202 SURFACES AND MEMBRANES 



temperature reading is identified on the scale and constitutes 0° C. 

 If there is a tendency to supercooling, a tiny crystal of ice may be 

 added through the side tube D to start the crystallization of the water. 

 If supercooling has taken place, the mercury in the thermometer will rise 

 rapidly on crystallization to the true freezing point, where it will remain 

 stationary for a time. 



The water is then removed from the freezing tube, and the tube is 

 dried and reassembled. About 25 cc of the solution whose freezing point 

 is to be determined is then introduced through the arm D and the freezing 

 point is determined in the same way as for water. 



TABLE V-10 



Freezing Point of Mammalian Blood 



A indicates the average freezing-point depression in degrees Centigrade. 

 O.P. the osmotic pressure calculated for body temperature, 37° C. 

 The unit of O.P. is the atmosphere. 



A = 0.1° C corresponds with an increase in 1.204 atmospheres at 0° C. 



A = 0.59 is equivalent to 8.0 atmospheres at 37° C. 



A point to be emphasized is that at no time does mammalian blood exert such a pressure; all that 

 is meant is that this is the osmotic pressure that would be produced by blood if it were separated from 

 pure water by a semi-permeable membrane. 



The difference between these two freezing points as read on the 

 thermometer is the desired depression A of the freezing point, from which 

 the osmotic pressure can be calculated. 



Some freezing-point-depression data obtained by cryoscopic methods 

 from mammalian blood are shown in Table V-10. The conclusion 

 drawn from these data is that the blood of all these mammals develops 

 about the same osmotic pressure, and that under normal circumstances 

 the osmotic pressure of blood never departs from this mean value. It has 

 been observed that shortly after meals the osmotic pressure of the blood 

 is slightly higher; it is reduced slightly by large ingestion of water. 

 The kidney as a mechanism exists for regulating the osmotic pressure of 

 the blood so that the tissues may remain immersed in a fluid of constant 

 osmotic pressure. This constancy of the osmotic pressure is attained 

 through the excretion of the necessary amounts of water and solids. 



Osmotic Pressure of Proteins 



Proteins are complex organic compounds of high molecular weight. 

 The serum albumins, for example, have a molecular weight of 68,000, 



