RESPIRATION 285 



be limited, so that the increased circulation required during mus- 

 cular exertion cannot be produced. The arterial blood pressure 

 will therefore fall during muscular work of more than a certain 

 amount. In consequence of this the coronary circulation may also 

 be impaired, with possibly dangerous consequences under the ex- 

 isting circumstances ; and there will be faintness along with hy- 

 perpnoea, owing to slowed circulation and hence diminished oxy- 

 gen pressure and increased CO 2 pressure in the capillaries of the 

 brain. During rest, however, or such muscular exertion as is pos- 

 sible without abnormal symptoms, the circulation will be carried 

 on in a normal manner. 



The alveolar CO 2 pressure in a number of cases of valvular 

 heart disease was investigated by Miss FitzGerald, and found to be 

 normal except in cases confined to bed with serious symptoms. 21 

 The absence of any fall in the alveolar CO 2 pressure constituted 

 good evidence of the absence of any impairment of the circula- 

 tion during rest. In cases with serious symptoms even during rest 

 there was a marked fall in the alveolar CO 2 pressure. This is also 

 the case in congenital heart affections, when the alveolar CO 2 

 pressure may be as low as 20 mm. 22 



We can see what is happening in these cases. Owing to the im- 

 paired or short-circuited circulation the oxygen pressure in the 

 tissues falls and the CO 2 pressure tends to rise. This, however, 

 increases the breathing, and so prevents the rise of CO 2 pressure 

 by abnormally diminishing the CO 2 pressure of the arterial blood 

 leaving the lungs. The fall in oxygen pressure cannot, however, 

 be prevented in this way, as the increased breathing will not 

 materially increase the oxygen in the arterial blood. Some anox- 

 aemia will therefore be present, and will probably show itself by 

 the color of the skin and lips, as well as by more frequent, and 

 possibly shallower, breathing, and other symptoms of anoxaemia. 

 The alkalosis produced by the increased breathing due to anox- 

 aemia will gradually be compensated for by increased excretion 

 of alkali and diminished formation of ammonia, just as at a high 

 altitude (see Chapter VII) ; and this will tend to diminish the 

 real anoxaemia though without diminishing the cyanosis. Unless 

 the breathing became shallow no material relief could be looked 

 for owing to active secretion of oxygen inwards by the lung ep- 

 ithelium, as this would only slightly increase the oxygen in the 



21 FitzGerald, Journ. of Pathol. and Bact., XIV, p. 328. 



* French, Pembrey, and Ryffel, Journ. of Physiol., XXIX, Proc. P&ysiol. Soc., 

 p. ix, 1909. 



