RESPIRATORY EXCHANGE. 205 



takes the oxygen from the capillary cells. The plasma loses the 

 oxygen thus obtained because the haemoglobin is very greedy for 

 oxygen. There is accordingly a difference in the oxygen pressure 

 in the plasma of the capillaries of the lungs, sufficient to account 

 for the absorption of oxygen by the haemoglobin of the blood. The 

 blood leaving the lungs is delivered into the left ventricle, from 

 which it is distributed over the body. Since oxidation takes 

 place within the tissue cells, oxygen is being continually called 

 for, and the lymph surrounding the cells must continually gain a 

 fresh supply of oxygen from the plasma of the blood. This re- 

 duces the tension of oxygen in the plasma and causes an evolution 

 of oxygen from the oxyhagmoglobin, which is taken up by the 

 plasma to be passed on to the lymph and then on to the cell. 

 There is thus a descending scale of pressure or tension of oxygen 

 from the air of the lungs, where its pressure may amount to 100 

 mm. of mercury, until it reaches the tissue elements, where the 

 pressure may be considered zero. Under ordinary conditions the 

 circulation is fast enough to prohibit the complete reduction of 

 the oxyhaemoglobin. In case it is not, or in case the oxygen sup- 

 ply is short, the phenomena of asphyxia develop (see p. 195). 



EFFECT OF CARBON DIOXIDE ON OXYHAEMOGLOBIN. As a result 

 of the oxidative changes which take place within the cells, carbon 

 dioxide is produced, and the tension of this gas rises in the tis- 

 sues. It will be remembered in the discussion of the dissociation 

 of oxyha3moglobin, that the effect of increased tensions of carbon 

 dioxide is to increase the rate of reduction of oxyhaemoglobin into 

 oxygen and haemoglobin. Since there is a high tension of car- 

 bon dioxide present in the tissues and at the site of the capil- 

 laries, the effect on the reduction of oxyhaemoglobin is very 

 marked, and has a great influence on the rate at which oxygen is 

 supplied to the tissues. Just as there is a descending pressure 

 of oxygen from the air in the lungs to the cell, so is there a de- 

 crease in pressure from the carbon dioxide in the cells to the air 

 of the lungs. This gas therefore passes through the lymph to the 

 plasma and out of the plasma through the pulmonary epithelium 

 by the simple process of diffusion. 



THE EXCHANGE OF CARBON DIOXIDE. Analysis of venous blood 



