74 Discussion 



rates of secretion. Certainly this is not tlie case in the parotid gland 

 (Fig. 4, p. 68). Conversely, I do not venture to claim that the cells in the 

 duct are impermeable to an extent that would completely prevent urea 

 from diffusing back into the blood stream along the concentration 

 gradient created by water reabsorption. But the amount of urea diffu- 

 sing back through the relatively impermeable duct epithelium is limited 

 by the short span of time during which the secretion remains in the duct. 

 Urea may equilibrate rapidly over some cellular membranes, more 

 slowly over others. This difference is not important when one measures 

 total body water as the volume of distribution of urea, because one waits 

 until complete equilibrium has been established before the measurement 

 is made. But the difference is important in the rate-dependent process of 

 secretion, where the time available for diffusion becomes limiting. 



Karvonen: In prolonged sweating the potassium concentration is 

 higher to start with and then gradually decreases. There is no similar 

 change in sodium or chloride and that would agree quite well with the 

 reabsorption and consequent storing of potassium in the tubule, whereas 

 sodium and chloride are not stored (Ahlman et al. (1953) Acta endocr., 

 Copenhagen, 12, 140). 



Thaysen : Yes, the first sample of sweat obtained after stimulation may 

 have a higher potassium concentration than the following ones. One 

 reason for this may be that the first sample is contaminated with cellular 

 debris, sebum and sweat residues on the skin surface. 



Karvonen: It is not just the rinsing factor, because we paid quite a lot 

 of attention to rinsing the skin and we still get this difference ; the potas- 

 sium is probably stored in the gland or at least in the tubule. 



Thaysen : In that case it cannot be contamination. Your finding is very 

 interesting to me, because we found exactly the same thing with the 

 parotid secretion. The first sample of saliva obtained after stimulation 

 invariably had a higher potassium concentration (and a higher urea con- 

 centration) than the following ones. This phenomenon occurred inde- 

 pendently of the rate at which the first sample was produced. We 

 speculated that the vigorous flow of saliva, caused by stimulation, 

 " pushed out" first a small amount of secretion, which had been produced 

 at the low secretory rates prior to stimulation, and which consequently 

 had a high concentration of potassium and urea and a relatively low 

 sodium concentration (1954. Amer. J. Physiol., 178, 155; 1955, Scand. J. 

 clin. Lab. Invest., 7, 231). Burgen (1956) also observed a high potassium 

 concentration in the first samples of saliva obtained after stimulation. 



Wallace: I have kept quiet here because a baby usually does not 

 sweat until the age of 3-4 months, nor does he shed tears — he only 

 learns to do that later. 



Karvonen : In Finland babies have hot sauna baths quite young and 

 Dr. Eila Kassila of the Children's Clinic, Helsinki, has made an investi- 

 gation on the composition of the sweat they produce during the saunas. 

 I do not think that much difference was found between baby and adult. 



Wallace : It is of interest that there is a very specific disease in children, 

 cystic fibrosis of the pancreas, in which the ability of the sweat glands to 

 reduce the sodium concentration in sweat seems to be lost. It is of 



