202 ADVANCED PHYSIOLOGY 



the bubbles of gas can be seen coming out of such water when 

 the pressure is released. 



The amount of oxygen which the blood absorbs in this way 

 is very small; by far the larger part is not simply absorbed 

 but chemically combined with the red coloring matter, or 

 haemoglobin. 



Haemoglobin. We have already seen that the blood con- 

 tains red corpuscles, which are like little sponges holding in 

 solution a material called haemoglobin; page 124. This sub- 

 stance has an affinity for oxygen, and provided the gas is 

 under slight pressure, will absorb it in large amounts when- 

 ever in contact with it. After haemoglobin has absorbed 

 oxygen, it is a bright crimson; but if then put in a place 

 where there is n6 oxygen, or where the oxygen pressure is 

 very slight, it will release the oxygen it has absorbed and its 

 color will change to a bluish red. Hence arterial blood, which 

 has just taken on oxygen in the lungs, is bright crimson, while 

 venous blood, which contains less oxygen, is 'bluish red ii 

 color. 



Oxygen can be absorbed by haemoglobin because of the iroi 

 element in it; by analysis it is found that a little over 0.3% 

 iron ; one molecule of oxygen will combine chemically with evei 

 atom of iron. The oxygen capacity of the blood is thus lim- 

 ited; but as each corpuscle hurries through the lung capillaries, 

 it seizes what it can absorb and hastens away to some part of 

 the body where the oxygen is needed. There is about nine 

 teen per cent of oxygen in arterial blood, most of it being ii 

 the red corpuscles. 



How the Blood Gives up its Oxygen. Let us follow the blood 

 and see what becomes of its oxygen. After going back to th( 

 heart, from the lungs, and flowing out through the arteries, i1 

 finally comes to the capillaries in the muscles, glands, etc., 

 somewhere in the body; for example, in the fingers; Fig. 91. 

 Although it spends only about one second in the capillaries it 

 loses in that short time 35% of its oxygen. This is becai 



