Urine Concentration and Dilution Chapter 29

Urine Concentration and Dilution

• Normal kidneys can vary the relative proportions of solutes and water in the urine in response to various challenges.
• The kidneys can excrete urine with an osmolarity as low as 50 mOsm/L (diluted urine) when there is excess water in the body and body fluid osmolarity is reduced.
• The kidneys can excrete highly concentrated urine with an osmolarity of 1200 to 1400 mOsm/L when there is a deficit of water in the body and extracellular fluid osmolarity is high.

Formation of Dilute Urine

• Tubular fluid remains isosmotic in proximal tubules.
• As fluid passes down the descending loop of Henle, water is reabsorbed by osmosis, and the tubular fluid becomes hypertonic.
• Tubular fluid is diluted (hypo-osmotic) in the ascending loop of Henle and early distal tubule, regardless of whether ADH is present or absent.
• Tubular fluid in late distal and collecting tubules and medullary collecting duct is further diluted (more hypo-osmotic) in the absence of ADH.

Formation of Concentrated Urine

• Two basic requirements for forming a concentrated urine are:
  • High level of ADH, which increases the permeability of the late distal tubules, cortical and medullary collecting ducts to water.
  • High osmolarity (high solute concentration) of the renal medullary interstitial fluid.

High Osmolarity of the Renal Medullary Interstitial Fluid

• Major factors that contribute to the buildup of solute concentration into the medulla include:
  • Active transport of Na ions and co-transport of K, Cl, and other ions out of the thick ascending limb of the loop of Henle (Countercurrent multiplier system).
  • Active transport of ions from the collecting ducts into the medullary interstitium.
  • Facilitated diffusion of urea from the inner medullary collecting ducts (urea recirculation).
  • Diffusion of only small amounts of water into the medullary interstitium—far less than the reabsorption of solutes.

Preservation of High Osmolarity

• Two special features of the renal medullary blood flow (vasa recta) that contribute to preserving high osmolarity into the medulla:
  • The vasa recta serve as countercurrent exchangers, minimizing the washout of solutes from the medullary interstitium.
  • Large amounts of water are reabsorbed into the cortex, rather than into the renal medulla.
  • Medullary blood flow is low (less than 5% of the total RBF), which is sufficient to supply the metabolic needs of the tissues but helps minimize solute loss from the medullary interstitium.

Urine Concentration and Dilution

• Normal kidneys can vary the relative proportions of solutes and water in the urine in response to various challenges.
• The kidneys can excrete urine with an osmolarity as low as 50 mOsm/L (diluted urine) when there is excess water in the body and body fluid osmolarity is reduced.
• The kidneys can excrete highly concentrated urine with an osmolarity of 1200 to 1400 mOsm/L when there is a deficit of water in the body and extracellular fluid osmolarity is high.

Formation of Dilute Urine

• Tubular fluid remains isosmotic in proximal tubules.
• As fluid passes down the descending loop of Henle, water is reabsorbed by osmosis, and the tubular fluid becomes hypertonic.
• Tubular fluid is diluted (hypo-osmotic) in the ascending loop of Henle and early distal tubule, regardless of whether ADH is present or absent.
• Tubular fluid in late distal and collecting tubules and medullary collecting duct is further diluted (more hypo-osmotic) in the absence of ADH.

Formation of Concentrated Urine

• Two basic requirements for forming a concentrated urine are:
  • High level of ADH, which increases the permeability of the late distal tubules, cortical and medullary collecting ducts to water.
  • High osmolarity (high solute concentration) of the renal medullary interstitial fluid.

High Osmolarity of the Renal Medullary Interstitial Fluid

• Major factors that contribute to the buildup of solute concentration into the medulla include:
  • Active transport of Na ions and co-transport of K, Cl, and other ions out of the thick ascending limb of the loop of Henle (Countercurrent multiplier system).
  • Active transport of ions from the collecting ducts into the medullary interstitium.
  • Facilitated diffusion of urea from the inner medullary collecting ducts (urea recirculation).
  • Diffusion of only small amounts of water into the medullary interstitium—far less than the reabsorption of solutes.

Preservation of High Osmolarity

• Two special features of the renal medullary blood flow (vasa recta) that contribute to preserving high osmolarity into the medulla:
  • The vasa recta serve as countercurrent exchangers, minimizing the washout of solutes from the medullary interstitium.
  • Large amounts of water are reabsorbed into the cortex, rather than into the renal medulla.
  • Medullary blood flow is low (less than 5% of the total RBF), which is sufficient to supply the metabolic needs of the tissues but helps minimize solute loss from the medullary interstitium.