156 THE ARMY FOR HOME DEFENCE 



opsonin either stimulated the leucocyte or sensitised the bacterium, 

 making it more chemio-attractive to the destroyer (Fig. 23). 



2. The kidneys are the great eliminating organs of the body. 

 Each of them is built up of a number of long unbranched tubes 

 closed at one end and, at the other, opening, along with several 

 other similar tubules, into a common collecting tubule. This 

 in turn opens into the pelvis of the kidney. The production of 

 urine goes on in these unbranched tubules, the collecting tubule 

 serving apparently only as a conduit to the pelvis. The closed 

 end of each tubule is invaginated within itself to form a Bowman's 

 capsule, where its epithelium lies in close contact with the capillary 

 tuft of blood vessels the whole end- structure being called a 

 Malpighian corpuscle (see over, Fig. 28). 



The kidney does not manufacture any of the constituents of 

 its secretion except hippuric acid, but merely eliminates unchanged 

 certain of the bodies brought to it by the blood. 



It is not a mere filter, as the concentration of the constituents 

 of the urine are vastly different from the concentrations of the 

 same substances in the blood. There seems to be a threshold 

 value for each and everv substance in the blood which when 



V 



exceeded is at once reduced to its normal value by the elimination 

 of the excess by the kidney. It keeps the salt content of the blood 

 steady ; therefore, abnormal constituents will be eliminated in 

 their entirety. Not only is there an alteration in the relative 

 concentration of the various substances eliminated, but there is 

 in general a concentration of solutes. This concentration of 

 solutes necessitates the expenditure of energy. 



The total energy used may be estimated, as in other glands, 

 by the consumption of oxygen. When the kidney is at rest, that 

 is, when little or no urine is being formed, very little oxygen is 

 consumed. During activity the amount of oxygen used increases 

 greatly. The following table taken from Barcroft and Brodie 

 shows this : 



TABLE XXIV. 



KESTINU KIDNEY. ACTIVE KIDNEY. 



Urine. Oxygen. Urino. Oxygen. 



c.c. 0-57 c.c. 0-85 c.c. 2-95 o.e 



0-07 0-64 0-34 1-14 



1-66 1-53 5-58 



0-05 0-04 2-1 0-09 



0-04 0-03 2-8 0-09 



Average 0-03 0-4 1-52 1-97 



Work per minute = 320 gram centimetres = 1650 gram centimetres. 



