RESPIRATION 543 



Tlte exchange of gases between the air of the lungs and the 

 blood may therefore be exjylained by simple diffusion. 



So perfect is the exchange that the tension of a gas in the 

 air of the alveoli may be taken as a measure of the tension in 

 the blood flowing through the lungs, Brodie has suggested 

 that a dead lung, through the vessels of which the blood is 

 allowed to flow, might be used a tonometer. 



Haldane maintains that at low partial pressures of oxygen, 

 the passage of the gas from the alveoli to the blood cannot 

 be explained by diffusion and that it must be due to some 

 as yet unknown factor. 



Certainly the accumulation of gas in the swim bladder of 

 fishes cannot be explained by the laws of diffusion of gases, and 

 it seems to be dependent on the activity of the cells lining the 

 bladder. It may be arrested by section of the nerves supplying 

 the bladder. 



A. The Effects of Decreased Almospheric Pressure. — The fact 

 that the haemoglobin in the blood is so nearly fully oxygenated 

 at a pressure of only 50 mm. Hg (p. 493), explains why the 

 pressure of this gas in the atmosphere may fall to about 

 one-half of its normal 152 mm. Hg without interfering with 

 the supply of oxygen to the blood, why men and animals can 

 live at high altitudes, and why aviation to such enormous 

 heights is possible. The record height is probably 30,500 feet, 

 or about 10,000 metres. 



The following table shows the relationship of the height, 

 partial pressure of oxygen in the alveolar air, and the per- 

 centage saturation of tlie haemoglobin with oxygen, and it shows 

 that at about 5000 metres (16,000 feet) the marked decrease in 

 the oxygen carrying capacity of the blood begins (consult p. 545). 



When an animal is suddenly subjected to a very marked 

 decrease of pressure, especially if it has to do muscular work, 

 as in climbing, the decreased supply of oxygen leads to shortness 

 of breath, palpitation, and even to sickness (mountain sickness). 

 These symptoms generally pass off, increased pulmonary venti- 

 lation and increased heart's action augmenting the intake of 

 oxygen. Hence, residence in high altitudes tends to increase 

 the power of the respiratory muscles and the strength of the 

 heart. It also increases the richness of the blood in erythro- 

 cytes and in haemoglobin. 



