216 URINE 



2. With the exception of hippuric acid, the kidney does 

 not elaborate the substances which it secretes. It merely 

 separates them from the blood and alters their concentration. 



3. The kidney being an excretory organ, its activity is 

 readily influenced by changes in the composition of the 

 blood with which it is supplied. 



To account for the manner in which the kidney performs 

 its work it is necessary to explain — 



1. Why some substances are separated from the blood 

 while others are not ; 



2. How the former came to attain a different degree of 

 concentration in the urine from that in which they exist 

 in the blood ; 



3. What part is played in the process by the glomerulus 

 and what part by the tubules ; 



4. Whether the process can be explained on physical 

 grounds or whether it is necessary to invoke the specific 

 activity of the cells ; 



5. How diuretics act. 



THE FORMATION OF URINE 



Structure of the Kidney 



Certain essential features of the renal anatomy must 

 be borne in mind. The functional unit of the kidney 

 consists of glomerulus, Bowman's capsule and the tubule. 

 The capsule is the dilated bhnd end of the tubule inva- 

 ginated to form a cup. In this cup is situated the glomeru- 

 lus or tuft of capillaries. The invaginated layer of the 

 capsule is formed of thin, flattened epithelium, which 

 embraces the glomerulus. The tubule follows a devious 

 route towards the pelvis of the kidney. In its first part, 

 the first convoluted tubule, it is, as its name imphes, much 

 twisted. Here it hes entirely in the cortex. This leads 

 into the second part, the descending limb, which pursues 

 a radial course into the medulla. Arrived there, it doubles 

 back upon itself at the loop of Henle, and returns to 



