CHANGES IX BLOOD. 219 



amount when present, that the whole of it may "be derived 

 from decomposing particles of food left in the mouth, or 

 from carious teeth or the like ; and that it is, therefore, 

 only an accidental constituent of expired air. 



The quantity of organic matter in the breath has been 

 lately investigated by Dr. Ransome, who calculates that 

 about 3 grains are given off from the lungs of an adult in 

 twenty-four hours. 



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. 85) : 2nd, and in 

 connection with the preceding change, it gains oxygen ; 

 3rd, it loses carbonic acid ; ^tli, it becomes l or 2 F. 

 warmer; $th, io coagulates sooner and more firmly, and, 

 apparently, contains more fibrin. 



The oxygen absorbed into the blood from the atmospheric 

 air in the lungs is combined chemically with the haemo- 

 globin of the red blood corpuscles. In this condition it is 

 carried in the arterial blood to the various parts of the 

 body, and with it is, in the capillary system of vessels, 

 brought into near relation or contact with the elementary 

 parts of the tissues. Herein co-operating probably in the 

 process of nutrition, or in the removal of disintegrated 

 parts of the tissues, a certain portion of the oxygen which 

 the arterial blood contains disappears, and a proportionate 

 quantity of carbonic acid and water is formed. 



But it is not alone in the disintegrating processes to 

 which all parts of the body are liable, that oxygen is con- 

 sumed and carbonic acid and water are formed in its 



