32J- RESPIRATORY FUNCTION OF THE BLOOD 



complex ; that is, only a certain amonnt of the water present 

 (the true disperse phase) is chemically free. The large bulk of the 

 fluid present is firmly bound and closely packed in orientated 

 layers to the colloid, or serves as water of hydration for the 

 crystalloidal components. The colloidally bound water is con- 

 sidered by R. A, Gortner to be capable of " dissolving " more gas 

 than free water. On the other hand, many experiments in which 

 various solutes were added to plasma tend to show that the 

 presence of free crystalloids decreases the amount of gas that it is 

 able to hold. 



Gas -holding power of plasma. 



It has been found that at the pressure of about 90 mm. Hg, 

 which we saw oxygen had in the lung, 100 c.c. of plasma will 

 dissolve 0-273 c.c. of oxygen (measured at N.T.P.). If we consider 

 that the tension of oxygen in the tissues cannot be less than zero, 

 and one has as a maximum amount 0-273 c.c. of oxygen for 

 every 100 c.c. of plasma passing through the tissue, a cat's 

 gastrocnemius muscle weighing 20 grams and using about 0-24 

 c.c. of oxygen per minute would, therefore, need to have at least 

 100 c.c. of plasma passing through it per minute. A warm- 

 blooded animal would need to have about twice as much plasma by 

 volume as the present volume of its body. The body would be 

 unable to cope with the weight of its own circulating fluid. For 

 example, the average man weighing 66 kg. would have to carry, in 

 addition, at least 140 kg. of plasma, thus increasing his total weight 

 to 206 kilos. As Barcroft puts it, " man would never have attained 

 any activity which the lobster does not possess, or had he done 

 so it would have been with a body as minute as the fly's." In 

 the experiment quoted above the actual amount of blood passing 

 through the cat's muscle was 4-5 c.c. per minute — just under a 

 twentieth of the amount necessary when plasma alone was con- 

 sidered. This is due to the specific oxygen capacity of the 

 haemoglobin in the blood. 



The following table gives the volume (in c.c. at N.T.P.) of 

 oxygen, nitrogen and carbon-dioxide which will dissolve in 100 c.c. 

 of fluid at 38° C. and 760 mm. pressure. 



TABLE XLVI 



Oxysen. Nitrogen. Carbon-dioxide. 



Water .... 2-37 1-2 55-5 



" Separated " plasma . 2-3 1-2 54-1 



" True " plasma . .2-2 1-1 51 -1 



In addition to the amount dissolved in " true " plasma one has to 

 consider the amount held by the haemoglobin. The table given 



