Regulation of Urine - Concentration and Dilution

1. Antidiuretic Hormone (ADH) - The Primary Regulator

2. Formation of Dilute Urine

• Thick ascending limb of loop of Henle (TAL): Actively pumps NaCl out of the lumen (Na⁺/K⁺/2Cl⁻ cotransporter). This segment is impermeable to water, so tubular fluid becomes hypo-osmotic (~100 mOsm/L) as it enters the distal tubule.
• Distal tubule and cortical collecting duct: In the absence of ADH, these segments remain impermeable to water, so dilute fluid passes through and is excreted.
• Result: The kidneys can excrete up to 20 L/day of dilute urine at ~50 mOsm/L.

3. Formation of Concentrated Urine - The Countercurrent Mechanism

A. Countercurrent Multiplier (Loop of Henle)

1. Fluid enters the descending limb at 300 mOsm/L.
2. The thick ascending limb actively pumps NaCl into the interstitium - creating ~200 mOsm/L gradient at each level.
3. The descending limb is permeable to water → water exits into the hyperosmotic interstitium → the descending limb fluid becomes progressively more concentrated toward the tip (up to ~1200 mOsm/L at the bend).
4. This concentrated fluid enters the ascending limb, which continues to pump NaCl out but is water-impermeable - progressively diluting as it ascends.
5. The countercurrent flow of the two limbs multiplies this effect, creating a steep osmotic gradient from cortex (~300) to inner medulla (~1200 mOsm/L).

B. Role of Urea

• The inner medullary collecting duct is permeable to urea (especially in the presence of ADH).
• As water is reabsorbed from the collecting duct, the tubular urea concentration rises.
• Urea diffuses out into the medullary interstitium, contributing up to ~600 mOsm/L of the total medullary osmolarity.
• Urea then enters the thin ascending limb and loops back through the medullary tubules - this is called urea recycling.

C. Countercurrent Exchanger (Vasa Recta)

• Blood flows down into the medulla in the descending vasa recta - it picks up solutes and loses water → becomes hyperosmotic.
• In the ascending vasa recta, solutes diffuse back out and water re-enters.
• Because of countercurrent exchange, the net result is that the vasa recta return solutes to the medulla and carry away only the water that was reabsorbed by the tubules.

4. Permeability Summary of Key Tubular Segments

5. Quantifying Urine Regulation: Free Water Clearance

• Osmolar clearance (C_osm) = (U_osm × V̇) / P_osm — volume of plasma cleared of all solutes per minute.
• Free water clearance (C_H₂O) = V̇ − C_osm
  • Positive C_H₂O → kidney excretes water in excess of solutes (dilute urine, water loss)
  • Negative C_H₂O → kidney returns water to circulation, conserving it (concentrated urine, water deficit state)

6. Disorders of Urinary Concentrating Ability

1. Abnormal ADH secretion:

• Too little ADH (central diabetes insipidus) → inability to concentrate → large volumes of dilute urine
• Too much ADH (SIADH) → water retention → concentrated urine, hyponatremia

1. Impaired countercurrent mechanism:

• Medullary damage (e.g., chronic kidney disease, analgesic nephropathy) → reduced medullary osmolarity → cannot concentrate urine even with normal ADH

1. ADH resistance (nephrogenic diabetes insipidus):

• Collecting duct does not respond to ADH → dilute urine despite high ADH levels
• Causes: mutations in V2 receptor or AQP2, hypercalcemia, hypokalemia, lithium toxicity

Summary Diagram (Conceptual)

Water deficit → ↑ Plasma osmolarity → Osmoreceptors in hypothalamus
                                         ↓
                            Posterior pituitary → ↑ ADH secretion
                                         ↓
              Collecting duct AQP2 channels inserted → ↑ water reabsorption
                                         ↓
                            Small volume, concentrated urine (↑ osmolarity)
                            
Water excess → ↓ Plasma osmolarity → ↓ ADH secretion
                                         ↓
              Collecting duct impermeable to water
                                         ↓
                            Large volume, dilute urine (↓ osmolarity)