RESPIRATION 199 



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is a sort of food, and we should undoubtedly so regard it, if it 

 were not for the fact that we require it constantly, instead of 

 taking it at stated intervals, as is the case with our articles 

 of diet. Again, as the demand of the system for food is 

 expressed by the sensation of hunger, so the demand for air is 

 marked by a painful sensation called suffocation. 



21. Interchange of Gases in the Lungs. But the air and the 

 blood are not in contact, ^s they are separated from each other 

 by the walls of the air-cells and of the blood-vessels. How 

 then do the two gases, oxygen and carbonic acid gas, exchange 

 places? Moist animal membranes have a property which 

 enables them to transmit gases through their substance, 

 although they are impervious to liquids. This may be beau- 

 tifully shown by suspending a bladder containing dark venous 

 blood in a jar of oxygen. At the end of a few hours the 

 oxygen will have diminished, the blood will be brighter in 

 color, and carbonic acid gas will Be found in the jar. 



22. If this interchange take place outside of the body, it 

 must take place more perfectly within it, where it is favored 

 by many additional circumstances. The walls of the vessels 

 and the air-ceils offer no obstacle to this process, which is 

 known as gaseous diffusion. Both parts of this process of 

 exchange are equally important. Without oxygen life ceases ; 

 if carbonic acid gas is not thrown off, it acts like a poison, pro- 

 ducing unconsciousness, convulsions, and death. 



23. Difference between Arterial and Venous Blood. The fol- 

 lowing table presents the essential points of difference in the 

 appearance and composition of the blood, before and after its 

 passage through the lungs : 



Venous Blood. Arterial Blood. 



Color, Dark blue, Scarlet. 



Oxygen, 8 per cent. , 18 per cent. 



Carbonic Acid Gas, 15 to 20 per cent., 6 per cent., or less. 



Water, More, Less. 



The temperature of the blood varies considerably; but the 

 arterial stream is generally warmer than the venous. The 



Jl . Moist animal membranes ? How shown with the bladder ? 

 22. Gaseous diffusion ? If oxygen be not received ? If carbonic acid be retained ? 

 28. Difference in the appearance and composition of the blood ? Temperature of the 

 blood ? The blood while passing through the lungs ? The consequence f 



