126 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



a general rule throughout the animal kingdom, including man. If, for 

 instance, a rabbit is treated with human blood, the serum ultimately 

 obtained from the rabbit contains a specific precipitin for human 

 blood ; that is to say, a precipitate is formed on adding such a rabbit's 

 serum to human blood, but not when added to the blood of any other 

 animal. 1 The great value of the test is its delicacy: it will detect 

 the specific blood when it is greatly diluted, after it has been dried 

 for weeks, or even when it is mixed with the blcod of other animals. 



CHEMISTRY OF RESPIRATION 



The consideration of the blood, and especially of its pigment, is so 

 closely associated with respiration that a brief account of that process 

 follows conveniently here. 



The air in the alveoli of the lungs, and the blood in the pulmonary 

 capillaries are only separated by the thin capillary and alveolar 

 walls. The blood parts with its excess of carbonic acid and watery 

 vapour to the alveolar air ; the blood at the same time receives from 

 the alveolar air the oxygen which renders it arterial. 



The intake of oxygen is the commencement, and the output of 

 carbonic acid the end, of the series of changes known as respiration. 

 The intermediate steps take place all over the body, and constitute 

 what is known as internal or tissue respiration. The exchange of 

 gases which occurs in the lungs is sometimes called in contradistinc- 

 tion external respiration. We have already seen that the oxyhaemo- 

 globin is only a loose compound, and in the tissues it parts with 

 its oxygen. The oxygen does not necessarily undergo immediate 

 union with carbon to form carbonic acid, and with hydrogen to form 

 water, but in most cases, as in muscle, is held in reserve by the tissue 

 itself. Ultimately, however, these substances pass into the venous 

 blood, and the carbonic acid and a portion of the water find an outlet 

 by the lungs. 



Inspired and Expired Air, The composition of the inspired and 

 expired air may be compared in the following table : 



1 There may be a slight reaction with the blood of allied animals ; for instance 

 with monkey's blood in the case of man. 



