584 LECTURE XXV. 



the beginning of the uriniferous tubes. According to this, the urine 

 would show a very similar composition to that of the plasma of the blood. 

 This, however, is not the case. Ludwig furthermore assumed that there 

 was a re-absorption in the uriniferous tubes, both of water and of dis- 

 solved substances. Very soon, however, important objections were raised 

 against this theory of a filtration of all of the constituents of urine. 

 Above all, it was pointed out that while it was comprehensible that the 

 proteins in the blood could not pass through the vascular endothelium, on 

 the other hand it would be expected that certain other substances, for 

 example sugar, which is always present in the plasma, would filter through 

 into the urine. This is, however, not the case under normal conditions, 

 and the same is true of certain other substances. To account for this it 

 has been suggested that perhaps the sugar may not circulate as such in 

 the blood, but that it is combined with some other compound which 

 will not filter through the medium. There is no support for any such 

 assumption. The quantitative composition of the urine also speaks 

 against any such simple filtration process taking place in the formation 

 of urine. The plasma contains about 0.05 per cent of urea. We would 

 then expect the urine to contain about the same amount of this sub- 

 stance, whereas in fact about 2 per cent is usually present. If we are to 

 retain the idea of a pure filtration, then we must necessarily assume 

 that the urine is concentrated in the uriniferous tubes to about ^ of its 

 original volume. It has, furthermore, been found that if the supply of 

 blood to the kidneys is entirely stopped, that the secretion of urine 

 ceases; but, on the other hand, when the blood is allowed to flow again 

 through the kidneys, it is about 45 minutes before the secretion of 

 urine begins. If we believe that the endothelium of the glomerulus 

 and that of Bowman's capsule is a mere filtering membrane, it is hard 

 to account for the long cessation of its function. 



Especially quite recently, more and more facts have become known, 

 which indicate that the kidneys act analogously to other glands during 

 the formation of their secretion. For one thing, it has been observed 

 that an increased secretion causes a rise of temperature, and that when 

 there is more work done by the kidneys, there is more oxygen consumed 

 and more carbonic acid produced. 1 Our present knowledge teaches us 

 that we cannot be far wrong in assuming that practically the whole 

 function of the kidneys is analogous to that of the other glands and that 

 no simple filtration 2 can take place; both the epithelium of the glome- 

 rulus, as well as that of the uriniferous tube, probably produce an actual 

 secretion. We know of various substances which increase the secretion 



1 J. Bancroft and T. G. Brodie: J. Physiol. 32, 18 (1904). 



2 Cf. Torald Sollmann: Ann. J. Physiol. 13, 241 (1905). W. Filehne and J. Biber- 

 feld: Pfliiger's Arch. Ill, 1 (1906). 



