Urine concentration and water balance

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Predict urine volume and concentration from the medullary gradient, segment permeability, and hormone-dependent collecting-duct water transport.

About 18 minutes, including the ADH response test and water-balance checks.

Concentrating urine requires a gradient and a water-permeable route

Water leaves tubular fluid only when two requirements are met: there must be an osmotic driving force, and the tubular segment must be sufficiently permeable to water. The kidney creates a hyperosmotic medullary interstitium and then regulates whether collecting-duct water can equilibrate with that environment.

This means a concentrated medulla alone is not enough. If the collecting duct is water-impermeable, tubular water cannot efficiently follow the gradient. Conversely, high water permeability is not useful for concentrating urine if the medullary osmotic gradient has been erased.

Concept: the loop of Henle builds a medullary osmotic gradient

The descending limb of the loop of Henle is relatively permeable to water, so water can leave as tubular fluid descends into the increasingly hyperosmotic medulla. The ascending limb is relatively impermeable to water while reabsorbing solute; the thick ascending limb actively reabsorbs Na⁺, K⁺, and Cl⁻. This separation of water movement from solute movement helps establish the corticomedullary osmotic gradient used later by the collecting duct.²

The arrangement is called a countercurrent multiplier because oppositely directed flow through the loop repeatedly amplifies a small transverse difference into a longitudinal medullary gradient. Urea recycling and medullary blood flow also contribute to preserving the concentrating environment.

Pattern: ADH controls collecting-duct water permeability

Antidiuretic hormone, also called vasopressin, increases water permeability of collecting-duct principal cells by promoting insertion of aquaporin-2 water channels into the apical membrane. With a hyperosmotic medulla present, more water then leaves the tubular fluid and returns to the body, producing a smaller volume of more concentrated urine.³

Low ADH produces the opposite pattern: the collecting duct remains less water-permeable, so more water stays in the tubular fluid and urine becomes more dilute. ADH secretion normally rises when plasma osmolality increases and can also increase when effective circulating volume falls substantially.

Worked application: low hormone versus receptor resistance

An original experiment studies two animals with high plasma osmolality and unusually dilute urine. Animal L has low endogenous ADH production. Animal R produces abundant ADH, but its collecting-duct ADH receptor is experimentally nonfunctional. Investigators then administer a stable ADH analog that activates a normal receptor but cannot bypass a defective receptor.

Animal L responds because the collecting duct can still respond to ADH; the problem was inadequate hormone. Animal R does not respond because adding more ligand cannot rescue a nonfunctional receptor. The response test therefore distinguishes a hormone-supply defect from receptor resistance.

The same logic applies broadly in endocrine physiology: if exogenous hormone restores the response, the defect is likely upstream of the responsive tissue. If a high hormone level and added hormone both fail to produce the expected tissue response, the defect is more consistent with receptor or downstream signaling failure.

Rule: sodium retention and water retention are related but not identical

Aldosterone increases sodium reabsorption in the distal nephron by increasing sodium-transport capacity in target cells. Retaining sodium tends to retain extracellular water when water is available, supporting extracellular-fluid volume and blood pressure. But aldosterone and ADH solve different physiological problems: aldosterone primarily changes sodium balance, while ADH directly changes collecting-duct water permeability.⁴

That distinction helps with osmolality questions. Pure water retention can lower plasma osmolality, whereas proportional sodium-and-water retention can expand volume with a smaller change in osmolality. The exact result depends on intake, other hormones, and renal function, so use the conditions supplied in the stem.

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Continue learning

• Kidney filtration and solute handling
• Endocrine feedback and hormone axes
• Membrane transport and osmosis
• Browse Biology & Biochemistry MCAT lessons

Sources

1. AAMC. What’s on the MCAT Exam? Content Outline. Content Category 3B.
2. NCBI Bookshelf. Physiology, Renal.
3. NCBI Bookshelf. Physiology, Vasopressin.
4. NCBI Bookshelf. Physiology, Aldosterone.