UKTSE 107 



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 filtration, yet it is no purely mechanical 

 process, but the cells exercise a selective influence, and prevent the 

 albuminous constituents of the blood from escaping. During the 

 passage of the water which leaves the blood at the glomerulus 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. 



The term excretion is better than secretion as applied to the 

 kidney, for the constituents of the urine are not actually formed in 

 the kidney itself (as, for instance, the bile is formed in the hver), but 

 they are formed elsewhere ; the kidney is simply the place where 

 they are picked out from the blood and eliminated from the body. 



GENERAL CHARACTERS OF URINE 



Quantity. — A man of average weight and height passes fi-om 1,400 

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

 (1^ oz.) of solids. The urine should be collected in a taU glass vessel 

 capable of holding 3,000 c.c, which should have a smooth-edged neck 

 accm-ately covered by a groimd-glass plate to exclude dust and avoid 

 evaporation. The vessel, moreover, should be graduated so that the 

 amount may be easily read off. 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 stercobihn leaves the body with the 

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

 urobilin. UrobiUn is very like the artificial reduction product of 

 bihrubin called hydrobihrubin (see p. 73). 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 



