10 - Urine Concentration and Dilution

Abstract

Body fluid tonicity is tightly controlled by the regulation of water excretion. The independent regulation of water and solute excretion is essential for the homeostatic functions of the kidney to be performed simultaneously. In the absence of changes in solute intake or metabolic production of waste solutes, the kidney is able to excrete different volumes of water upon changes in water intake. This ability to excrete the appropriate amount of water without marked perturbations in solute excretion is dependent on renal concentrating and diluting mechanisms. Renal water excretion is tightly regulated by the peptide hormone arginine vasopressin (also named antidiuretic hormone). Under normal circumstances, the circulating vasopressin level is determined by osmoreceptors in the hypothalamus that trigger increases in vasopressin secretion (by the posterior pituitary gland) when the osmolality of the blood rises above a threshold value. The kidney responds to the variable vasopressin levels by varying urine flow (i.e., water excretion). For example, during extreme antidiuresis (high vasopressin), water excretion is greater than 100-fold lower than during major water diuresis (low vasopressin). These major changes in water excretion are obtained without substantial changes in steady-state solute excretion. This phenomenon is dependent on the kidney's ability to concentrate and dilute the urine. During low circulating vasopressin levels, urine osmolality is less than that of plasma: the diluting function of the kidney. In contrast, when the circulating vasopressin level is high, urine osmolality is much higher than that of plasma: the concentrating function of the kidney.