title: The osmotic pressure of body fluids in health and in diseased conditions
Creators: name: Harrison, I.M.
Publisher: name: University of Glasgow
Year: 1966
Subjects: subject: Cellular biology, physiology.

description: The maintenance of the normal composition and osmotic pressure of the extracellular fluid is essential for the existence of living cells. The kidney plays a major role in maintaining the homeostasis of the extra-cellular fluid by varying the output and the composition of the urine. The osmotic pressure of the extracellular fluids and hence that of the cells they bathe is maintained in the region of 290 mOsm. per Kg. water. The human kidney is capable of producing urine which can vary in osmolality from about 50 mOsm. per Kg. water under conditions of maximal diureais to 1400 mOsm. per Kg. water under conditions of antidiuresis. During antidiuresis, one of the major functions of the kidney is the production of a concentrated urine. Urinary osmotic pressure reflects this concentrating ability of the kidney, and hence should bear some relationship to the functional state of the kidney. The aim of the present study was to define more clearly the limits within which urine osmolality could be used as an index of renal concentrating ability, and to establish a test of renal function based on urine osmolality which could be applied to subjects with chronic renal disease. Urine osmolality following 14 - 15 hours dehydration was found to be a poor index of renal concentrating ability. In normal subjects under standard conditions, a very wide range of normal values was obtained. Many of the subjects could achieve a higher urine osmolality throughout the day, without long periods of dehydration. The most significant finding in normal subjects was a linear relationship between solute excretion rate and urine flow rate in the range 14 - 75 ml. per 60 min. This range of flow rates could be attained under normal hydration or mild dehydration. At higher urine flow rates, this relationship ceased abruptly, and there was a random scatter of results. From the equation relating urine flow rate and solute excretion in the range 14 - 75 ml. per 60 min., the theoretical osmolality corresponding to a given urine flow rate was derived. The ratio observed: theoretical osmolality expressed as a percentage was taken as an index of renal concentrating ability. This measure of concentrating ability was applied to a group of normals, a group of control patients, and a series of patients suffering from chronic renal diseas. Simultaneous urea clearance tests were carried out on all subjects to assess the reliability of this test as an index of renal function. In normal subjects and control patients, both tests gave a comparable index of renal function. In the early stages of chronic renal disease there was good agreement between the tests. In the more advanced oases of chronic renal disease, there was a marked discrepancy between the two assessments, and urea clearance values were consistently lower than concentrating ability. Changes in urine flow rate and solute excretion rate were also studied in normal subjects during a water diuresis, and were compared with a diuresis induced by urea or Fruaemide. Findings in subjects with polyuria due to renal disease were compared with normal subjects. The possible application of the concentrating ability teat to other clinical conditions was considered.