Concentration of Urine and Countercurrent Mechanism - Anatomy and Physiology
- Which part of the loop of Henle is permeable to water but relatively impermeable to solutes?
- The thin descending limb. Water leaves it by osmosis into the hyperosmotic medulla, so tubular fluid becomes more concentrated as it descends.
- Besides NaCl, which solute, recycled from the inner medullary collecting duct, adds substantially to inner medullary osmolarity?
- Urea. ADH increases the urea permeability of the inner medullary collecting duct, so urea diffuses out and helps hold the inner medulla hyperosmotic.
- Which nephrons, whose long loops of Henle reach deep into the medulla, are mainly responsible for creating the medullary gradient?
- Juxtamedullary nephrons. Their long loops generate the steep corticomedullary osmotic gradient; cortical nephrons have short loops.
- A person has no ADH activity but an otherwise normal medullary gradient. Will the urine be concentrated or dilute, and why?
- Dilute. The collecting duct stays impermeable to water, so the hypo-osmotic fluid from the ascending limb passes through without equilibrating with the medulla.
- What role do the vasa recta play in urine concentration?
- They act as a countercurrent exchanger. Their hairpin blood flow carries away reabsorbed water and solute without washing out the medullary gradient.
- How does the osmolarity of fluid leaving the thick ascending limb compare with plasma osmolarity?
- It is hypo-osmotic (about 100 mOsm/L versus about 300 mOsm/L in plasma), because the segment removes solute but not water.
- Why can final urine osmolarity never exceed the interstitial osmolarity at the papilla tip?
- Water leaves the collecting duct only by osmosis, so tubular fluid can at most equilibrate with the surrounding interstitium (about 1200 mOsm/L).
- Which segment of the loop of Henle actively transports Na+, K+, and Cl- out of the tubule while remaining impermeable to water?
- The thick ascending limb, using the Na+-K+-2Cl- (NKCC2) cotransporter. Removing solute without water builds the medullary osmotic gradient.
- What is the approximate interstitial osmolarity at the corticomedullary junction and at the tip of the renal papilla in humans?
- About 300 mOsm/L at the corticomedullary junction, rising to about 1200 mOsm/L at the papilla tip.
- A drug blocks the NKCC2 cotransporter in the thick ascending limb. What happens to the medullary gradient and the ability to concentrate urine?
- The medullary gradient dissipates, so the kidney cannot concentrate urine and excretes a large volume of roughly isosmotic urine. This is how loop diuretics work.
- Why is the loop of Henle mechanism called countercurrent multiplication?
- Fluid flows in opposite directions in the two limbs, so the small gradient that active transport creates at each level is multiplied along the loop into a large vertical gradient.