374 - Multicellular Animals, Especially Man 



lar walls allow considerable quantities of 

 water, glucose, urea, and all other of the 

 crystalloidal components of the plasma to 

 escape from the blood into the capsule. How- 

 ever, all the more complex components of 

 the blood, particularly the plasma proteins 

 and other colloids (and, of course, the cor- 

 puscles), cannot pass through the glomerular 

 walls, and do not appear in the filtrate. These 

 colloids remain in the blood as it leaves the 

 glomerulus via the efferent vessel. Each sam- 

 ple of blood passes rapidly through a glomer- 

 ulus; and while it is passing, it loses only a 

 fraction (5 to 10 percent) of its volume. This 

 fraction becomes the filtrate, which is side- 

 tracked into the nephric tubule; but simul- 

 taneously the main flow of blood continues 

 through the glomerulus and efferent vessel, 

 and finally through the capillaries surround- 

 ing the other parts of the nephron (see Fig. 

 20-5). 



Determining the composition of the neph- 

 ric filtrate is a very difficult task. The kidney 

 of a frog, for example, can be exposed; and 

 a special microscope is then used to examine 

 the glomeruli while the nephric filtrate pours 

 into the capsules of the nephrons. A very 

 delicate glass syringe is required to withdraw 

 samples of the filtrate from the capstdes; and 

 new microchemical methods were devised to 

 analyze the extremely small samples of filtrate 

 obtained. In fact, the collection and analysis 

 of nephric filtrate by A. N. Richards and 

 co-workers at the University of Pennsylvania 

 represented one of the most important 

 achievements in physiology of the decade 

 1921 to 1930. 



Analysis of the capsular fluid (Table 20-1) 

 revealed essentially that the nephric filtrate 

 is a colloid-free filtrate of the blood plasma. 

 The filtrate contains all the plasma crystal- 

 loids, including water, glucose, amino acids, 

 and salts, as well as purely waste substances, 

 such as urea and uric acid. Moreover the 

 concentration of these crystalloids is tlie same 

 in the filtrate as it is in the plasma (see Table 

 20-1). But the colloids of the plasma, mainly 

 the proteins and lipoids, are held back. 



Under normal conditions no colloids are 

 present in the filtrate as it enters Bowman's 

 capsule. 



The work of forcing filtrate from the blood 

 into the capsule is done, not by the kidney, 

 but by the heart. Blood pressure in the 

 glomerulus must be maintained at a level 

 above 30 mm of mercury if any filtrate is to 

 be formed; and other factors being equal, 

 the quantity of filtrate and urine formed in 

 a given time tends to parallel the blood pres- 

 sure. 



The filtrate contains the same solutes as 

 the plasma, except that the colloids of the 

 plasma are lacking in the filtrate — and con- 

 sequently filtrate is hypotonic to the plasma. 

 Normally the glomerular blood pressure 

 amounts to about 70 mm of mercury. About 

 30 mm of this pressure is expended in over- 

 coming back-pressure— due to the hypoto- 

 nicity of the filtrate— and only 40 mm is the 

 effective pressure, which drives more filtrate 

 into the nephric capsule. If the blood pres- 

 sure in the glomeruli rises or falls, the effec- 

 tive filtration pressure follows, and the quan- 

 tity of filtrate varies accordingly. 



Reabsorption. Normal urine (Table 20-1) 

 contains virtually no glucose or amino acids, 

 because the convoluted tubules extract these 

 useful compounds from the filtrate and re- 

 turn them to the blood that flows through 

 surrounding capillaries. 



TABLE 20-1 -The Nephric Filtrate As 

 Compared to the Plasma and Urine 



