GLANDS AND THE WORK OF EXCRETION 205 



Work of the Kidneys. Why should the blood pass 

 through two systems of capillaries in the kidneys? This 

 is because the separation of waste is done in part by the 

 Malpighian capsules and in part by the uriniferous tubules. 

 Water and salts are removed chiefly at the capsules, while 

 the remaining solid constituents of the urine pass through 

 the secreting cells that line the tubules. It was formerly 

 believed that the kidneys obtained their secretion by a 

 process of filtration from the blood, but this belief has 

 been gradually modified. The prevailing view now is that 

 the processes of .filtration and secretion are both carried on 

 by the kidneys, that the capillary clusters in the Mal- 

 pighian bodies serve as delicate filters for the separation of 

 water and salts, while the secreting cells of the tubules 

 separate substances by the process of secretion. 



On account of the large volume of blood passing through the kidneys 

 this liquid is still a bright red color as it flows into the renal veins (Fig. 

 90). The kidney cells require oxygen, but the amount which they 

 remove from the blood is not sufficient to affect its color noticeably. 

 The blood in the renal veins, having given up most of its impurities 

 and still retaining its oxygen, is considered the purest blood in the body. 



Urea is the most abundant solid constituent of the urine 

 and is the chief waste product arising from the oxidation 

 of nitrogenous substances in the body. Although secreted 

 by the cells lining the uriniferous tubules, it is not formed 

 in the kidneys. The secreting cells simply separate it from 

 the blood where it already exists. The muscles also have 

 been suggested as a likely source of urea, for here the pro- 

 teids are broken down in largest quantities ; but the muscles 

 produce little if any urea. Its production has been found 

 to be the work of tJie liver. In the muscular tissue, and 

 in the other tissues as well, the proteids are reduced to a 

 lower order of compounds, such as the compounds of 



