PRINCIPLES OF PHYSIOLOGY 95 



as the liquid flows through the tubules and past the additional capil- 

 laries there. The kidney must expend energy to move certain of the 

 substances excreted or reabsorbed against a diffusion gradient. The ex- 

 cretory process would be very wasteful and inefficient if the urine leaving 

 the body had the same composition as the fluid in the Bowman's capsule. 

 However, as the urine passes down the kidney tubule, water, sugar, salts 

 and many other substances are reabsorbed, whereas the waste products 

 such as ammonia and urea are not. It is by this selective reabsorption 

 of certain substances, and by the addition of others to the urine, that 

 the kidney tubules regulate the composition of the blood and body 

 fluids. In the higher animals such as man, the lungs, skin and digestive 

 tract also remove certain wastes from the body. 



The most important waste products excreted by animals are the 

 nitrogenous ones which result from the deamination of amino acids and 

 from the breakdown of nucleic acids. The ammonia formed by de- 

 amination is toxic, but quite soluble and readily diffusible. If plenty 

 of water is available, as it is for fresh-water animals, the ammonia 

 diffuses out as such, either directly through the body surface, or through 

 gills and excretory organs if these are present. Animals living on land 

 cannot afford to excrete the amount of water which would be required 

 to eliminate ammonia. In land animals, ammonia is converted meta- 

 bolically to some other substance to be excreted. In mammals, the 

 nitrogenous wastes from amino acid metabolism are excreted largely as 

 urea, which is a soluble, small molecule that diffuses readily and is less 

 toxic than ammonia. Urea requires a moderate amount of water for its 

 excretion. Reptiles and birds convert their nitrogenous wastes largely 

 to uric acid for excretion. This substance is only slightly soluble so that 

 once it has been formed and excreted into the kidney tubules, water 

 may be reabsorbed and the uric acid is excreted as a paste or dry 

 powder. Insects, which are largely terrestrial animals, also excrete uric 

 acid. The uric acid is excreted into the malpighian tubules whence it 

 leaves the body via the digestive tract as a dry paste. Some animals 

 simply accumulate precipitated uric acid in some organ of the body— 

 the "fat body" of the insect is an example. Nitrogenous wastes in this 

 form are removed from the body fluids as effectively as those actually 

 excreted from the body in urine. 



30. Protection 



The complex physicochemical system we know as protoplasm requires 

 protection against the many adverse effects of the surrounding environ- 

 ment. The ameba is an exception to the general rule that animals have 

 some protective device to cover the protoplasm. The ameba's proto- 

 plasm is separated from the surrounding environment only by the 

 plasma membrane. Many other protozoa have a tough, flexible, non- 

 cellular pellicle surrounding the cell and some secrete hard, durable, 

 calcareous or siliceous shells. All the multicellular animals have some 

 protective covering or skin over the body. The skin may consist of one 

 or of many layers of cells, and may be reinforced by scales, hair, feathers, 



