138 



PRINCIPLES OF ANIMAL BIOLOGY 



under average conditions. Urine consists of about 96 per cent water, 2 

 per cent urea, 0.5 per cent uric acid, and 1.5 per cent inorganic salts. 



A small amount of waste material may be added to the forming urine 

 in the tubules, by excretory action of the cells of the tubules; but this 

 addition is unimportant in relation to the amount filtering in at the 

 renal corpuscle. 



The Skin as Excretory Organ. — Excretion in the skin is done by the 

 sweat glands, of which there are about two millions in man. These 

 glands are of the simple tubular type (page 84), the deeper portion of 

 the tube being closely coiled, while the outer part forms a duct which 

 empties on the surface. Around the coiled bottom is a network of 



capillaries (Fig. 114). The amount of sweat excreted 

 varies greatly with the temperature and the amount 

 of muscular exertion; in mild weather and with 

 moderate or slight exercise, about 600 cc. may be 

 produced in a day, but five times that amount is not 

 uncommon in hot weather and with great exertion. 



Sweat is much more dilute than urine, about 99 

 per cent of it being water. Of its solids, sodium 

 chloride is the most important. Urea is not very 

 abundant; at the minimum production of sweat (600 

 cc. per day) only about 1.5 per cent of the total 

 urea is lost through the skin in man. Other soluble 

 wastes, of the same kinds as are eliminated by the 

 kidneys, are found in the sweat, but in much smaller 

 amounts. Since the sweat evaporates as rapidly as 

 it is formed under ordinary conditions, these solids 

 dry on the surface of the skin. As is pointed out on 

 page 121, in connection with heat regulation, many 

 mammals have only a few sweat glands, or none at 

 all. In them the kidneys bear the whole burden of 

 The sweat glands even in man are not an important 

 Their chief service is regulation of temperature. 



The liver shares in the excretion of urea. 



Fig. 114.— Hu- 

 man skin, dissected 

 to show sweat gland. 

 At left, complete 

 gland, much coiled 

 at bottom. At 

 lower right, network 

 of capillaries from 

 the midst of which 

 the coiled portion of 

 another gland has 

 been removed. 



urea elimination, 

 excretory device. 



Other Means of Excretion, 

 since it helps convert protein wastes into urea. When proteins are 

 broken down, ammonium salts are among the products. These salts 

 are converted into urea partly in the liver, but the actual excretion is 

 elsewhere. The liver performs, however., a primary act of excretion in 

 the removal of the hemoglobin of worn-out red l)lo<)d corpuscles. The 

 bile pigments are produced from this hemoglobin and are eliminated with 

 the bile into the intestine, where they eventually pass out with the feces. 

 Cholesterol is another waste substance excreted by the liver and elimi- 

 nated into the intestine with the bile. ' 



