THE LUNGS AND RESPIRATION. 229 



soda. As this substance is very soluble in water, and con- 

 sequently soluble in blood plasma, large quantities of it 

 could be easily carried in solution. 



By way of summary what has occurred in the capillaries 

 of the tissue may be re-stated thus: First, the CO 2 has 

 resulted from an actual breaking down of the living tissue. 

 Second, from the tissue it has been absorbed by the lymph 

 which bathes the tissues. Third, from the lymph it streams 

 into the blood plasma. Here a certain quantity of it is car- 

 ried, merely absorbed or dissolved in it. Fourth, in the 

 plasma a larger quantity of the CO 2 enters into chemical 

 combination with some of the alkaline substances, possibly 

 Na 2 CO 3 , (sodium carbonate,) and forms Na H CO 3 (so- 

 dium bicarbonate) . This sodium bicarbonate is easily sol- 

 uble in the plasma, and so in solution is carried lungward 

 in the venous stream. 



3. The Elimination of Co 2 in the Lungs. The final 

 scene in this drama of the respiration is, of course, the elimi- 

 nation of this CO 2 from the blood into the lungs. When 

 the venous blood reaches the pulmonary capillaries the CO 2 

 dissolved in the plasma of the blood at once begins to pass 

 out of the blood into the air, since the pressure of the CO 2 

 in the air is practically nothing. This streaming out of 

 the CO 2 is, therefore, in regular obedience to the law of 

 Dalton. In this way all the CO 2 merely dissolved in the 

 plasma is finally eliminated. If venous blood were put 

 under the receiver of an air pump and the air above it 

 then exhausted, it would be possible to pump out of the 

 blood practically all of the CO 2 absorbed in it. This por- 

 tion of the gas then presents no difficulty. It streams out 

 into the lung for the same reason that the bubbles of gas 

 stream out of a bottle of mineral-water when that mineral- 

 water conies in contact with the air. But the difficulty 

 presents itself when we try to explain in what manner the 

 CO 2 , which is chemically combined in the sodium salt, is 

 liberated. 



