Final answer:
The specific gravity of urine reflects the kidney's ability to concentrate urine by measuring the solutes per unit volume. A normal value is around 1.020, while lower values indicate an issue with water reabsorption in the kidneys. Urine osmolality is also a key measurement, offering more precision in assessing the concentration and dilution capabilities of the kidneys.
Step-by-step explanation:
The ability of the kidneys to concentrate urine is best reflected by the specific gravity of urine. Specific gravity measures the quantity of solutes per unit volume of a solution, which is directly related to the concentration ability of the kidneys. A simple test to measure this involves injecting a known quantity of pituitrine, which acts like antidiuretic hormone (ADH), and then measuring the urine's specific gravity over a 24-hour period. A specific gravity of 1.020 indicates normal kidney function, whereas a lower specific gravity suggests a defect in the reabsorption of water by the kidneys and thus in concentration. Additionally, urine osmolality is a more precise measurement than specific gravity. With restricted fluid intake, urine osmolality should exceed 800 mOsm/kg, indicating the kidneys can effectively concentrate urine, while with increased fluid intake resulting in osmolality of less than 100 mOsm/kg, the kidneys can dilute urine properly.