THE CHEMISTRY OF RESPIRATION 1185 



haemoglobin is only 29-5 per cent. In consequence of this fact, in the 

 tissues, where the carbon dioxide tension is high, the oxyhsemoglobin 

 will be dissociated with greater ease, so that oxygen will be set free 

 where it is most wanted. 



We are now in a position to understand how the oxygen is taken 

 up by the blood as it circulates round the pulmonary alveoli. Arterial 

 blood, such as that which fills the pulmonary veins and the systemic 

 arteries, is very nearly (i.e. about 90 per cent.) saturated with oxygen, 

 and will only take up about 2 volumes per cent, more on shaking 

 it with air at the body temperature. Venous blood requires 8 to 

 10 volumes per cent, of oxygen to saturate it ; but we have 

 already mentioned that, at a tension of 30 mm. oxygen, the 

 blood becomes nearly saturated. The tension of oxygen in the 

 alveoli is considerably above this. In the trachea the tension of 

 oxygen is about ^ of an atmosphere (since the air here contains 16 

 volumes per cent.), and the tension in the alveoli will be only a little 

 lower than this. If we take the oxygen tension in the alveoli at ^ of 

 an atmosphere,* it will still be something over 100 mm. [Hence the 

 venous blood brought to the alveoli by the pulmonary artery will, on 

 there coming into intimate contact with the atmosphere, take up 

 oxygen from it to saturation, or to a point not far removed from it. 



The blood, thus laden with oxygen, travels to the left side of the 

 heart, and from there is sent through the arteries to all parts of the 

 body. It must be remembered that neither in the lungs nor in 

 the tissues does the haemoglobin come in actual contact with the source 

 of the oxygen, nor with the cells which it is to supply. In both cases the 

 interchange is effected through the intermediation of the plasma and, 

 in the tissues, of the lymph as well. Since the tissue-elements are con- 

 stantly using up oxygen, they absorb any oxygen that is present in the 

 surrounding lymph. There is, in consequence, a descending scale of 

 oxygen tensions from red blood-corpuscle through plasma, vessel wall, 

 lymph, and tissue-element. The cell draws from the lymph, and the 

 lymph from the plasma, so that the oxygen tension in the plasma sinks. 

 This has the same effect as if we put the red corpuscles in a mercurial 

 pump and lowered the pressure of gas. The immediate result is an 

 evolution of oxygen, which is taken up by the plasma, to be in turn 

 passed on to the lymph and the tissue-cell. 



Under normal circumstances a blood-corpuscle never stays long 

 enough in the proximity of the tissues to lose its whole store of oxygen. 

 If, however, the further supply of oxygen to the blood be prevented, 

 as in asphyxia, the last traces of oxygen disappear from the blood. 

 The enormous avidity of the tissues for oxygen is shown by the 



* The oxygen tension in the alveoli has been reckoned at about 12-6 per cent. 



to 13-5 per cent, of an atmosphere. 



75 



