KIDNEY AND SKIN AS EXCRETORY ORGANS. 751 



ber of factors and their relations to one another, but any change 

 which will increase the difference in pressure between the blood in 

 the renal artery and the renal vein will tend to augment the flow 

 of blood unless it is antagonized by a simultaneous constriction in 

 the small arteries of the kidney itself. On the contrary, any vas- 

 cular dilatation of the vessels in the kidney will tend to increase 

 the blood-flow through it unless there is at the same time such a 

 general fall of blood-pressure as is sufficient to lower the pressure 

 in the renal artery and reduce the driving force of the blood to an 

 extent that more than counteracts the favorable influence of dimin- 

 ished resistance in its small arteries. 



The Composition of Urine. The urine of man is a yellowish 

 liquid that varies greatly in depth of color. It has an average 

 specific gravity of 1.020 and usually an acid reaction. This acid 

 reaction is attributed generally to the presence of acid phosphates, 

 particularly acid sodium phosphate (NaH 2 P0 4 ); but, according to 

 Folin,* the acidity is due partially and indeed in larger part to or- 

 ganic acids. When tested by the usual indicators (litmus) human 

 urine may show an alkaline reaction, and, in fact, observations 

 indicate that the reaction may vary in accordance with the 

 character of the food. Among carnivora the urine is uniformly 

 acid, and among herbivora it is alkaline so long as they use a veg- 

 etable diet. During starvation, however, or when living upon the 

 mothers' milk, that is, whenever they are existing upon a purely 

 animal diet the urine becomes acid. The general explanation of 

 this effect of food that has been suggested (Drechsel) is that upon 

 an animal diet more -acids are formed (from the oxidation of the 

 sulphur and phosphorus of the proteids) than in the case of the 

 vegetable foods in which the alkaline salts of the vegetable acids 

 give rise on oxidation in the body to alkaline carbonates. The 

 kidney separates from the alkaline (neutral) blood and lymph the 

 excess of salts and thus maintains a normal balance between the 

 acid and basic equivalents in the blood. 



The composition of the urine is very complex. In addition to 

 the water and inorganic salts the following elements are important, 

 namely, urea, the purin bodies (uric acid, xanthin, hypoxanthin), 

 creatinin, hippuric acid, oxalic acid (calcium oxalate), several 

 conjugated sulphates and conjugated glycuronates, several aromatic 

 oxyacids and nitrogenous acids, fatty acids, dissolved gases ( N and 

 C0 2 ), and the urinary pigments urochrome and urobilin. This list 

 is not complete; a number of additional substances have been de- 

 scribed as occurring constantly or occasionally in traces within the 

 limits of health. Under pathological conditions the composition 

 may be still further modified. The complexity of the composition 

 * " American Journal of Physiology, " 9, 265, 1903. 



