BLOOD 317 



pressure it can be seen that the arterial blood contains more 

 oxygen and less carbon dioxide than the venous blood. 



Table XIX. — Composition of Gases in Blood 



100 volumes 



Arterial. Venous. 



Oxygen 20 12 



Nitrogen 1 to 2 1 to 2 



Carbon dioxide 40 45 



The blood going into the lungs through the capillaries is 

 charged with carbon dioxide but it does not contain its full 

 quota of oxygen. The inspired air with which this blood 

 comes in contact contains on the other hand an excess of 

 oxygen and little or no carbon dioxide. Consequently, 

 oxygen passes through the capillary walls, dissolves in the 

 blood plasma, and then combines with the haemoglobin to 

 form oxyhemoglobin. Carbon dioxide meanwhile has been 

 carried in the blood in the form of sodium bicarbonate and 

 dissolved to a slight extent in the blood plasma. The dis- 

 solved carbon dioxide passes through the capillary walls 

 into the lung cells and with the reduction in the amount 

 of dissolved carbon dioxide, sodium bicarbonate breaks up 

 into sodium carbonate and carbon dioxide, the latter pass- 

 ing into the lung cells, as above described. 



The blood, now charged with oxygen and containing less 

 carbon dioxide, passes to the tissues where oxygen is needed. 

 The oxyhemoglobin now breaks up, oxygen dissolving in the 

 plasma and passing through the capillaries. It oxidizes 

 dextrose with the elimination of carbon dioxide, which passes 

 through the capillaries, first dissolving in the plasma and 

 then combining with the sodium carbonate to form sodium 

 bicarbonate. 



The above gaseous exchange in the lungs and in the 

 tissues is caused by a difference in the pressure of oxygen 

 and carbon dioxide, and is the result of mass action as can 

 be easily seen from the following reversible equations: 



Haemoglobin + O2 ^ Oxyhemoglobin 

 2NaHC0 3 ^ Na 2 C0 3 + C0 2 + H2O. 



