334 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



G. The Kidney and the Skin as Excretory Organs. 



The secretion of the kidneys is the urine. The means by which this secretion 

 is produced, it- relations to the histological structure of the kidney, and its con- 

 nections with the blood- and nerve-supply of that organ \\ ill !><• found described 

 in the section <>n Secretion. In this section will be discussed only the chemical 

 composition of urine, and especially the physiological significance of itsdiffer- 

 em constituents. Theurineof man isa yellowish liquid varying greatly in depth 

 of color. It has an average specific gravity of 1020, and an acid reaction. The 

 acid reaction is not due to a free acid, but is usually attributed to an acid salt, 

 the acid phosphate of sodium (Xall.POj). Under certain normal conditions 

 human urine may show a neutral or even a slightly alkaline reaction, especially 

 after meals. In fact, the reaction ot* the urine seems to depend directly on the 

 character of the food. Among carnivorous animals the urine is uniformly 

 acid, and among herbivorous animals it is uniformly alkaline, so long as 

 they are using a vegetable diet, hut when starving or wheu living upon the 

 mother's milk — that is, whenever they are existing upon a purely animal diet — 

 the urine becomes acid. The explanation, as given by Drechsel, is that upon 

 an animal diet more acids are produced (from the sulphur and phosphorus) 

 than the bases present can neutralize, whereas upon a vegetable diet carbonates 

 are formed from the oxidation of the organic acids of the food in quantities 

 sufficient to neutralize the mineral acids. The chemical composition of urine is 

 very complex. Among the constituents constantly present under the conditions 

 of normal life we have, in addition to water and inorganic salts, the following 

 substances: Urea; uric acid; xanthin ; creatinin ; hippuric acid; the urinary 

 pigments (urobilin); sulphocyanides in traces; acetone; oxalic acid, probably 

 as calcium oxalate ; several ethereal sulphuric acids, such as phenol and cresol 

 sulphuric acids, indoxyl sulphuric acid (indican), and skatoxvl sulphuric acid; 

 aromatic oxy-acids ; some combinations of glycuronic acid ; some representa- 

 tives of the fatty acids; and dissolved gases (X and C0 2 ). This list would be 

 very much extended if it attempted to take in all those substances occasion- 

 ally found in the urine. The complexity of the composition and the fact that 

 so many different organic compounds occur or may occur in small quantities 

 is readily understood when we consider the nature of the secretion. Through 

 the kidneys there are eliminated not only what we might call the normal end- 

 products of the metabolism of the tissues, excluding the C0 2 , but also, in 

 large part, the products of decomposition in the alimentary canal, the end- 

 products of many organic substances occurring in our foods and not usually 

 classed a- food-stufls, foreign substances introduced as drugs, etc., all of which 

 are eliminated either in the form in which they arc taken or as derivative 

 products of some kind. "We shall speak briefly of the most important of the 

 normal constituents, dwelling especially upon their origin in the body and their 

 physiological significance. For details of chemical properties and reactions, 

 reference musl be made to the Chemical section. 



Urea. — Urea, which is given the formula CH 4 N 2 0, is usually considered 



