inhales the gas, 

 it slowly disappears from the blood. This is due not to oxidation, 

 but to dissociation of the carboxy-hsemoglobin by the mass influ- 

 ence of the oxygen in the pulmonary capillaries. 



It will be necessary later to consider the relation of pressure 

 to the absorption of carbon monoxide by the blood, for the prin- 

 ciple of Haldane's method for the determination of the pressure 

 of oxygen in the arterial blood of man or animal is the calculation 

 of the pressure of oxygen from the pressure of carbon monoxide 

 in samples of the blood and the final saturation of the haemoglobin 

 with carbon monoxide. The subject of the experiment breathes 

 a known very small percentage of carbon monoxide until the per- 

 centage saturation of the haemoglobin with the gas is found to 

 be constant. Here it need only be stated that there is consider- 

 able want of agreement between the results obtained by Hiifner, 

 Haldane and Lorrain Smith, and Bock for the relation between 

 the pressure of the gas and its absorption by the blood. 



From the resemblances between the combinations formed with 

 haemoglobin by carbon monoxide and oxygen, it would be ex- 

 pected that differences should be found in the absorption of carbon 

 monoxide by blood and by solutions of haemoglobin, and there 

 should be different forms of haemoglobin with specific carbon 

 monoxide capacities resembling those for oxygen. Such results 

 have been obtained by Bohr and others. 



The Alveolar Air of the Lungs. In the preceding pages the 

 gaseous composition of the mixed venous blood in the right 

 ventricle and the arterial blood have been compared in order to 

 show the effect of the circulation of the blood through the lungs, 

 where separated only by thin walls the blood flowing through 

 the extensive capillary network is exposed to the air of the lungs. 

 This air is not the inspired air, it is not even the air about to be 

 expired ; it is the air of the alveoli, and, owing to its more 

 stationary character, contains less oxygen and more carbon dioxide 

 than the expired air. The alveoli of the lungs are exceedingly 



