14 PHYSICOCHI^MICAL BASIS OF PHYSIOLOGICAL PROCESSES 



substances a permeability which varies, not at all with the physical 

 diffusibility of the substance, but with its value from a physiological 

 standpoint. Thus, sodium sulphate and sodium chloride diffuse through 

 ordinary membranes with about equal facility, and yet if a solution con- 

 taining these two salts is placed in the intestine, the chloride will be 

 absorbed into the blood much more quickly than the sulphate. Sodium 

 sulphate in watery solution diffuses through a membrane fifteen times 

 more quickly than cane sugar, but from the intestinal lumen, cane 

 sugar is absorbed ten times more quickly than sodium sulphate. If. 

 however, the vitality of the epithelium is destroyed, as by first of all 

 bathing it with a solution of sodium fluoride, then the sulphate and 

 chloride will be absorbed at an equal rate. 



Although diffusion and osmosis can not therefore play any significant 

 role in the normal process of absorption from the intestine, we must 

 not entirely discount them; under certain circumstances, these physical 

 forces may assert their influence as, for example, when concentrated 

 saline solutions are present. Such solutions will attract water from the 

 blood, and, other things being equal, more will be attracted the less 

 permeable the epithelium happens to be towards the saline employed. 

 Sulphates and phosphates will attract more water than chlorides or 

 acetates. This property of the saline solutions to attract water coun- 

 teracts the natural tendency for the water to be absorbed, and the 

 large volume of fluid stimulates peristalsis. 



2. Do the physical processes of filtration, 'diffusion and osmosis suf- 

 fice to account for the production of urine ~by the kidneys? Under normal 

 conditions the molecular concentration of the urine, as determined by 

 the depression of freezing point, is considerably greater than that of 

 the blood. This indicates that excretion must have occurred contrary 

 to the laws of osmosis; in other words, that the renal cells must have 

 compelled dissolved molecules to be transferred from the blood to the 

 urine, although the difference in osmotic pressure would cause them to 

 pass in the opposite direction. This force, sometimes called for want 

 of a better name "vital activity," must depend on the operation of 

 processes that are quite distinct from those of diffusion, etc.; but that 

 they are necessarily of a nonphysical nature (e. g., vital) is less probable 

 than that they depend on some physical process the nature of which our 

 present knowledge does not permit us to understand. 



By comparing the osmotic pressures of urine and blood, attempts 

 have been made to measure the work done by the kidney in the produc- 

 tion of urine. Thus, it has been found that A for normal urine (human) 

 is about 1.8, and for blood about 0.6, from which it may be calculated 

 that in the production of 1 kilogram of urine 150 kilogrammeters of 



