URINE 143 



through the thin walls of these vessels into the surrounding Bow- 

 man's capsule, which forms the commencement of each renal tubule. 

 Bowman's capsule is lined by a flattened epithelium, which is reflected 

 over the capillary tuft. Though the process which occurs here is 

 generally spoken of as a nitration, yet it is no purely mechanical 

 process, bufc the cells undoubtedly exercise a selective influence, and, 

 among other things, prevent the albuminous constituents of the blood 

 from escaping. During the passage of this dilute urine through the 

 rest of the renal tubule it gains the constituents, urea, urates, &c., 

 which are poured into it by the secreting cells of the convoluted 

 tubules. 



GENERAL CHARACTERS OF URINE 



Quantity, A man of average weight and height passes from 1,400 

 to 1,600 c.c., or about 50 oz., daily. This contains about 50 grammes 

 (li oz.) of solids. The urine should be collected in a tall graduated 

 glass vessel capable of holding 3,000 c.c., which should have a 

 smooth-edged neck accurately covered by a ground-glass plate to 

 exclude dust and avoid evaporation. From the total quantity thus 

 collected in the twenty-four hours samples should be drawn off for 

 examination. 



Colour, This is some shade of yellow which varies considerably 

 in health with the concentration of the urine. It appears to be due 

 to a mixture of pigments : of these urobilin is the one of which we 

 have the most accurate knowledge. Urobilin has a reddish tint and 

 is ultimately derived from the blood pigment, and, like bile pigment, 

 is an iron-free derivative of haemoglobin. The bile pigment (and 

 possibly also the haematin of the food) is in the intestines converted 

 into stercobilin ; most of the stercobilin leaves the body with the 

 faeces ; but some is reabsorbed and is excreted with the urine as 

 urobilin. Urobilin is very like the artificial reduction product of 

 bilirubin called hydrobilirubin (see p. 92). Normal urine, however, 

 contains very little urobilin. The actual body present is a chromogen 

 or mother substance called urobilinogen, which by oxidation (such as 

 occurs when the urine stands exposed to the air) is converted into 

 the pigment proper. In certain diseased conditions the amount of 

 urobilin is considerably increased. 



The most abundant urinary pigment is a yellow one called 

 urochrome. It shows no absorption bands. It is probably an 

 oxidation product of urobilin (Eiva, A. E. Garrod). (See Lesson 

 XXVI.) 



Reaction. The reaction of normal urine is acid. This is not due 



