CHANGES IN BLOOD. 229 



exhalation of the water of the blood, taking place from the 

 surfaces of the air-passages and cells, as it does from the 

 free surfaces of all moist animal membranes, particularly 

 at the high temperature of warm-blooded animals. It is 

 exhaled from the lungs whatever be the gas respired, con- 

 tinuing to be expelled even in hydrogen gas. 



Carbonic acid and water are, however, not the only prin- 

 ciples given off from the lungs. The Rev. J. B. Reade 

 showed, some years ago, and Dr. Richardson's experiments 

 confirm the fact, that ammonia is among the ordinary 

 constituents of expired air. And Wiederhold has stated, 

 that not only ammonia, but chloride of sodium, and even 

 uric acid and urate of soda and ammonia, may be readily 

 detected in the condensed vapour of expired air. His 

 experiments, therefore, seem to prove that the lungs may 

 furnish a channel for the excretion of some of the same 

 kind of solid principles that are met with in the secretions 

 of the skin and of the kidneys. 



Changes produced in the Blood by Respiration. 

 The most obvious change which the blood undergoes in 

 its passage through the lungs is that of colour, the dark 

 crimson of venous blood being exchanged for the bright 

 scarlet of arterial blood. The circumstances which have 

 been supposed to give rise to this change, the conditions 

 capable of effecting it independent of respiration, and some 

 other differences between arterial and venous blood, were 

 discussed in the chapter on BLOOD (p. 93). The change 

 in colour is indeed the most striking, and may appear the 

 most important, which the blood undergoes in its passage 

 through the lungs ; yet, perhaps, its importance is very 

 little, except so far as it is an indication of other and 

 essential alterations effected in the composition of the 

 blood. Of these alterations the principal are, \st, that the 

 blood, after passing through the lungs, is i or 2 warmer, 

 than it was before ; 2nd, that it coagulates sooner and 



