146 ELEMENTARY PHYSIOLOGY 



up by the blood and transported by that fluid to those organs 

 which are destined to remove them. 



When the dark venous blood reaches the capillaries which 

 surround the air-cells of the lungs, it is immediately changed in 

 colour, becoming bright scarlet. Carbonic acid gas passes through 

 the thin layer of membrane which separates the air from the 

 blood, and oxygen gas passes from the air into the blood. We 

 see, then, that the lungs do not prepare carbonic acid gas, but 

 simply serve as a means by which this gas, brought by the blood, 

 is separated from the system. 



The readiness with which the diffusion of gases may take place through 

 moist membranes may be illustrated as follows : Fill a bladder with venous 

 blood, and suspend it in ajar of oxygen. The oxygen will be absorbed, and 

 the blood will soon acquire the properties of arterial blood. Again, suspend 

 a bladder of arterial blood in a jar of carbonic acid gas, and the blood will 

 soon become venous. 



The extent to which these changes take place in the body may 

 be judged by comparing expired with inspired air. The latter 

 contains about 79 per cent, of nitrogen, 21 per cent, of oxygen, 

 with only about '04 per cent, of carbonic acid gas, and a variable 

 proportion of water vapour ; while the expired air contains about 

 5 per cent, more carbonic acid gas, 5 per cent, less oxygen, and an 

 increased proportion of water vapour, the nitrogen remaining the 

 same. 



It follows from this, that if a person were to shut himself up 

 in a room without any means of ventilation whatever, the air he 

 breathed would gradually lose its oxygen, and gain a correspond- 

 ing proportion of carbonic acid gas. When one or two per cent, 

 of the oxygen had thus been removed, a feeling of general uneasi- 

 ness would arise, accompanied by headache; and as the loss 

 increased, there would not be sufficient oxygen in the inspired air 

 to change the colour of the blood. This would certainly be the 

 case when the loss of oxygen and the gain of carbonic acid gas 

 rose to ten per cent. The blood would then be venous through- 

 out the system. Carbonic acid gas, which is in itself slightly 

 poisonous, would saturate the blood ; and this, together with the 

 oxygen starvation, would speedily cause death from asphyxia (Gr. 

 a, without; and sphuzo, I throb cessation of pulse, caused by 

 suffocation). 



It is probable that in this case death would be caused by the 

 deprivation of oxygen rather than by the poisonous effects of car- 

 bonic acid gas, for it has been proved that air containing 10 or 

 15 per cent, of this gas produces no immediate effect on the 

 animal system providing the supply of oxygen be proportionately 

 increased. 



