RESPIRATION 541 



dioxide was about 42 mm., the amount varying from about 4 

 to 5'5 per cent. 



At sea-level the partial pressure or tension in the alveoli 

 may be taken as about — 



O., = 100 iTim. Hg. 

 CO.' =42 mm. Hg. 



From the great inequality in the expansion of the lungs in 

 different parts of the thorax and the resulting differences in the 

 ventilation of the air vesicles, the samples of alveolar air taken 

 by Haldane's method will tend to give too low a carbon dioxide 

 and too high an oxygen figure, since the sample is chiefly derived 

 from the better ventilated part of the lung. 



This means that in the less expanding parts of the lung the 

 blood is subjected to a higher partial pressure of COo, and a 

 lower partial pressure of 0„. 



2. The Partial Pressure or Tension of the Gases in the Blood- — 

 The tension of 0^ and 00^ in the blood has been already 

 considered (p. 492). Whether a gas is simply dissolved, or 

 whether it be held in loose chemical combination, its amount 

 will depend upon the teTn-perature of the fluid and upon the 

 pressure of the gas over the fluid. If the temperature is 

 raised, the fluid will hold less of the gas. 



If the pressure of a gas over a fluid is increased, some will 

 be taken up by the fluid ; if it be decreased, the gas will tend 

 to come off from the fluid, as occurs when a bottle of soda 

 water is opened. 



Thus, for every temperature, there is a certain pressure of 

 the gas in the atmosphere at which the solution or chemical 

 combination exposed to it will neither give off nor take up 

 more of the gas, and this gives the measure of the tension of 

 the gas in the fluid. 



Theoretically, the determination of the tension of a gas in 

 a fluid is simple (p. 492). But when it has to be carried out in 

 circulating blood it becomes extremely ditficult. It must be 

 carried out without marked disturbance of the circulation, 

 and a thorough exposure of the air to the blood must be 

 secured. The trouble of clotting has also to be faced. 



The best results have been obtained by the aerotonometer 

 of Krogh, in which a bubble of air is exposed to the blood. 



