354 PRINCIPLES OF GENERAL PHYSIOLOGY 



We have found sufficient evidence to show that the first stage is a mere 

 filtration from the blood. It is clear that this, if copious enough, would be able 

 to get rid as far as necessary, of all the non-colloidal metabolic products. But, 

 since these are present in very small concentration in the blood, a large amount 

 of water must be filtered with them. 



In the case of water animals, this would not be a serious matter. In the fish, Lophiu* 

 pixcatorius, Denis (1913) found that the urine in the bladder had a specific gravity of 1 ()!; 

 and contained only 0*083 per cent, of total nitrogen, but 1'08 per cent, of chlorine. Contrast 

 these figures with those of one of the higher land vertebrates. In man, the percentage of 

 nitrogen is about 1, and that of chlorine about 0'6 per cent. It is obvious that the sea fish has 

 no need to be careful as regards water and chlorides. 



In land animals, where water is often difficult to obtain, its loss would be 

 serious to the organism. Salts are also of importance, so that we find that 

 arrangements have been evolved to diminish losses of both kinds. According 

 to the theory put forward by Ludwig (1844), water is absorbed by the cells 

 lining the tubules, as the dilute glomerular filtrate passes over them. The 

 evidence for this must be examined. Further, we have to remember that 

 mere concentration by removal of water will not account for the fact that the 

 concentration of urea goes up very much more than that of sodium chloride, to 

 take the two chief constituents. To do this, cither sodium chloride must be 

 absorbed, or urea must be excreted in the tubules It will be noticed that 

 these two compounds represent two distinct classes of substances which air 

 present in the glomerular filtrate, which is an indiscriminate mixture of all the 

 diffusible substances in the blood. Urea represents the various metabolic products 

 which require removal as far as possible; sodium chloride represents valuable 

 food-stuffs, inclusive of glucose, which should not be lost more than is avoidable 

 and are present in the glomerular filtrate because they cannot help being there, 

 if one may use the phrase. Sodium chloride itself is probably chiefly of 

 importance for the maintenance of the correct osmotic pressure of the blood. 

 This could be done by other salts, but, as we have seen above, those of sodium 

 are the least toxic. Where they are not to be obtained from earth or sea, 

 there is great desire for them, especially in animals taking a diet in which 

 vegetable matter preponderates, since the food does not contain sufficient. The 

 tubules of an ideal kidney, therefore, would absorb, together with water, the 

 useful substances of the filtrate, leaving the metabolites untouched, or even 

 adding to them by active secretion. 



In contradistinction to the theory of Ludwig, Bowman (1842), while also 

 regarding the glomerular function as that of filtration, believed that the cells of 

 the tubules secreted the specific contents, such as urea, uric acid, etc., while 

 the filtrate itself, which would be much less copious than that required by 

 Ludwig's theory, contained only water and salts. On p. 75, Bowman speaks of 

 the " escape of water " from the blood, and, from the description given, it 

 seems evident that he regarded the process as a filtration effected by the blood 

 pressure. In any case, the total process must involve work on the part of the 

 cells, since the osmotic pressure of the urine is higher than that of the blood. 



Absorption of Water. If we calculate the amount of glomerular filtrate which 

 must be concentrated in order to give the daily output of urea, as is done by 

 Starling (1912, p. 1288), we find that 28 litres of water must be reabsorbed by 

 the tubules from 30 litres of filtrate. While it is not impossible for so large a 

 quantity of fluid to be filtered by the glomeruli, it seems a wasteful process. On 

 the other hand, if we confine our attention to the sodium chloride, and assume 

 that the excess of urea is secreted by the tubules, only 6 litres of filtrate would 

 be necessary, since 1 litre of blood contains about 2 - 7 g. of sodium chloride and 

 15 g. are excreted daily. These 6 litres would only need concentrating down 

 to 1*5 litres. It seems to be forgotten by some opponents of the reabsorption 

 of water, as is pointed out by Cushny, that the cells of the tubules are not 

 comparable to those of a secreting gland which elaborate new substances, and 

 that their function consists in the separation of urea, etc., from the blood. 

 Since, therefore, the urea is only present in a very small definite amount in 



