416 THE RESPIRATION 



rarefied atmosphere, and the alveolar C0 2 tension assumes a value con- 

 siderably below the normal. For example, at sea level the minute vol- 

 ume of air breathed in one individual was 10.4 liters, and the alveolar 

 C0 2 tension 39.6 mm. Hg. After being some time on Pike's Peak, where 

 the barometer registers only 459 mm. Hg, Douglas 26 found the minute 

 volume of air to be 14.9 liters, and the alveolar C0 2 tension 27.1 mm. Hg. 

 At first sight the above statement may seem to contradict one pre- 

 viously made, to the effect that the alveolar C0 2 tension tends to remain 

 constant at varying barometric pressures. This applies, however, to 

 the slight variations occurring at ordinary elevations. It is important 

 to consider the significance of these changes because it will assist us 

 in the investigation of the clinical conditions of hyperpnea, in which 

 likewise a diminished C0 2 alveolar tension is often observed. Mountain 

 sickness may indeed ~be considered as an intermediate condition between 

 the physiological and the pathological. 



From what we have learned we should expect the above result to be 

 dependent upon stimulation of the respiratory center by deficiency of 

 oxygen, that is anoxemia (page 374). This excitation may be aggravated 

 by a hyperexcitability of the center due to constant irritation of the 

 sensory nerve terminations in the skin by a greater chemical activity of 

 the light rays at high altitudes. The erythema of the skin observed at 

 high altitudes is cited as evidence for this irritative action of light rays. 

 A similar increase in respiratory activity has been observed by Lindhard 63 

 to be produced by light baths. This author believes that this action 

 of light is the main cause for a demonstrably greater excitability of 

 the respiratory center during summer than winter. 



The increased breathing brings about a blowing off of C0 2 from the 

 blood with a consequent decrease of alveolar C0 2 -tension, and a tend- 

 ancy to alkalosis (page 373) as a result of which the kidney excretes 

 less acid and ammonia (page 381). Until this compensation is fully 

 effected it would be expected that P H would tend to be raised and this 

 has, as a matter of fact, been demonstrated by the calorimeter method 

 using particular care to see that the blood is kept at a tension of C(X 

 equal to that of the alveolar air. (Barcroft et al.) Since the adjustment 

 of the acid-base equilibrium by means of alteration in the acid excretion 

 by the kidneys must take some time it is to be expected that the alveolar 

 C0 2 will gradually attain its new levels both on the mountain and after 

 returning to sea level. That this is actually the case is shown in 

 Fig. 142-A. 



Thus, on Pike's Peak, where the barometric pressure is 459 mm. Hg., 

 the C0 2 -tension after an initial fall took about seven days before it came 

 to its permanent level for that barometric pressure, and fourteen days 

 elapsed after descending from the mountain before the sea level tension 

 had been regained. 



