41 6 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 03 

 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, to com- 

 pensate for the resulting tendency to a lowering of C H , the kidney ex- 

 cretes less acid and ammonia (page 381). This compensation has been 

 found by Barcroft to be adequate to maintain C H at its normal value, 

 as judged by the magnitude of the dissociation constant for hemoglobin 

 (page 402) when the blood is exposed to a tension of C0 2 equal to that of 

 the alveolar air. 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. 



