WORK OF KIDNEY' 193 



3. 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 pelvds. 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 (Fig. 46). 



The kidney does not manufacture any of the constituents of 

 its secretion except hippuric acid and probably a small quantity of 

 urea, 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 every substance in the blood. That is, when- 

 ever any substance capable of elimination by the kidney oversteps 

 its threshold value, it is, in general, excreted till the excess has been 

 removed. In this way, the kidney acts as a regulator of the water 

 and salt content of the system. Abnormal constituents of the 

 blood, except those entangled in the reticulo-endothelial cells and 

 excreted via bile and faeces, pass in their entirety into the urine. 

 Not only is there an alteration in the relative concentrations of the 

 various substances eliminated, but there is in general an increase 

 in the concentration of solutes. This process of concentration 

 necessitates the expenditure of energy. 



It is very difficult to get reliable experimental results from the 

 kidney. Its nature, blood supply and position do not lend- them- 

 selves to surgical interference, and the student ought to be keenly 

 critical of results which are produced under uncontrolled abnormal 

 conditions. Some facts, however, are obtainable and may be 

 detailed here shortly. 



1. Function. No one doubts that the kidney as a cell-com- 

 munity has specialised in excretion. Every cell in the body has 

 the power of eliminating waste products. Most of these substances 

 find their way into the blood and most of those that are non-volatile 

 are voided by the kidney cells. 



2. Structure. The functioning parts seem to be structurally, 

 two, (a) the capsules, and (h) the tubules. Each capsule is lined 

 by flattened pavement cells supported by a delicate basement 

 membrane, A tubule passes from each capsule, joins with other 



B. Vi 



