CH. XXIV.] OXYGEN IN THE BLOOD 379 



that gas in simple solution forms only a small fraction of the total 

 present. This small amount is of course doubled by doubling the 

 pressure, but such an increase is insignificant, the bulk of the 

 oxygen being in chemical union with haemoglobin. The oxygen of 

 oxyhaemoglobin can be replaced by equivalent quantities of other 

 gases like carbonic oxide. The tension or partial pressure of 

 oxygen in the air of the alveoli is less than that in the atmosphere, 

 but greater than that in venous blood ; hence oxygen passes from the 

 alveolar air into the blood-plasma ; the oxygen immediately combines 

 with the haemoglobin, and thus leaves the plasma free to absorb more 

 oxygen ; and this goes on until the haemoglobin is entirely, or almost 

 entirely, saturated with oxygen. The reverse change occurs in the 

 tissues when the partial pressure of oxygen is lower than in the 

 plasma, or in the lymph that bathes the tissue elements ; the plasma 

 parts with its oxygen to the lymph, the lymph to the tissues; the 

 oxyhsemoglobin then undergoes dissociation to supply more oxygen to 

 the plasma and lymph, and thus in turn to the tissues once more. 

 This goes on until the oxyhaemoglobin loses a great portion of its 

 store of oxygen, but even in asphyxia it does not lose all. 



The following values are given by Fredericq for the tension of 

 oxygen in percentages of an atmosphere. His experiments were made 

 on dogs. 



External air 20'96 m 



Alveolar air 18 



Arterial blood 14 



Tissues 



The arrow shows the direction in which the gas passes. 



When the gases are being pumped off from the blood, very little 

 oxygen comes off till the pressure has been greatly reduced, and then, 

 at a certain point, it is disengaged at a burst. This shows that it 

 is not in simple solution but is united chemically to some constituent 

 of the blood, which is suddenly dissociated at the reduced pressure. 

 This constituent of the blood is haemoglobin. 



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 

 circulation of a living animal, and the animal killed a few minutes 

 later, the blood is found dark blue, but the organs (especially those 

 which like glandular organs are in a state of activity) colourless. On 

 exposure 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 



