Boycott and Haldane4 found in experiments on themselves when they were confined in a steel pneumatic cabinet that if the atmospheric pressure was reduced to 356 mm. of mercury, corresponding to a height of 6000 meters (= 20,000 feet) the oxygen pressure in the alveoli fell to 30 mm. and pronounced cyanosis occurred, accompanied first by loss of memory and then by unconsciousness. There was only slight hyperpnea. Greater attenuation of the atmosphere on mountains and in balloons may often be tolerated. This they ascribe to a gradual production of lactic acid within the organism which renders the respiratory center especially sensitive to the stimulus of carbon dioxid. They recommend that one frequently partake of carbohydrates when among the higher mountains in order that a maximum amount of carbon dioxid be furnished to the blood-stream. The carbon dioxid pressure in the alveoli falls as an accompaniment of the rising acid content of the body. This changed condition of the blood does not pass off at once on return to a lower level.1 The respiratory stimulus persists and the beneficial effects of descending are promptly felt. At a given altitude on the descent the alveolar oxygen pressure will therefore probably be higher than at the same altitude on the ascent on account of the greater stimulation of the respiratory center.

4 Boycott and Haldane: Ibid., 1908, xxxvii, 355.

These relations are shown in the following table compiled from Ward's experiments on himself:

One may compare the statement of Boycott and Haldane that cyanosis occurred in them when the oxygen pressure in the alveoli fell to 30 mm. with the statement of Loewy and Zuntz2 that when the oxygen pressure is 31.8 human blood will absorb oxygen so that 56 per cent, of its hemoglobin is saturated. This agrees well with the finding of Ringer3 in the author's laboratory that dogs lose consciousness when their hemoglobin becomes half-saturated with carbon monoxid gas. Ringer's dogs, however, were not beyond the power of resuscitation until 70 per cent, of the hemoglobin was combined with the poisonous gas.

This observation is similar to that of Bornstein and Muller,4 who have shown that death occurs when 70 per cent, of the hemoglobin of the blood is converted into methemoglobin by magnesium chlorid. Rapid recovery takes place if the process is not carried so far as this.

1 Ward: "Journal of Physiology," 1908, xxxvii, 378.

2 Loewy and Zuntz: "Archiv fur Physiologie," 1904, p. 214. 3 Ringer: Unpublished.

4 Bornstein and Muller: "Archiv fur Physiologie," 1907, p. 470.

The discovery of Viault1 that at an altitude of 4000 meters the number of red blood-cells increased to 7,000,000 and 8,000,000 per cubic mm. of blood appeared at first to indicate a compensatory increase in oxygen-combining power during life in rarefied air. An increase in the quantity of hemoglobin has been positively shown by Zuntz and his coworkers.2

While in the high altitudes of Monte Rosa, von Wendt3 noticed a retention of nitrogen, iron, and potassium which he suggests was in part used for the construction of new red blood-corpuscles, in part for the upbuilding of new musculature.

The Pike's Peak Expedition already referred to does not fully agree with the interpretations of the Zuntz school. The numerous visitors who reached the summit of Pike's Peak by train and remained only about three-quarters of an hour showed blueness of the lips and cheeks, accompanied by great hyperpnea on exertion. Only a few became miserable and faint and required oxygen for their restoration. Among those who arrived on foot, frequently after ten hours of effort, the symptoms were much more severe: nausea, vomiting, headache, and fainting being common. The nose-bleed traditionally assigned as characteristic of life in rarefied atmospheres is mythical. The process of acclimatization follows these lines:

(1) The production of acids which reduce the alkalinity of the blood, this in turn stimulating the respiratory center with a resultant increase in ventilation of the lungs, a fall in the alveolar carbon dioxid tension and an increase in the oxygen tension;

(2) an increase up to 150 per cent, of the normal amount of hemoglobin. These factors are of such influence that even more than the normal quantity of oxygen may be carried to the tissues. The hemoglobin was found to be saturated with oxygen to an extent of 95 per cent., which is contrary to the teachings of Zuntz. The authors believe that only adherence to the theory that the alveolar epithelium secretes oxygen from the air into the blood will explain this phenomenon. The pulse and blood-pressure were but little affected. On passing from the summit of the mountain to the sea-level a fortnight is required before the stimulus to the respiratory center disappears and the alveolar carbon dioxid tension becomes normal, and several weeks pass before the total quantity of hemoglobin in the body returns to the normal. It is evident that in unacclimated persons balloon ascents and the like are to a greater extent dangerous to life than in those who have undergone climatic adaptation to high altitudes. Into all phases of the fascinating work of the Pike's Peak Expedition it is impossible to go.

1 Viault: "Comptes rendus de l'academie des sciences," 1890, cxi, 917.

2 Zuntz, Loewy, Muller, and Caspari: "Hohenklima und Bergwanderungen in ihrer Wirkung auf den Menschen," Berlin, 1906.

3 von Wendt: "Skan. Archiv fur Physiologie," 1911, xxiv, 247.

The work was ably supplemented by that of Miss Fitzgerald,1 who worked among acclimated mine attendants and their wives, persons who had lived a year or more at different heights above the sea-level in Colorado, some of them having been born in these localities. The records included, among others, some made at Denver (5100 ft.), Colorado Springs (6000 ft.), Cripple Creek (10,000 ft.), Camp Bird Mine (11,300 ft), Lewis (12,500 ft.), and Pike's Peak (14,100 ft.). Miss Fitzgerald showed that for every 100 mm. fall in barometric pressure there was an increase of 10 per cent, above the amount of hemoglobin present in the body at the level of the sea, the law holding true for both sexes. Also, for every fall of 100 mm. in the atmospheric pressure there is a fall of 4.2 mm. in the pressure of alveolar carbon dioxid, accompanied by a progressive fall in the oxygen pressure.

From the facts she makes the following computation:

1 Fitzgerald: "Transactions of the Royal Society of London," 1913, Series B, cciii, 351.

Table Showing The Tension Of The Alveolar Gases In Acclimated Individuals

Each successive diminution of 100 c.c. of barometric pressure causes a greater absolute increase in the ventilation of the lung and this introduces more oxygen. The full reaction, however, is not effected in short periods. Thus, in the experiment by Boycott and Haldane (see p. 433) in which they subjected themselves to a barometric pressure of 350 mm., their alveolar carbon dioxid tensions were 31.2 and 27.3 mm. respectively. In an acclimatized individual the carbon dioxid tension at this level of the barometer would have been 22.4 mm. and he would have had 3.2 per cent, more oxygen in his alveoli than Haldane had. Acclimatization involving this reaction, as well as increasing the quantity of hemoglobin, would have prevented the cyanosis and unconsciousness which followed in the experiments of Boycott and Haldane when they were in the respiration chamber.