In the present work we study the modifications in hydrosaline metabolism which occured in two groups of healthy male individuals, one consisting of 15 young men (mean age of 20 years) and the other of 30 elderly subjects (mean age of 75 years) when the thirst test was performed for 48 hours. After performing the thist test we observed that the elderly were less thirsty than the young group and that the former presented more marked alterations in the hydrosaline metabolims. Thus, the elderly presented at the end of the test higher serum and urine sodium levels, a greater serum osmolarity and higher plasmatic ADH leves both basal and at the end of the test. However, inspite of all this, the elderly scarsely reduced their diuresis and urinary osmotic values were much lower (almost half) than the young subjects. The mechanism for which these alterations occure is probably multifactorial, in which the decrease in glomerular filtrate, the increase in filtration fraction, the proportional increase in medullar flux together with some tubular resistance to ADH action could intervene.

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