Maximal Urine Osmolarity and Concentration

There is an upper limit to how concentrated urine can become, set by the highest osmolarity the inner medulla can reach; in healthy adults this corresponds to a maximal urine osmolarity on the order of well above plasma. Because a fixed daily solute load must be excreted, this limit also fixes a minimum, obligatory volume of water that must leave the body in urine.

Definition

Maximal urine osmolarity is the highest concentration to which the kidney can raise the urine, set by the peak osmolarity of the inner-medullary interstitium with which the collecting-duct fluid equilibrates under maximal antidiuretic hormone action; together with the daily solute load it determines the obligatory urine volume.

Scope

This topic covers what determines the maximal achievable urine osmolarity, the concept of obligatory water loss that follows from it, and the opposite extreme of maximal dilution. It integrates the gradient, vasa recta, and urea topics into the overall concentrating performance of the kidney. It is reference physiology, not clinical guidance.

Core questions

• What sets the ceiling on urine osmolarity?
• How does maximal concentration translate into an obligatory minimum urine volume?
• What determines the most dilute urine the kidney can make?
• Which factors lower the maximal achievable concentration?

Key concepts

• Maximal urine osmolarity ceiling
• Equilibration of collecting-duct fluid with the inner-medullary interstitium
• Obligatory water loss
• Daily solute load and minimum urine volume
• Maximal urinary dilution
• Determinants of concentrating ability (gradient, urea, vasopressin, aquaporin-2)

Mechanisms

Under maximal antidiuretic hormone action the collecting duct becomes highly water-permeable through abundant apical aquaporin-2, and the fluid leaving the duct equilibrates osmotically with the inner-medullary interstitium; the maximal urine osmolarity therefore cannot exceed the peak interstitial osmolarity, which is itself set by countercurrent multiplication, urea recycling, and preservation by the vasa recta. Because a roughly fixed quantity of solute must be excreted each day, dividing that solute load by the maximal osmolarity yields a minimum or obligatory urine volume below which the kidney cannot go; less efficient concentration raises this obligatory volume. At the opposite extreme, when antidiuretic hormone is absent the collecting duct stays water-impermeable while the ascending limb keeps removing solute, so the urine can be made markedly hypo-osmotic, defining the kidney's diluting limit. Any factor that lowers the medullary gradient, reduces aquaporin-2 insertion, or diminishes urea availability lowers the maximal osmolarity that can be reached.

Clinical relevance

Maximal concentrating ability and the resulting obligatory water requirement explain why solute and water balance are linked, and reduced concentrating ability is a recognised feature of several physiological and disease states discussed in clinical nephrology; this entry describes the physiology and is not a basis for diagnosis or treatment.

Evidence & guidelines

The account draws on physiological reviews of urinary concentration, on reviews of renal aquaporins, and on standard physiology textbooks; numerical values for maximal osmolarity vary with method and species and are given qualitatively here.

History

Once the countercurrent mechanism explained how the medullary gradient is built, attention turned to what limits the gradient and hence the maximal urine concentration, and to the related idea of an obligatory minimum urine volume needed to clear the daily solute load. The discovery and characterisation of aquaporin-2 later gave a molecular account of the regulated water permeability that allows collecting-duct fluid to equilibrate with the medullary interstitium during maximal antidiuresis.

Key figures

• Jeff M. Sands
• Harold E. Layton
• Mark A. Knepper
• Søren Nielsen

Seminal works

• sands-2009
• nielsen-2002

Frequently asked questions

What limits how concentrated urine can become?

The maximal urine osmolarity cannot exceed the peak osmolarity of the inner-medullary interstitium that the collecting-duct fluid equilibrates with under maximal antidiuretic hormone action; that interstitial peak is set by countercurrent multiplication, urea recycling, and vasa recta exchange.

What is obligatory water loss?

Because a roughly fixed daily solute load must be excreted and urine can be concentrated only up to a maximal osmolarity, there is a minimum urine volume that must be lost to carry that solute; this minimum is the obligatory water loss.