THE BLOOD 101 



The following values are given by Ipredericq for the tension of 

 oxygen in percentages of an atmosphere. His experiments were made 

 on dogs : — 



External air 2096 



Alveolar air 18 ¥ 



Arterial blood 14 t 



Tissues 



The arrow shows the direction in which the gas passes. 



The methods of obtaining the gases of the blood and analysing 

 them are described in the Appendix. When the gases are being 

 pumped off fiom the blood, very little oxygen comes off until the 

 pressure is greatly reduced, and then, at a certain point, it is suddenly 

 disengaged. This shows it is not in simple solution, but is united 

 chemically to the haemoglobin as oxyhaemoglobin, which is dissociated 

 when the pressure is extremely low. 



The avidity of the tissues for oxygen is shown by Ehrlich's experi- 

 ments with methylene blue and similar pigments. Methylene blue is 

 more stable than oxyhaemoglobin ; but if it is injected into the circu- 

 lation of a living animal, and the animal killed a few minutes later, 

 the blood is found dark blue, but the organs colourless. On ex- 

 posure to oxygen the organs become blue. In other words the tissues 

 have removed the oxygen from methylene blue to form a colourless 

 reduction product; on exposure to the air this once more unites 

 with oxygen to form methylene blue. 



Carbonic Acid in the Blood. — What has been said for oxygen holds 

 good in the reverse direction for carbonic acid. Compounds are 

 formed in the tissues where the tension of the gas is high : these pass 

 into the l^Tnph, then into the blood, and in the lungs the compounds 

 undergo dissociation, carbonic acid passing into the alveolar air where 

 the tension of the gas is comparatively low, though it is greater here 

 than in the expired air. 



The relations of this gas and the compounds it forms are more 

 complex than in the case of oxygen. If blood is divided into plasma 

 and corpuscles, it wiU be found that both yield carbonic acid, but the 

 jdeld from the plasma is the greater. If we place blood in a vacuum 

 it bubbles, and gives out all .its gases ; addition of a weak acid 

 causes no further liberation of carbonic acid. If plasma or serum is 

 similarly treated the gas comes off, but from 10 to 18 per cent, of the 

 carbonic acid is fixed — that is, the addition of some stronger acid, 

 like phosphoric acid, is necessary to displace it. Fresh red cor- 

 puscles wiU, however, take the place of the phosphoric acid, and thus 



