100 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



4-5 

 Normally it is -v~ =0*9, but this varies considerably with diet, as 



just stated. It varies also with muscular exercise, when the output 

 of carbonic acid is much increased both absolutely and relatively to 

 the amount of oxygen used up. 



Gases of the Blood. — From 100 volumes of blood about 60 volumes 

 of gas can be removed by the mercurial air-pump (see Appendix). 

 The average composition of this gas from dog's blood is — 



The nitrogen in the blood is simply dissolved from the air just 

 as water would dissolve it : it has no physiological importance. The 

 other two gases are present in much greater amount than can be 

 explained by simple solution ; they are, in fact, chiefly present in loose 

 chemical combinations. Less than one volume of the oxygen and about 

 two of carbonic acid are present in simple solution in the plasma. 



Oxygen in the Blood. — The amount of gas dissolved in a liquid 

 varies with the pressure of the gas ; double the pressure and the amount 

 of gas dissolved is doubled. Now this does not occur in the case of 

 oxygen and blood ; very nearly the same amount of oxygen is dissolved 

 whatever be the pressure. We thus have a proof that oxygen is not 

 merely dissolved in the blood, but is in chemical union : and the fact 

 that the oxygen of oxyhsemoglobin can be replaced by equivalent 

 quantities of other gases, like carbonic oxide, is a further proof of the 

 same statement. The tension or partial pressure of oxygen in the air 

 of the alveoli is less than that in the atmosphere, but greater than that 

 in venous blood ; hence oxygen passes from the alveolar air into 

 the blood ; the oxygen immediately combines with the haemoglobin, 

 and thus leaves the plasma free to absorb more oxygen ; and this goes 

 on until the haemoglobin is entirely, or almost entirely, saturated with 

 oxygen. The reverse change occurs in the tissues where the partial 

 pressure of oxygen is lower than in the plasma, or in the lymph that 

 bathes the tissue elements ; the plasma parts with its oxygen to the 

 lymph, the lymph to the tissues; the oxyhaemoglobin then undergoes 

 dissociation to supply more oxygen to the plasma and lymph, and this 

 in turn to the tissues once more. This goes on until the oxyhaemo- 

 globin loses a great portion of its store of oxygen, but even in asphyxia 

 it does not lose all. 



