138 INTRODUCTION TO GENERAL PHYSIOLOGY 



vessels, " glomcruli," suspended in cavities which are themselves the 

 beginnings of long-looped and twisted tubes, lined with cells, along 

 which the filtrate passes in its way to the duct, called " ureter," 

 which conducts it to the bladder. As the filtrate from the glomcruli 

 passes along the " tubules," it is subjected to certain operations which 

 convert it into urine, the name given to the liquid which leaves the 

 kidney. It is to be understood that the glomeruli and tubules are 

 in very large number, so that a great area of surface is provided for 

 filtration and the subsequent operations (E., p. 223). 



Fig. 8 represents, diagrammatically, what takes place in the 

 glomeruli. 



In order that the filtration referred to may take place at a 

 perceptible rate, the liquid to be filtered, the blood, must be sub- 

 jected to pressure. The heart provides this pressure in the arteries. 

 Now it is found that if the arterial pressure is lower than about 40 



FIG. 8. Diagram of the Filtration in the Glomeruli. 



The clear space represents water. 

 The dots, the crystalloids. 

 The small circles, the colloids. 



mm. Hg. no filtration takes place at all. Why is this ? We saw 

 (E., p. 172) that the colloids, almost entirely proteins, of the serum 

 possess an osmotic pressure of about 40 mm. of mercury. This 

 means that they attract water through a membrane impermeable to 

 them, unless the entrance of the water is opposed by a pressure of 

 40 mm. Hg. If the pressure is greater than this, then water, con- 

 taining in solution any substances to which the membrane is 

 permeable, is filtered through at a rate proportional to the height 

 of the filtration pressure above 40 mm. Hg. 



The glomerular filters are able, then, to get rid of waste pro- 

 ducts without any other mechanism ; but, as remarked above, with 

 the loss of large quantities of water, a matter of great importance 

 to land animals, together with solutes which are of value, especially 

 sodium chloride, glucose, and amino-acids. In marine animals the 

 loss of water and of sodium chloride would not matter ; but even 

 here that of food materials is more serious. Accordingly, we find 



