140 PHYSIOLOGY. 



Greek delta A- A solution of NaCI of 0.95-per-cent. strength gives 

 the same A, hence the two solutions have the same osmotic 

 pressure and 0.95 per cent, of NaCl is isotonic with mammal's 

 serum. 



The A of any given solution may be expressed in terms of a 

 gram-molecular solution by dividing it by the constant 1.87, since 

 a gram-molecular solution of a nonelectrolyte is known to lower the 

 freezing-point 1.87 C. Now, blood-serum gives A 0.56 C.; then 

 its concentration in terms of a gram-molecular solution will be 

 ~~- or 0.3. Hence blood-serum has 0.3 of the osmotic pressure 

 exerted by a gram-molecular solution of a nonelectrolyte that is, 

 22.32 X 0.3 = 6.696 atmospheres. 



A molecule in solution exerts an osmotic pressure that is 

 exactly equal to the gas pressure exerted by a gas molecule moving 

 in the same space and at the same temperature. Hence the osmotic 

 pressure of a gram-molecular solution of any substance that is not 

 an electrolyte is equal to the gas pressure of 2 grams of H. when 

 compressed to the volume of 1 liter. 



A gram-molecule of H., that is, 2 grams of H. when brought 

 to the volume of 1 liter, will exert a gas pressure equal to 22.32 

 liters compressed to 1 liter, that is, a pressure of 22.32 atmospheres. 

 The pressure of an atmosphere is 760 millimeters of mercury. 



Now, solutions of any substance can be made to possess the 

 same osmotic pressure as any solution of another substance simply 

 by changing the concentration, either increasing it, if the molecule 

 of the substance is of large size, or decreasing it if it is of small 

 size. Solutions which have the same osmotic pressure as blood- 

 serum are isotonic. A solution which has a higher osmotic pres- 

 sure is hypertonic, and that with a lower osmotic pressure hypotonic. 



The osmotic pressure of urine has the highest isotonic coeffi- 

 cient of any fluid in the body, and its A is equal to 1.85 C. 



The most important electrolytes present in blood-serum are the 

 inorganic salts XaCl and Xa 2 C0 3 . 



The freezing-point of defibrinated blood is the same as that of 

 serum; m other words, the presence of blood-corpuscles has no 

 effect upon the freezing-point. This ensues because proteids have 

 an exceedingly low osmotic pressure, although a high molecular 

 weight. The freezing-point of blood does not change during 

 haemorrhage. 



The osmotic pressure of the lymph is somewhat greater than 

 that of the blood. An excess of carbon dioxide in the blood ele- 

 vates the osmotic pressure, 



