Respiration. 89 



learned, this is combined in the red cells, but the fact of 

 its yielding large quantities of CO., points to the blood 

 plasma as the chief means by which this substance is 

 carried. 



It has been determined experimentally that blood plasma 

 will absorb more CO., than the same quantity of water, 

 and it is evident, therefore, that there is something in the 

 plastna which assists in carrying the C0 2 ; this something 

 has been variously stated, but it is generally believed that 

 the sodium carbonate of the blood unites with a portion 

 of the carbonic acid, though other substances may assist. 

 Between the amount absorbed in the plasma, and that 

 held in chemical combination by certain salts of the plasma, 

 the total amount is carried along in the venous blood 

 stream, the partial pressure of the C0 2 in the fluid being 

 high; on arriving at the lungs it circulates through the 

 capillary network spread over the walls of the alveoli, 

 the same wet membrane existing between it and the 

 external air as was described in speaking of the oxygen. 



The partial pressure of the CO._, in the air of the air- 

 sacs being much lower than that of the blood, diffusion 

 occurs between the blood and the air, the COo passing out 

 until equilibrium is established. The air now in the alveoli 

 of the lungs having lost some of its oxygen, and consider- 

 ably gained in its carbonic acid — in other words, having 

 the partial pressure of its gases altered— diffusion between 

 the air in the ultimate air-cells and bronchial tubes rapidly 

 occurs until the balance is restored, and the air in the 

 alveoli rendered fit for further blood-reviving processes. 



Before closing this part of the subject it is necessary to 

 allude to the manner in which the combiued oxygen is 

 liberated in the tissues, and the combined CO., liberated 

 in the lungs, so that the law of diffusion may act, this 

 being, as we have now learned, the chief process by which 

 the balance of gases in these regions is restored. 



Certain gases have a tendency to leave the substances 

 with which they are united when the pressure upon them 

 becomes reduced, this process is termed 'dissociation'; it 



