kl M'IRATION 109 



aitrnal liloiul. It a large clot of blood be cut into and 

 examined, it is found that the clot is brighter red near the 

 surface than it is deeper in. When the clot was first formed 

 it was bright red throughout, but, on standing, the deeper 

 parts of the clot become dark, because certain substances, 

 occurring in minute quantity in the blood, are slowly oxidised by 

 the oxygen which they take from the oxyhaemoglobin, and this 

 consequently turns to the dark- coloured haemoglobin ; sub- 

 stances which thus take oxygen from another substance are 

 i ailed reducing substances. Near the surface of the clot the 

 oxyhaemoglobin, when it gives up oxygen to the reducing sub- 

 stances can get a fresh supply from the air, and so remains the 

 bright-coloured oxyhaemoglobin. 



Diffusion. If, instead 'of shaking up the defibrinated 

 blood with carbonic acid gas, it be put in a bladder and hung 

 up in the gas, the same change will take place ; and if it is 

 hung up in air in which the quantity of oxygen and of carbonic- 

 acid can be varied, the colour can be changed from one to 

 the other as often as wished. The gases pass through the 

 bladder membrane by what is called diffusion, just as they 

 would if a bladder containing oxygen were hung up in carbonic 

 arid gas. If this were done you would find that, after a short 

 time, the gases inside and outside the bladder would be the 

 same, each containing carbonic acid gas and oxygen in the 

 same proportion. The gases diffuse through the membrane 

 until the pressure of each gas inside and outside the bladder 

 is the same. 



The Change of Arterial Blood to Venous Blood. 

 The change of arterial blood to venous blood takes place as 

 it passes through the capillaries of the tissues. 



The tissues are constantly using up oxygen for their life, 

 so that there is no, or only a little, free or loosely-combined 

 nxygcn in them. The blood in the capillaries, on the other 

 hand, contains much oxygen held loosely by the haemoglobin. 

 Some of this is given off and passes through the capillary 

 walls into the lymph, and thence is taken up by the tissues. 

 Some of the oxyhaemoglobin is thus reduced to haemoglobin 

 again. The longer the blood stays in the capillaries the more 

 of its oxygen it gives up, but it rarely happens that it stays 

 long enough to give up all its loosely-combined oxygen. 



