Reabsorption, Secretion, Loop of Henle

• Highly selective reabsorption percentages

  • Water: 1%
  • Glucose: 0%
  • Sodium: 0.5%
  • Urea (waste product): 50%
  • Phenol (waste product): 100%
• 100% of filtered glucose reabsorbed
• Basic Renal Processes

  1. Glomerular filtration

  2. Tubular reabsorption

  3. Tubular secretion
• Terminology reminder

  • Transcellular: through epithelial cells (thru 2 cell membranes)
  • Paracellular: between cells, through tight junctions (no cell membranes)
• 80% of total energy spent by kidneys is used for Na+ transport
• Reabsorption in Proximal Convoluted Tubule

  1. Reabsorbs 100% of glucose and amino acids

  2. Reabsorbs 50% of urea

  3. Reabsorbs 60-70% of Na+, K+, PO4-3, Ca2+, H2O, and Cl-

  4. Reabsorbs 90% of HCO3-
• Glucose acts as an osmotic force resulting in water to remain in filtrate
• Reabsorption of filtered Na+ results in very little being excreted in urine
• Transport Maximum (Tm) is the maximum rate of transporting solutes
• PCT reabsorption of filtered Na+

  1. Process is isosmotic

  2. Na+ reabsorbed by different mechanisms in 1st & 2nd half of PT
• 99% of filtered water reabsorbed
• Proximal Convoluted Tubule is the major site for reabsorption of solutes
• 99.5% of filtered salt reabsorbed
• Na+ is reabsorbed throughout the nephron, but to varying extents
• Even if ADH is very high, most H2O is reabsorbed in PCT
• Variable reabsorption of Ions, Solutes, and Water


  • Transporters and channels
  • Aquaporins
  • "Leakiness" of tight junctions
  • Responsiveness of transporters, channels, and aquaporins to hormones
• Water follows reabsorbed Na+ affecting blood volume and blood pressure
• PCT reabsorption of filtered K+


  K+ reabsorbed in the PCT mainly by the paracellular route
• Transport maximum is the total transport maxi
• Transport maximum is reached when carriers are saturated
• Diuresis in the PCT

  1. Filtered load >> capacity of tubules to reabsorb it

  2. Transporters become saturated, glucose ends up in the urine

  3. Glucose is a solute that draws water into the urine by osmosis
• Salt concentration in the PCT

  1. Amino acids, glucose, salt, and water are reabsorbed throughout the length of the PCT

  2. PCT filtrate [salt] = the plasma [salt]

  3. Saltiness diminishes after the Loop of Henle
• Urea cycling


  Help concentrate the interstitium
• Glucose transporters


  • Transport maximum is reached when carriers are saturated
  • Threshold = the appearance of glucose in the urine before reaching the overall Tm of the kidney
  • Hyperglycemia causes excessive urine production (diuresis) as transporters become saturated
  • Glucose acts as an osmotic force resulting in water to remain in filtrate
• Loop of Henle function

  1. Concentration of urine by separating solutes and water

  2. Loop of Henle has 3 segments: Impermeability to ions, Permeable to water (Thin) &&, Impermeable to water, Permeable to ions (Thick)
• Urea is 50% reabsorbed in the PCT
• Reabsorption of HCO3-

  1. Kidney reabsorbs all the filtered HCO3-

  2. HCO3- reabsorption mainly in PT (~90%); rest in TAL and CD

  3. All reabsorption involves H+ secretion
• Tm for glucose = 375 mg/min
• Descending Limb of Loop of Henle

  1. Receives isotonic fluid from PT

  2. Loses water to the higher concentration outside the loop and increases in osmolarity
• The function of the Loop of Henle is the concentration of urine to conserve water
• Loop of Henle: Ascending Limb

  1. Na+ and Cl- are reabsorbed

  2. Na+ -K+ -2Cl- cotransporter is the site of action of loop diuretics like furosemide

  3. Na+ & Cl- move down gradient; K+ moves against the gradient

  4. Na+, K+ ATPase is a key element

  5. K+ returns to the lumen through K+ channel (ROMK)

  6. ROMK creates a positive lumen, causing reabsorption of several cations

  7. Furosemide decreases Na+ -K+ -2Cl- transport, leading to less positive lumen and increased loss of K+, Ca2+, & Mg2+
• Permeability to ions


  Ability to allow ions to pass through
• Ions


  • Na, K, 2Cl cotransport; Ca2+, Mg2+
• Thin


  Impermeable to water
• Late distal tubule/CD

  1. Major site of regulated (hormone stimulated) reabsorption

  2. Principal cells involved in Na+ reabsorption (via ENaC) and K+ secretion (via ROMK)

  3. Aldosterone increases ENaC and ROMK activity, as well as Na+/K+ pumps

  4. ADH increases water permeability

  5. Intercalated cells (α and β) secrete H+ (via H+ -ATPase), with aldosterone increasing activity
• Thick

  Impermeable to water
• Permeable to water

  Ability to allow water to pass through
• Potassium

  • Freely filtered
  • 60-70% of filtered K+ is reabsorbed in PT via solvent drag
  • 25-35% of filtered K+ is reabsorbed in LoH via the Na-K-2Cl cotransporter
  • 5-15% of filtered K+ reaches the distal nephron, with reabsorption by α-intercalated cells
• Loop of Henle: Descending Limb

  1. Receives isotonic (300 mOsm/L) fluid from PT

  2. Loses H2O to the higher concentration outside the loop and increases in osmolarity until it reaches its maximum at the hairpin turn

  3. This area represents the highest concentration in the nephron, but the collecting duct can reach this same osmolarity with maximum ADH effect
• Distal convoluted tubule: Early DCT

  1. Reabsorbs NaCl by a Na+-Cl- cotransporter

  2. Still impermeable to water, leading to continued dilution of tubular fluid

  3. Thiazide diuretics inhibit Na+ -Cl- cotransporter