RESPIRATION. 149 



As there is always more oxygen consumed than carbonic acid 

 exhaled, and as oxygen unites with carbon to form an equal volume 

 of carbonic acid, it is evident that a certain quantity of oxygen 

 disappears within the body. In all probability it unites with the 

 sulphur hydrogen of the food to form water. 



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

 expired air is estimated at 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 necessitates 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 

 consequence of its alkalinity, where it exists in a state of simple solu- 

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

 soda and potassa, forming the bicarbonates. 



Nitrogen is simply held in solution in the plasma. 



Exchange of Gases in the Air-cells. From the difference in ten- 

 sion of the oxygen in the air-cells (27.44 mm - of Hg) and of the 

 oxygen in the venous blood (22 mm. Hg), and from 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 is due solely to pressure. The absorption of oxygen, 

 however, does not follow absolutely the law of pressure ; that chemic 

 processes are involved is shown by the union of oxygen with the 

 hemoglobin of the blood corpuscles. The exhalation of CO 2 is also 

 partly a chemic process, as it has been shown that the quantity 

 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 be excreted only by the addi- 

 tion 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 of venous blood to the bright red of arterial blood. 



