248 RESPIRATORY FUNCTION OF THE BLOOD 



Effect of solutes. 



Plasma contains crystalloids and colloids. The crystalloids are 

 all more or less hydrated. They combine loosely with a certain 

 amount of water and, therefore, reduce the effective absorption 

 volume. Similarly, the emulsoid colloids present reduce the 

 effective concentration of water by imbibition. They, however, 

 are able to adsorb a quantity of gas (see p. 60). 



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.). The tension 

 of oxygen in the tissues cannot be less than zero, and, therefore, 

 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 gastro- 

 cnemius muscle weighing 20 grams uses about 0-24 c.c. of oxygen 

 per minute and, therefore, would 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 wieght of its own circulating fluid. For example, the 

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

 at least 140 kgs. 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 min. just under a 

 twentieth of the amount necessary when plasma alone was 

 considered. 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 XXXVII. 



Water 



Plasma 



Blood 



In addition to the amount dissolved one has to consider the 

 amount held by the haemoglobin. The table given below contains 

 the results of a series of experiments on the blood of a horse, 



