RESPIRATION 295 



creases, so that the total haemoglobin in the body increases more 

 than the percentage of haemoglobin. Thus the corpuscles dcrnot 

 simply increase at the expense of the space occupied by plasma, 

 but the total space occupied by the blood is increased. It seems 

 probable, however, that when a rapid increase in the percentage 

 of haemoglobin occurs, as shown in Figure 73, the increase is 

 mainly brought about at first by disappearance of plasma owing 

 to a pressor reaction of the vasomotor center, with consequent in- 

 creased filling of the capillaries and resulting loss of liquid from 

 the blood. In acute anoxaemia produced by asphyxial conditions 

 there appears to be a rapid loss of fluid from the blood, and this 

 is probably due to a pressor reaction. Schneider and his colleagues 

 have recently observed that in a considerable proportion of airmen 

 exposed for a quite short time to low pressures of oxygen there is 

 a small but quite appreciable rise in the haemoglobin percentage. 32 



There appears to be no doubt that the cause of the increased 

 total amount of haemoglobin and red corpuscles in the body at 

 high altitudes is increased activity of the bone marrow in forming 

 red corpuscles. On this point direct evidence was obtained by 

 Zuntz and his colleagues. 33 They found that in dogs the blood- 

 forming red marrow was markedly increased at a high altitude. 

 The stimulus to this increase was undoubtedly fall in the oxygen 

 pressure of the blood, and it is doubtless in the same way that in- 

 creased formation of red corpuscles is brought about by loss of 

 blood, especially if repeated. From the experiments of Boycott and 

 Douglas on repeated blood transfusions, we can also infer with 

 great probability that with increased oxygen pressure in the tis- 

 sue capillaries, owing to an increased proportion of haemoglobin, 

 there is a corresponding increase in the blood-destroying tissues. 

 The proportion of haemoglobin in the blood appears, therefore, 

 to be dependent on the oxygen pressure in tissue capillaries. This 

 inference is confirmed by the fact that, as Nasmith and Graham 

 showed, 34 the haemoglobin percentage rises markedly in animals 

 which are kept exposed to a small percentage of CO. 



In cases of chronic heart disease, and more particularly in cases 

 of congenital heart defects accompanied by cyanosis, there is often 

 a great increase in the total haemoglobin and also in the blood 

 volume. Thus in a congenital case of "Morbus coeruleus," brought 



82 Gregg, Lutz, and Schneider, Amer. Journ. of Physiol., L, p. 216, 1919. 



33 Zuntz, Loewy, Muller, and Caspar!, Hohenklima und Bergwanderungen, 

 Berlin, 1906. 



34 Nasmith and Graham, Journ. of Physwl., XXXV, p. 32, 1906. 



