RES FIR A TION 627 



in the blood-plasma, and how oxyhaemoglobin gives up oxygen to places where 

 the tension of the gas is lower than that where the oxygen was taken up by 

 the haemoglobin. It remains to consider the mechanism by which haemoglobin, 

 after being robbed of the greater part of its oxygen by the tissues, replenishes 

 its supply from the external air. Incidentally, the carbon dioxide which 

 has been given off to the blood by the cells escapes to the atmosphere at 

 the same time. Here we may say that, owing to its greater solubility, and 

 to the fact of its being taken up as bicarbonate in the blood-plasma, any 

 special provision for its carriage is not so necessary as for oxygen. 



It is generally known that, in air-breathing animals, there are arrangements 

 by which a large surface of blood is brought into contact with air, which is 

 itself repeatedly changed. A thin membrane is all that intervenes, so that 

 the distance through which the gases have to diffuse is extremely short. The 

 organs in which this interchange takes place are known as lungs. 



I quoted above the experiment of Hooke, in which he showed that a 

 renewed supply of air is necessary to preserve an animal from death by asphyxia 

 It does not belong to the subject matter of this book to describe the details 

 of the muscular mechanisms by which the air is sucked in and expelled from 

 the lungs. Suffice it to say that their capacity is periodically increased and 

 diminished by the action of muscles on the walls of the cavity in which they 

 are contained. 



It will be obvious that the whole of the air cannot be expelled in expira- 

 tion unless the lungs are squeezed flat, a mechanical impossibility in the 

 construction of an animal. The air in the final terminations of the branching 

 air tubes, the alveolar air sacs, must possess, therefore, a tension in oxygen 

 lower than that of the atmosphere, and one of carbon dioxide higher than 

 that of the atmosphere. It is with this air that the gases in the blood enter 

 into exchange. The problem before us is, then, how do the oxygen and carbon 

 dioxide tensions of the arterial blood leaving the lungs compare with those 

 of the alveolar air? 



Since a gas always diffuses from a place of higher tension to one of lower 

 tension, it is clear that if the pulmonary exchange is regulated by the laws of 

 diffusion alone, the oxygen tension of the arterial blood can never exceed that 

 of the alveolar air, and that its carbon dioxide tension can never fall below it. 



In the first place, it is important to grasp the meaning of the tension of a gas 

 in a fluid, as opposed to its actual concentration. In a mixture of gases at 

 atmospheric pressure the matter is simple, the tension of any one is proportional 

 directly to its relative concentration. Thus, oxygen makes up 21 per cent, 

 of the air, and therefore its tension at the ordinary atmospheric pressure is 

 21 per cent, of 760 mm., that is, 159-6 mm. of mercury; and, in fact, it is 

 21 per cent, of any pressure under which air may be placed. Now suppose that 

 we have a volume of gas at 760 mm. pressure containing 10 per cent, of carbon 

 dioxide, its tension is 76 mm. Place the gas next in contact with a layer 

 of water, and allow equilibrium to be attained, keeping the tension of the carbon 

 dioxide in the gas phase constant by adding more as required. The water 

 dissolves a certain quantity of the carbon dioxide, and, at its contact surface with 

 the gas phase, a certain number of molecules of carbon dioxide are continually 

 entering the water and a certain number leaving it, so that equilibrium means that 

 the same number enter and leave in the same time. It follows that the tension 

 of the carbon dioxide is the same in both phases, although if we determine 

 its total concentration in the water and in the gas, we shall not find it to be the 

 same. Further, let us add some alkali to the water, still keeping the carbon 

 dioxide tension in the gas phase constant; as is well known, carbon dioxide 

 combines with alkali to form carbonate and bicarbonate, so that the liquid phase 

 will contain much more carbon dioxide than the gas phase per unit volume, but 

 again the tension at the surface, and therefore throughout the liquid, must 

 be identical with that in the gas mixture when equilibrium is reached. We 

 may deal with oxygen in the same way, supposing haemoglobin to be present 

 in the liquid instead of the bicarbonate. 



