Concentrating and Diluting the Urine
Key facts on countercurrent multiplication, the medullary gradient, urea recycling, vasa recta and ADH.
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Questions Covered in This Set
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What is the range of urine osmolality the human kidney can produce?
About 50 mOsm/kg (maximally dilute) to 1200 mOsm/kg (maximally concentrated) — a ~24-fold range.
How much obligatory solute must be excreted daily, and why does it matter?
About 600–900 mOsm/day. If urine could only reach 300 mOsm/kg, ~2–3 L of water would be obligatorily lost each day.
What is the 'single effect' of the thick ascending limb?
The water-impermeable TAL pumps NaCl out via NKCC2, making the interstitium ~200 mOsm/kg more concentrated than the tubular fluid at any horizontal level.
Define countercurrent multiplication in one sentence.
A small repeated horizontal (transverse) 200 mOsm effect is multiplied along the axis of the loop by hairpin countercurrent flow, creating a 300→1200 mOsm/kg longitudinal gradient.
What are the permeability properties of the thin descending limb?
Water-permeable but solute-impermeable — it passively equilibrates with the salty interstitium, concentrating fluid before the bend.
Why do loop diuretics impair both concentrating AND diluting ability?
Furosemide blocks NKCC2, abolishing the single effect; this collapses the medullary gradient (no concentrating) and prevents dilute fluid generation in the TAL (no diluting).
Which nephrons do the heavy lifting for urine concentration?
Juxtamedullary nephrons (~15%) with long loops of Henle — longer loops produce steeper gradients (kangaroo rat reaches ~6000 mOsm/kg).
What contributes about half of the 1200 mOsm/kg at the papilla?
Urea, delivered into the inner medullary interstitium via ADH-stimulated UT-A1/UT-A3 transporters and recycled through UT-A2.
Why can protein-malnourished patients not maximally concentrate urine?
Low urea production means less urea available for medullary recycling, so the medullary osmotic gradient cannot be fully built.
What is the function of the vasa recta?
Hairpin capillaries acting as passive countercurrent exchangers — they supply the medulla with blood while minimising washout of the osmotic gradient.
