150 Discussion 



DISCUSSION 



Black : There seems to be some conflict between Dr. Talbot, who says 

 that large intakes should produce retention, and Prof. Wallace, who tells 

 us that large intakes produce large arithmetical errors. In this matter I 

 am on Dr. Talbot's side, and that is not entirely the emotional reaction 

 of someone who has done a certain amount of balance experiments. I 

 think we have some supporting evidence in that if balance experiments 

 are done on an adult person who has just had an operation and is on a 

 milk intake (in which the errors of measurement should be much the 

 same as those of excreta), there is quite a definite correlation between 

 intake and retention (Davies, H. E. F., Jepson, R. P., and Black, 

 D. A. K. (1956). Clin. Sei., 15, 61). 



Bull : What was the nature of the load imposed in the experiments on 

 the tolerance of loading? 



Talbot : The rate of intake of the substances in question is increased in 

 a stepwise manner which allows time for compensatory homeostatic ad- 

 justment in rate of output to take place. At each step, measurements are 

 made to find out whether the body content and /or concentration of the 

 substance is being kept within physiological limits by appropriate adjust- 

 ments of the rate of output. As rate of input is increased, it eventually 

 reaches a point where the body is unable to keep its content and concen- 

 tration values within normal limits by suitable adjustment of rate of out- 

 put. This point is considered to be the upper limit of physiological 

 tolerance or physiological ceiling for the substance in question. Rates of 

 input in excess of this ceiling level produce a tendency to abnormal 

 retention. For example, in the case of potassium, when the rate of input 

 exceeds the physiological ceiling value, body potassium content increases 

 above normals levels and hyperkalaemia develops, together with signs of 

 potassium intoxication. 



McCance : I would like a firm definition of what you mean by tolerance 

 and capacity to eliminate. De Wardener did some experiments in which 

 he took large amounts of water every day for 7 or 14 days and although 

 he did not succumb and appeared to tolerate them perfectly well, there 

 were finite changes in his responses, sensitivities, etc. (de Wardener, 

 H. E., and Herscheimer, A. (1957). J. Physiol, 139, 42 and 53). 



Talbot : In the case of water, the body normally can tolerate up to 

 approximately 15 litres per square metre or about 25 litres per adult per 

 day. These large quantities are eliminated simply by increasing the ratio 

 of water to solutes in urine to levels of 20 to 30 ml. per m-osm. It is diffi- 

 cult to exceed this ceiling value in the normal individual. On the other 

 hand, it is easy to exceed the water tolerance ceiling value in pan-ne- 

 phritics and postoperative patients who are unable to increase the water/ 

 solute ratio of their urine above a few ml. per m-osm. and whose rate of 

 solute output may be low. Such individuals may be unable to take more 

 than 2 or 3 litres of water per square metre per 24 hours without retaining 

 water and developing water intoxication. 



Kennedy : Some of these substances were orally administered, and some 



