100 



RESPIRATORY MECHANISMS 



dissociation curve than the blood of the mother, which has 

 turned out to be due to the presence of a different haemoglobin. 

 It is well known that the mechanism of haemoglobin for- 

 mation in the foetus differs 

 from that in the mature 

 animal, haemoglobin being 

 formed in the foetus along 

 with the blood vessels, but 

 later only in the bone mar- 

 row, and it appears that the 

 gradual change in dissocia- 

 tion curve shown in Fig. 56 

 can be explained by assum- 

 ing that foetal haemoglobin 

 is slowly replaced by the 

 mature kind. The foetal 

 haemoglobin is definitely 

 adapted to the peculiar con- 

 ditions of foetal life. 



Adaptation of the blood to high 



Fig. 56. Oxygen dissociation curves 

 of haemoglobin from chicks during de- 

 velopment, at p H 7.38. Age expressed altitudes. A few mammals, 

 as from the beginning of incubation, especially the llama and 



vicuna {Lama huanachus and 

 vicugna), and some birds (Chloephaga melanoptera and Rhea 

 americana) live habitually at high altitudes. These were care- 

 fully studied by Hall, Dill, and Barron (1936) and the dis- 

 sociation curves showed a distinct shift towards the left as 

 compared with their low altitude relatives (Fig. 57). 



Several other warm-blooded animals, including man, are 

 exposed to low oxygen pressures when occasionally ascending 

 to high altitudes, and the corresponding adaptations have 

 been extensively studied in man. Keys (1938), who has given 

 a very good summary of the literature, estimates that about 

 10 million humans live permanently at heights above 3,000 m, 

 but only a few hundred above 5,000 m. 



Whether the stay at high altitude is of short or of lifelong 



