ABSORPTION. 141 



In metabolism the large proteid molecules, which in solution 

 exert an exceedingly low osmotic pressure are split into smaller 

 ones. In consequence, the number of dissolved molecules in the 

 tissue fluids and in the blood is increased, which causes an increase 

 in the depression of the freezing-point of these fluids. The loss of 

 water by the body, through evaporation, has a similar effect. It is 

 the function of the kidneys to rid the body of this excessive num- 

 ber of molecules, and so keep the osmotic pressure of the blood and 

 of the other fluids constant. If the activity of the kidneys is 

 decreased, the depression of the freezing-point of the blood will 

 become greater. A beginning renal insufficiency will therefore be 

 manifested by an abnormally great depression of the freezing-point 

 of the blood. The work done by the secretory cells of the kidneys 

 in secreting the urine, the osmotic pressure of which is much higher 

 than that of the blood, can be calculated by utilizing the laws of 

 osmotic pressure. If the kidneys secrete 200 cubic centimeters of 

 urine, the energy required amounts to 37 kilo-grammeters; that is, 

 the energy required is equal to that expended in raising a weight 

 of 37 kilograms to the height of 1 meter. The freezing-point of a 

 solution of any substance in water is lower than that of the water 

 alone. The kidney-cells separate urine from the blood against a 

 pressure of a force about six times greater than the maximum force 

 of muscle. The molecular weight of a body can be determined by 

 the depression of the freezing-point. 



Another theory has been proposed to explain the low freezing- 

 point of urine. Ludwig proved that the glomerulus filters a nearly 

 pure solution of sodium chloride, and that in the urinary tubules 

 the water is in part reabsorbed. The theory of Koranyi is that in 

 the urinary tubules there is a molecular exchange in such a manner 

 that, for each molecule of urinary constituents coming from the 

 blood, there is a molecule of sodium chloride passing from the 

 tubules into the blood. 



Loeb has shown that rhythmical contractions can be produced 

 at will in striped muscles of the frog by a single salt in solution. 

 This is not produced by the salt itself, but the ions, because it 

 occurs only in solutions of electrolytes; that is, substances which 

 dissociate. Among the ions found in the blood, he thinks those of 

 sodium are the producers of rhythmical activity. Pure sodium 

 chloride he regards as a poison. If rhythmical activity begun by it 

 is to persist, these poisonous properties must be neutralized by cal- 

 cium salts. Loeb thinks calcium and potassium salts prevent 



