CHANGES IN BLOOD. 179 



with watery vapor, yet when respiration is performed very 

 rapidly the air has scarcely time to be raised to the highest 

 temperature, or be fully charged with moisture ere it is expelled. 



For ordinary cases, however, it may be held that the ex- 

 pired air is saturated with watery vapor, and hence is derivable 

 a means of estimating the quantity exhaled in any given time : 

 namely, by subtracting the quantity contained in the air in- 

 spired from the quantity which (at the barometric pressure) 

 would saturate the same air at the temperature of expiration, 

 which is ordinarily about 99. And, on the other hand, if the 

 quantity of watery vapor in the expired air be estimated, the 

 quantity of air itself may from it be determined, being as much 

 as that quantity of watery vapor would saturate at the ascer- 

 tained temperature and barometric pressure. 



The quantity of water exhaled from the lungs in twenty- 

 four hours ranges (according to the various modifying circum- 

 stances already mentioned) from about 6 to 27 ounces, the or- 

 dinary quantity being about 9 or 10 ounces. Some of this is 

 probably formed by the combination of the excess of oxygen 

 absorbed in the lungs with the hydrogen of the blood ; but the 

 far larger proportion of it must be the mere 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 what- 

 ever be the gas respired, continuing to be expelled even in 

 hydrogen gas. 



5. 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. It seems 

 probable, however, both from the fact that this substance can- 

 not be always detected, and from its minute 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 color, the dark crimson of 

 venous blood being exchanged for the bright scarlet of arterial 

 blood. (The circumstances which have been supposed to give 



