1150 PHYSIOLOGY 



bolism, of soluble non- volatile substances in every cell of the body. These 

 substances, together with the excess of water taken in with the food above 

 that lost by respiration and cutaneous transpiration, are turned out by the 

 kidney as urine. The activity of this organ must therefore be determined in 

 the first place by chemical stimuli. It must react to the slightest deviation 

 from normal of the blood composition by excreting water or dissolved 

 substances. This delicate sensibility is displayed in two directions : 



(1) Under the influence of certain substances, such as urea, uric acid, 

 or water, the cells of the convoluted tubules take up the substance, which 

 is in excess, from the surrounding lymph and accumulate it in vacuoles, 

 which are discharged on the inner surface of the cells into the lumen of the 

 tubules. 



(2) Besides this specific secretory activity of the cells of the convoluted 

 tubules, the tubules as a whole are endowed with the power of absorbing 

 both water and dissolved substances from the fluid in their lumen. Whether 

 this absorptive power is limited to the cells of Henle's loop, as was first 

 suggested by Ludwig, or occurs coincidently with secretion in the cells of the 

 convoluted tubules, as might be imagined from the close analogy between 

 the structure of these cells and that of the intestinal epithelium, we have 

 not sufficient evidence to decide. We do know, however, that the quality 

 of the absorption is strictly regulated according to the needs of the organism, 

 so that the constituents which are precious are reabsorbed for service in the 

 body, while those which are in excess or are of no value to the organism 

 are allowed to pass out into the ureters. The process of resorption is 

 indeed, as is shown by Cushny's experiments, largely dependent on the 

 physical qualities of the substances undergoing absorption, and especially 

 on the permeability of the renal cells to these substances. The physical 

 conditions are, however, subordinated to the physiological, so that a salt so 

 diffusible as potassium iodide is left in the fluid, while sodium chloride may 

 be reabsorbed in large quantities. 



The necessity for the endowment of the tubular epithelium with a resorp- 

 tive as well as a secretory function is determined by the presence at the 

 beginning of the tubule of a mechanism the glomerulus, devoid of the 

 fine selective power or chemical sensibility possessed by the cells of the 

 convoluted tubules. The production of urine by the glomerulus is ap- 

 parently regulated entirely by the pressure and velocity of the blood through 

 its capillaries and by the colloid content of the blood-plasma. We may 

 assume that in Bowman's capsule there is under normal conditions a constant 

 production of a fluid free from protein, but having the same crystalloid 

 concentration as the blood-plasma. With any rise of general blood -pressure 

 the amount of this transudate is increased ; .with any fall it is diminished. 

 The small qualitative changes which are constantly occurring in the blood 

 as the result of the taking of food or the activity of different organs, probably 

 produce but little effect on the amount of glomerular fluid. Only indirectly, 

 as the result of these events on the general blood-pressure, or possibly in con- 

 sequence of the production of substances having a vaso- dilator effect on 



