636 PRINCIPLES OF GENERAL PHYSIOLOGY 



to oxygen in presence of salts or acid. The difficulty of interpreting this formula 

 in terms of mass action is pointed out. The intervention of phenomena of 

 aggregation are suggested by the fact that haemoglobin behaves as if in colloidal 

 solution. 



Organs, lungs or gills, are provided by which a large surface of blood is exposed 

 to the medium which contains oxygen, that is, to the air or water. Mechanical 

 means of periodic change of the medium enable efficient oxygenation, together 

 with escape of carbon dioxide. 



The question as to whether sufficient oxygen can be taken up by mere diffusion 

 is discussed. It is found that the only results which cannot, as yet, be explained 

 thus are those of Douglas, Haldane, and their co-workers on the condition attained 

 after some days' acclimatisation to high altitudes. Even in vigorous muscular 

 work there is no need to assume special oxygen secreting power on the part of the 

 cells of the alveoli of the lungs. 



The amount of air pumped in and out of the lungs is regulated by the action 

 on the respiratory centre of the hydrogen ion content of the arterial blood. This 

 concentration in hydrogen ions is determined, under ordinary conditions, by the 

 carbon dioxide tension in the alveolar air of the lungs. Under special conditions, 

 as in acclimatisation to low barometric pressures or by feeding on substances which 

 increase the acidity of the urine and blood, other acids, non-volatile, formed in 

 tissue metabolism, assist in the stimulation of the centre. 



There seems to be no evidence that the excitability of the respiratory centre 

 to carbon dioxide is sensibly affected by want of oxygen, until the oxygen tension 

 has become very low. 



In asphyxia, at a certain stage, products of disintegration of the cells of the 

 nerve centres themselves act as exciting agents on these centres. But these 

 products are not to be regarded as normal stimulants. 



The function of the reflex nervous mechanism, whose afferent fibres are 

 contained in the vagus nerves, is to regulate the rate of respiration. An expansion 

 of the lungs, resulting from stimulation of the centre by carbon dioxide, is cut 

 short by the inhibiting reflex from the vagus endings in the lungs, and the centre 

 prepared for another stimulation by carbon dioxide. The reflexes are subject to 

 the laws of double reciprocal innervation. 



A short account is given of the phenomena of mountain sickness, due to want 

 of oxygen only. " Acapnia," or want of carbon dioxide, plays no part in them. 



LITERATURE 



Genera]. 



Babak (1912). Winterstein (1912, 2). 



Tissue Respiration. 



Warburg (1914). Barcroft (1908). 



Hemoglobin. 



Barcroft (1914). 



Exchange in the Lungs. 

 Krogh (1910). 



Regulation of Respiration. 



Haldane and Priestley (1905). Douglas (1914). Head (1889). 



