66 HUMAN PHYSIOLOGY. 



The amount of watery vapor which passes out of the body with the 

 expired air amounts to from one to two pounds. 



The organic matter, though slight in amount, gives the odor to the breath. 

 In a room with defective ventilation, the organic matter accumulates and 

 gives rise to headache, nausea, drowsiness, etc. Long continued breathing 

 of such air produces general ill health. It is not so much the presence of 

 CO 2 in increased amount, as the presence of organic matter which neces- 

 sitates thorough ventilation. 



Condition of the Gases in the Blood. 



Oxygen is absorbed from the lungs into the arterial blood by the coloring 

 matter, hemoglobin, with which it exists in a state of loose combination, 

 and is disengaged during the process of nutrition. 



Carbonic acid, arising in the tissues, is absorbed into the blood, in conse- 

 quence of its alkalinity, where it exists in a state of simple solution and 

 also in a state of feeble combination with the carbonates, soda and potassa, 

 forming the bi'carbonates. 



Nitrogen is simply held in solution in the plasma. 



Exchange of Gases in the Air Cells. From the difference in tension 

 of the oxygen in the air cells (27.44 mm. of Hg), and of the oxygen in the 

 venous blood (22 mm. Hg), and of the difference of the carbonic acid tension 

 in the venous blood (41 mm. Hg), and in the air cells (27 mm. Hg), it might 

 be concluded that the passage of the gases might be due solely to pressure. 

 The absorption of oxygen, however, does not follow absolutely the law of 

 pressures ; that chemical processes are involved is shown by the union of 

 oxygen with the haemoglobin of the blood corpuscles. The exhalation of 

 CO 2 is also partly a chemical process, as it has been shown that the quan- 

 tity excreted is greatly increased when oxygen is simultaneously absorbed. 

 Oxygen not only favors the exhalation of loosely combined CO 2 , but favors 

 the expulsion of that which can only be excreted by the addition of acids 

 to the blood. 



Changes in the Blood during Respiration. 



As the blood passes through the lungs it is changed in color, from the 

 dark purple hue of venous blood to the bright red scarlet of arterial 

 blood. 



The heterogeneous composition of venous blood is exchanged for the 

 uniform composition of the arterial. 



It gains oxygen and loses carbonic acid. 



Its coagulability is increased. Temperature is diminished. 



Asphyxia. If the supply of oxygen to the lungs be diminished and 



