720 PHYSIOLOGY 



(1) In one set of experiments the blood from a dog that had been starved 

 for five days was led through the organs of a well-fed dog. In these experi- 

 ments Schondorff found that, without exception, the urea in the blood was 

 largely increased at the end of the experiment. 



(2) In a second series of experiments the blood of a fasting animal was led 

 through the hind limbs and liver of a fasting animal. In these the amount of 

 urea in the blood was unaltered. 



(3) In a third set blood of a well-fed animal was led through organs and liver of 

 a fasting animal. In these cases the amount of urea was always diminished. 



It was concluded from these experiments that the extent of protein metabolism 

 depends on the nutritive condition of the cells and not on the protein contained 

 in the circulating fluids. It must be remarked, however, that the absolute quan- 

 tities of urea obtained in these experiments were minute in comparison with 

 those which would be formed by the same weight of tissues in the whole normal 

 animal. Thus in one of Schondorff 's experiments, the dog, which was extremely 

 well fed, was turning out nitrogen at the rate of 38 grni. a day, i.e. 1-5 grm. 

 an hour. In the same animal the blood perfused for four and a half hours 

 through the hind limbs and liver picked up only 25 mg. of nitrogen in the 

 form of urea. During this time the whole animal would have formed and 

 excreted 6 grm. of nitrogen as urea, i.e. 200 times as much as that actually 

 obtained. Folin remarks that 25 mg. of nitrogen is not sufficient foundation 

 for so weighty a superstructure as the theories which have been based on this 

 experiment. 



There can be little doubt, however, that Voit was substantially 

 correct in assigning a twofold fate to the ingested protein, and that, 

 as Speck has pointed out, we can consider protein metabolism under 

 two headings, viz. tissue or nutritional metabolism and energy 

 metabolism. Since the proteins form the main constituent of living 

 protoplasm, the death and destruction of cells, which are constantly 

 going on in the body, must result occasionally in the production of 

 substances which are not available for resynthesis, and are therefore 

 turned out of the body with the urine.* A certain proportion of 

 the proteins of the food must therefore be applied to replacing this 

 waste of tissue. The proportion will be larger in cases where a 

 growth of the nitrogenous tissues is occurring, as in the young animal 

 or during convalescence from a wasting disorder. On the other 

 hand, a certain proportion of the nitrogen in the urine will be derived 

 from the breakdown of tissues, and this in its turn will be increased 

 under any conditions which bring about an augmented tissue 

 disintegration, such as the toxsem'a of fevers, poisoning by arsenic 

 or phosphorus, or partial asphyxia by deprivation of oxygen, as after 

 inhalation of carbon monoxide gas. In this function protein cannot 



* Tigerstedt suggests that the irreducible minimal protein consumption 

 may be due, not to a special metabolic cycle of the protein on its way into and 

 out of the living cell, but to the fact that the circulating fluids of the body 

 contain large amounts of protein as essential and constant ingredients, and 

 the living cells therefore, which are bathed by these fluids, cannot refrain, 

 even in times of protein scarcity, from feeding to a certain extent on these 

 the predominant constituents of their nutrient medium. 



