THE LUNGS AND RESPIRATION. 225 



external respiration that the presence of such a membrane 

 does not in any way hinder gaseous interchange, and so we 

 may again imagine the plasma in the capillaries in immedi- 

 ate contact with the lymph bathing the tissues. As there 

 is a three-pound pressure in the plasma, and no pressure in 

 the lymph, the oxygen will stream from the plasma into the 

 lymph, just as in opening a bottle of mineral- water where 

 the pressure of the gas in the bottle is much greater than 

 the pressure of that gas on the outside of the bottle, the 

 gas will stream out into the air surrounding the bottle. 

 This streaming of oxygen out of the plasma will continue , 

 until finally the oxygen pressure of the plasma sinks to a 

 half pound. 



It will be noticed that up to this point the red corpuscles 

 have taken no part at all in the process of the gaseous 

 interchange, but as soon as the oxygen pressure in the 

 plasma sinks to or below a half pound the haemoglobin is 

 no longer able to remain united with the oxygen, but dis- 

 associates. The oxygen so liberated from the oxy haemo- 

 globin flows into the plasma with which it is surrounded, 

 and from the plasma in turn the oxygen streams into the 

 lymph. This will continue until finally all of oxyhaemo- 

 globin has been disunited, and until almost all the oxygen 

 from the plasma has streamed into the lymph. The oxygen 

 does not accumulate in the lymph, for in the tissues this 

 gas is used up almost as fast as it is brought, and so the 

 oxygen pressure in the lymph, in spite of the amount of 

 gas carried to it, remains practically nothing. The fact that 

 the oxygen does not accumulate but is used up as fast as it 

 is brought explains why it is impossible, even for a very 

 short interval of time, to be deprived of air. It takes but 

 a minute or more of a loss of air to induce the fatal effects 

 of suffocation. About the instant when all the oxygen has 

 been taken out of the blood, this /is pushed into the veins 

 and sent back to the heart for a fresh supply. 



Here, also, by way of summary, the condition of things 

 in the venous blood as it leaves the tissues is re-stated. 



