GENERAL ZOOLOGY 



is now known as urine. The amino acids and inorganic ions (Na , K , Gl , 

 HCO3", HPO3", and S04^) also re-enter the blood by diffusion, as does 

 about 40 per cent of the urea. Urea, CO(NH2)2, is the chief nitrogenous 

 waste of mammals and results from amino acid metabolism. It is built up in 

 the liver in a series of reactions, driven by the energy of ATP, and directed 

 by at least seven different enzymes. Glucose moves back as a result of active 

 transport (p. 74); that is, energy is required to move it across the membrane 

 of the tubule. Only transitorily after a meal rich in carbohydrates, or in the 

 diseased state of diabetes (p. 120), can glucose be found in the voided urine. 

 Cells of the tubular membrane also add certain substances to the tubular 

 fluid. In addition to the waste products of metabolism, certain drugs, such as 

 penicillin, and compounds detoxified by the liver leave the body in the urine. 

 The kidneys function in the manner indicated to preserve the constancy of 

 the internal environment of the animal with respect to water and electrolyte 

 balance which is necessary for optimal body function, as well as to eliminate 

 nitrogenous waste products of metabolism. 



There are substances other than those we have just discussed which must 

 also be eliminated if the organism is to remain normal. One of these is the 

 pigment bilirubin, formed by the disintegration of hemoglobin when red 

 blood cells die and are destroyed. Bilirubin leaves the body by way of the 

 liver and is the pigment chiefly responsible for the color of the bile. Jaundice 

 results if for any reason this pigment remains in the body. The liver also 

 eliminates cholesterol, which arises, in part, from the destruction of red blood 

 cells. Gertain types of gallstones are almost pure cholesterol, which is a 

 steroid (p. 26). Various drugs, certain poisons, and metals, such as copper 

 and iron, none of which are excreta as we have defined the term, are eliminated 

 from the body in the bile secreted by the liver. These substances are dis- 

 solved in the bile and are carried to the large intestine, where they are found 

 in the feces and eliminated when defecation occurs. 



Summary 



In this chapter, we have considered the ways in which multicellular animals 

 like the vertebrates have met the metabolic requirements of the living cells 

 of which they are composed. We have described systems of organs — digestive, 

 circulatory, respiratory, and excretory — the functions of which, in the final 

 analysis, are simply to provide an internal environment in which individual 

 cells, specialized in many different ways, can maintain themselves and con- 

 tribute to the life of the whole group. It may have occurred to you that in 

 such a complex and interrelated series of events there must needs be some 

 insurance of orderliness, of checks and balances, because externally the indi- 



84 



