A Comparative Study of the Excretion of Water and Solids by Normal and Abnormal Kidneys

Abstract

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

Critical Statistical Analysis of Data on Renal Function in Grouped Subjects With Essential Hypertension

“…13 Although definite, the difference between the means in the specific gravity tests is not great. A decrease in the six hour output of urine may possibly be present, indicating an increase in renal impairment in patients with excessive diastolic pressures, although such changes in function may be due to a delay in excretion of urine.…”

The Effect of Urine Volume on Urea Excretion

“…Significance of urine volume in maintaining eliminatwin by the nephritic kidney Lashmet and Newburgh (13) and Marriott (14) have emphasized that in nephritis maintenance of a large flow of urine is necessary when ability to concentrate is decreased. Figure 2 indicates how a study of the urea clearance with different urine volumes in the individual patient can indicate the volume output that is needed to approximate maximal efficiency of urea excretion.…”

The void in using urine concentration to assess population fluid intake adequacy or hydration status

“…These relatively wide ranges are a function of both biological variation and experimental research design, as well as the desired a priori level of diagnostic confidence for decision making. Many of these thresholds overlap those of normal spontaneous urine voids or 24-h urine volume concentrations (4-7, 10) because it is ultimately the ratio of water to total solids in urine that determines concentration, regardless of whether the specimen is obtained under conditions of renal water retention or renal solute excretion (21). For this reason, concentrated urine may or may not alone reflect inadequate fluid intake (or dehydration).…”