PART VI 

 THE EXCRETION OF URINE 



CHAPTER LX 



THE EXCRETION OF URINE 



(Partly contributed by R. G. PEARCE) 



It will be advisable to introduce the subject by a brief review of the 

 essential structural features of the kidney, in so far as they apply to 

 che excretory function of the organ. 



STRUCTURE OF THE KIDNEY 



The kidney is mainly derived from the surface of the celom, and is a 

 mesodermal structure. In this respect it differs from ordinary secreting 

 glands, which are endodermal in origin. Just as it is more or less 

 unique in its development as a gland, it is also unique in its method 

 of functioning. The physiological theories of the mechanism of urinary 

 secretion are closely related to the highly characteristic structure of the 

 kidney. For this reason a brief survey of the structure of the different 

 parts of the uriniferous tubules and the epithelial cells with which these 

 are lined, is advisable. 



The uriniferous tubule, which is the secreting unit of the kidney, 

 takes its origin in the capsule of Bowman, which may be likened to a 

 hollow sphere of very delicate epithelium, one side of which is 

 invaginated by a very much convoluted capillary mass, the glomerulus. 

 The capsule opens up by a narrow twisted neck into a tubule, which is 

 rather tortuous in the cortex (the proximal convoluted tubule), but soon 

 takes a sharp descending course in the medulla towards the pelvis of the 

 kidney, and doubles back (loop of Henle) in a straight course again to 

 the cortex, where it again makes a twisted course (the distal convoluted 

 tubule), and terminates in a collecting tubule, which, uniting with other 

 tubules, collects the urine and conducts it to the pelvis of the kidney (Fig. 

 170). The capsule is lined with very thin epithelial cells, especially over 

 the capillaries comprising the glomerulus. The proximal and distal tubules 



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