THE URINARY SYSTEM. 389 



stances are given as such by the mouth. Thus the whole of the tyrosine 

 and phenylalanine taken into the body is excreted as homogentisic acid. 

 Cystinuria and alcaptonuria, when they occur, are present at birth 

 and persist through life ; they are due to " inborn errors " of metabolism, 

 probably to the lack of certain ferments, and do not lead to any 

 disturbance of health. 



SECTION II. 

 THE FORMATION OF URINE. 



Broadly speaking, the function of the kidney is to keep the com- 

 position of the blood constant by excreting into the urine either 

 abnormal constituents which enter the blood, or any excess of substances 

 normally present, such as water, urea, and sodium chloride. The 

 abolition of this function by the complete removal of the kidneys leads 

 to the retention of the urinary constituents in the blood, and the 

 animal dies in two or three days ; the kidneys are, therefore, essential 

 to life. A very important part of their function is to regulate the 

 reaction, that is, the H ion concentration of the blood and tissues. 

 The respiratory mechanism prevents any accumulation of carbonic acid 

 in the blood, while the kidneys control the H ion concentration due to 

 other acids. These organs are extraordinarily sensitive to the slightest 

 change in the reaction of the blood, and respond by excreting in the 

 urine any excess of acid or alkali which is present. Recent observations 

 in man show that in some diseases of the kidneys the normal balance 

 of acid and base in the blood is no longer maintained, and the amount 

 of lactic and other acids in the blood is increased. This increase 

 stimulates the respiratory centre, as already described (p. 268), 

 leading to severe hyperpncea, and to a fall in the tension of carbonic 

 acid in the blood. 



So far as is known, there are no secretory nerves to the kidney ; its 

 functional activity is excited solely by any change in the chemical 

 composition and amount of the blood flowing through it, and is thus 

 largely determined by metabolic changes occurring in other parts of 

 the body. 



The structure of the renal tubule is extremely complex, much more 

 so than that of most of the other glands of the body ; many views have 

 been held as to the function of its different parts, and even now the 

 problem is not completely solved. The structure of the convoluted 

 tubule and of the capsule of Bowman is so different that it seems 

 certain that their functions must also be different; and Bowman, on 

 purely histological grounds, suggested that the glomeruli filtered off 



