Urine Concentration & Volume Regulation

Vocabulary flashcards covering the major terms and mechanisms involved in renal regulation of urine concentration and volume.

Osmolarity

The concentration of solute particles in a solution, expressed in milliosmoles per liter (mOsm/L); blood plasma is kept near 300 mOsm/L.

Antidiuretic Hormone (ADH)

Posterior-pituitary hormone required for water reabsorption in the collecting duct; presence allows formation of concentrated urine, absence produces dilute urine.

Juxtamedullary Nephron

Type of nephron whose long loop of Henle extends deep into the renal medulla and is essential for establishing the medullary osmotic gradient.

Cortical Nephron

Nephron located mainly in the renal cortex with short loop of Henle and no vasa recta; it does not significantly concentrate urine.

Vasa Recta

Hairpin-shaped capillaries surrounding juxtamedullary loops; participate in countercurrent exchange to preserve the medullary gradient.

Countercurrent Flow

Fluid movement in opposite directions within parallel limbs of the same loop (e.g., descending vs. ascending limb), enabling efficient exchange.

Countercurrent Multiplier

Mechanism in the loop of Henle that creates the medullary osmotic gradient through differential permeability and active salt pumping.

Countercurrent Exchanger

Process in the vasa recta that maintains the medullary osmotic gradient by passive exchange of water and solutes without washing it out.

Medullary Osmotic Gradient

Progressive increase in interstitial osmolarity from cortex (~300 mOsm/L) to inner medulla (~1,200 mOsm/L); drives water reabsorption.

Loop of Henle

U-shaped portion of the nephron consisting of descending and ascending limbs; central to urine concentration.

Descending Limb

Segment of the loop of Henle permeable to water but not salt; water exits by osmosis, concentrating tubular fluid.

Ascending Limb

Segment impermeable to water that actively pumps NaCl into the medulla via NKCC2 transporters, diluting tubular fluid.

Na⁺-K⁺-2Cl⁻ Symporter (NKCC2)

Transport protein in the thick ascending limb that moves sodium, potassium, and chloride into the interstitium, raising medullary osmolarity.

Iso-osmotic

Having the same osmolarity as another solution; initial filtrate entering the loop of Henle is iso-osmotic with surrounding fluid.

Concentrated Urine

Small-volume urine with high osmolarity produced during dehydration; results from maximal water reabsorption under ADH influence.

Dilute Urine

Large-volume urine with low osmolarity formed when overhydrated or when ADH levels are low/absent.

Medullary Interstitial Fluid

Extracellular fluid in the renal medulla whose osmolarity gradient is established by countercurrent multiplication.

Hairpin Loop (in Vasa Recta)

U-turn configuration of vasa recta capillaries that prevents washout of the medullary gradient by allowing countercurrent exchange.

Slow Blood Flow (Vasa Recta)

Reduced velocity through vasa recta that permits adequate time for solute and water exchange, preserving the gradient.

Osmosis

Passive movement of water across a semipermeable membrane from low to high solute concentration, key to water reabsorption in kidney.

Dehydration (Kidney Response)

Triggers ADH release; kidneys produce low-volume, high-osmolarity urine to conserve water and restore plasma osmolarity.

Overhydration (Kidney Response)

Results in suppressed ADH; kidneys excrete high-volume, low-osmolarity urine to eliminate excess water.

Peritubular Capillaries

Capillary network surrounding cortical nephron tubules; lacks countercurrent exchange function of vasa recta.

Osmotic Equilibrium

State in which two solutions reach equal osmolarity; drives water movement out of descending limb and collecting duct.