378 RESPIRATION. [CH. xxiv. 



amount of oxygen is dissolved whatever be the pressure. We 

 have thus a proof that oxygen is not merely dissolved in the 

 blood, but is in chemical union; and the fact that the oxygen of 

 oxyhsemoglobin can be replaced by equivalent quantities of other 

 gases, like carbonic oxide, is a further proof of the same state- 

 ment. 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 hsemo- 

 globin, 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 oxyhsemoglobin 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 ...... 



Alveolar air 



Arterial blood 



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 

 experiments with methylene blue and similar pigments. Methy- 

 lene blue is more stable than oxyheemoglobin ; 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^he 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. 



