396 MANUAL OF PHYSIOLOGY. 



The amount of liquid passing out at the kidneys is in direct 

 proportion to the blood-pressure, whereas the excretion of the 

 specific constituents of urine is independent of the pressure, but is 

 related to the amount existing in the blood, and the condition of 

 the epithelium. This is shown by the increased elimination of 

 urea when that substance is artificially introduced into the circu- 

 lation, even after the flow of the fluid has been checked by sec- 

 tion of the spinal cord. 



Another view has been put forward, which, with some modifi- 

 cation, appears plausible, or at least worthy of mention. Paying 

 attention to the fact that where vascular filtration i.e., the pas- 

 sage of liquid under pressure through the capillary wall occurs 

 elsewhere in the body it is not only water and salts, but plasma 

 that passes out of the vessels into the interstices of the tissues, we 

 may then assume that the fluid part of the blood, as such, and 

 not merely its watery part, escapes at the glomerulus. That is to 

 say, the solid ingredients of the urine in a diluted form, plus 

 serum-albumin, pass into the tubules. But on its way down the 

 long and circuitous route through the tubules the albumin with 

 much water is reabsorbed by the capillaries of the convoluted 

 tubes. The first step in this case is a mechanical filtration ; the 

 second is a vital process of reabsorption of a solution of serum- 

 albumin carried on by the gland cells in the tubules, aided by the 

 low pressure in the peri-tubular capillary plexus. This view 

 seems supported by pathological experience, which teaches that 

 the removal of the epithelium of the tubes (the glomeruli remain- 

 ing perfect), is followed by the appearance of albumin in the 

 uriue, and cysts formed by the destruction of the epithelium and 

 occlusion of the tubules commonly contain a fluid somewhat like 

 plasma. 



Doubtless much remains to be found out as to the exact method 

 of secretion of the urine, and possibly future research may show 

 us that all the views here enumerated have some truth in them. 

 That a filtration, not mere osmosis, takes place, is made certain 

 by the special vascular mechanism of the glomerulus. Why 

 simply water and salts without albumin should pass through 

 the capillaries of the glomerulus, and not through any other 



