LEC 27: Glomerular filtration & tubular reabsorption

• Intrinsic controls of glomerular filtration rate

  Known as renal autoregulation
• Intrinsic mechanisms


  1. Myogenic mechanism

  2. Tubuloglomerular feedback
• Myogenic mechanism


  • Constriction of afferent arterioles reduces blood flow into nephron and protects glomeruli from damaging high BP
• Tubuloglomerular feedback


  • Directed by macula densa cells which responds to filtrate & NaCl concentration
  • Flow rate increase in renal tubules = less time to reabsorb NaCl which increases Na levels in filtrate
  • Afferent arteriole constrict lowers net filtration pressure and glomerular flow rate which gives tubules more time to reabsorb NaCl
• Goal of intrinsic controls

  Maintain glomerular filtration rate (GFR)
• Intrinsic regulation occurs within the kidney and maintains stable GFR despite changes in BP
• Intrinsic controls are regulatory mechanisms intrinsic to the kidney
• Extrinsic controls of glomerular filtration rate

  Involves neural and hormonal regulation
• Extrinsic mechanisms


  1. Sympathetic nervous system responses

  2. Renin-angiotensin-aldosterone system (RAAS)
• Goal of extrinsic controls

  Maintains blood pressure
• Extrinsic regulation involves nervous system and hormonal responses and can override intrinsic controls in response to extreme conditions
• Extrinsic controls are regulatory mechanisms originating outside the kidney
• Mechanisms underlying water and solute reabsorption from renal tubules

  Involves both active and passive transport, occurring through transcellular and paracellular routes
• Active reabsorption


  • Requires ATP to move solutes across cell membrane
  • Sodium is actively transported out of tubule cells into interstitial fluid through basolateral membranes via sodium-potassium pumps
  • Active pumping of Na+ generates negative charge inside tubule cells which facilitates movement of positively charged ions (K+ and Mg2+)
  • ATP-dependent pumps drive reabsorption of organic nutrients (glucose, amino acids, vitamins)
• Passive reabsorption


  • Occurs down electrochemical gradients without energy
  • Water reabsorption is passive and occurs through osmosis, driven by movement of Na+ solutes
  • Aquaporin facilitates water movement across cell membrane
  • Some solutes (K+, Mg2+, Ca2+) slip between cells through tight junctions
  • Solute concentration in filtrate increases as water is reabsorbed which creates a concentration gradient which further drives their movement across tubule membrane
• Hormonal regulation of sodium and water reabsorption in distal and collecting duct


  • Aldosterone increases sodium reabsorption and potassium secretion
  • ADH increases water reabsorption by inserting aquaporins into cell membranes
• Fine-tuning of sodium and water balance

  • Aldosterone acts on principal cells to increase sodium reabsorption and potassium secretion
  • ADH acts on collecting ducts to increase water reabsorption
• Overall effect of hormonal regulation

  Helps adjust sodium and water balance in response to the body's needs and conditions
• Tubular secretion


  The process of moving substances from the blood into the renal tubules to be excreted in urine
• Importance of tubular secretion

  • Remove substances from the blood that were not filtered by the glomerulus
  • Eliminate unwanted substances or excess ions to maintain homeostasis
• Substances secreted

  • Protein-bound substances like drugs and metabolites
  • Urea and uric acid
  • Excess potassium ions
  • Hydrogen ions to regulate blood pH
• Proximal convoluted tubule is most active in reabsorption and tubular secretion
• Transport carriers


  Reabsorption of specific substances from filtrate to the blood. Has a limit to reabsorption capacity if concentration of solute exceeds saturation point of transporters
• Sympathetic Nervous system response

  1. Constricts arterioles

  2. Decreases glomerular filtration rate

  3. Increases systemic BP

  4. Redirects blood flow to other organs
• Renin-angiotensin-aldosterone system


  1. Renin from kidneys

  2. Constrict arterioles

  3. Increase BP
• Tubular reabsorption - Transcellular route


  Solute -> apical membrane -> cytosol of tubular cells -> exits via basolateral membrane -> enters blood via endothelium of peritubular capillaries
• Tubular reabsorption - Paracellular route


  1. Solute -> between tubular cells

  2. Limited by tight junctions

  3. Leaky in proximal region of nephron