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A previous report by us (1) presented evidence to show that normal kidneys are able to excrete concentrated urine, while diseased kidneys are not. The manner in which this information was obtained was as follows: The subjects were on a constant diet for a preparation period of 3 days. Fluid intake was restricted to 1500 cc. daily. Beginning at 6:00 P.M. of the third day, all intake of food and fluid was stopped for 18 hours. Urine was collected at intervals during this fast and the specific gravity determined. It was found that normal kidneys were able to concentrate the urine to a specific gravity of 1.026 or above. Diseased kidneys were unable to reach 1.026.In order to observe these facts in more detail, further studies (2) have been made. This time a greater load was imposed upon the kidneys, but for a shorter period of time. The procedure was as follows: Beginning at 10:00 P.M., all intake of fluid and food, except a special diet, was withheld for 38 hours. Urine was collected at intervals during this period and the specific gravity determined.In the previous study, the total available water was approximately 2800 cc. daily. In the latter one, the total available water was reduced to 700 cc. daily.The solid intake of the body was practically the same in both studies. Under these latter conditions, it was found that normal kidneys are able to concentrate the urine to a specific gravity of 1.029 or above. Diseased kidneys cannot reach 1.029. The more severe the renal damage is, the lower the concentrating ability of the kidneys was found to be.The specific gravity of any solution is an expression of a ratio between water and solids in solution. As the kidneys become unable to concentrate the urine, the ratio of grams of water per gram of solid excreted must increase. Figure 1 illustrates this ratio of water to total solids in the urine at different specific gravities. This ratio holds for each specific gravity regardless of whether it is the maximal specific gravity attainable or that of a specimen obtained under submaximal conditions.Since, in disease, there is an increase in the proportion of water per gram of solid excreted, it is important to determine what effect this high ratio has on the total 24 hour excretion of water and solids. 1003

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