Renal physio 1

• Kidneys


  Receive ~ 25% cardiac output

  
• Functions of the kidneys

  • Excretory - eliminate toxins/harmful substances, metabolic waste and excess substances
  • Regulatory - maintain a constant volume and composition of the body fluids by varying the excretion of solutes and water (regulate water; ion, osmotic and pH balance; blood pressure)
  • Endocrine - synthesize and secrete (i) Renin (ii) Erythropoietin (iii) 1,25-dihydroxycholecalciferol

  
• The renal artery and vein, lymphatics, nerves and the ureter pass through a single indentation or hilus

  
• Sympathetic nervous system input

  Triggers vasoconstriction of the arterioles reduce renal blood flow into the glomerulus and decrease urine production
• Parasympathetic nervous system input

  Via the renal branches of the vagus nerve, causes vasodilation and increased blood flow of the arterioles; increase urine production

  
• Dogs ~ 500,000 nephrons/kidney, Cats ~ 200,000 nephrons/kidney
• Two main types of nephrons

  • Cortical nephrons
  • Juxtamedullary nephrons
• Cortical nephrons


  • Glomeruli located in the outer and middle cortices
  • Associated with the loop of Henle that extend to the junction of the cortex and medulla or into the outer zone of the medulla, e.g. marine aquatic mammals
• Juxtamedullary nephrons


  • Glomeruli in the cortex close to the medulla
  • Associated with loops of Henle that extend more deep into the medulla, e.g. mammals in arid region, such as the Kangaroo rat
• Avian kidneys contain both mammalian-type and reptilian-type nephrons
• Glomerulus


  • Filters blood, Solutes, water, urea, creatinine
  • Tuft of capillaries enclosed within the glomerular/Bowman's capsule

  
• Layers of the capillary wall

  • Capillary endothelium
  • Basement membrane
  • Visceral epithelium
• Capillary endothelium


  • Single layer of thin cells
  • Endothelial fenestrae - transcellular pores that conduct water and other components into the blood

  
• Basement membrane


  • Thick, acellular
  • Composed of various glycoproteins, primarily laminins, type IV collagens and heparin sulfate (proteoglycan)
• Visceral epithelium


  • Layer of interlocking cells called podocytes
  • Interdigitation of podocyte appendages creates filtration slits between the cells
  • The size of these filtration slits is the primary determinant of which particles can filter out of the blood
• Proximal tubule


  Reabsorbs most filtered water and solutes (Na+, K+, Cl-, HCO-3, H2O, glucose, amino acids)
• Descending/thin limb of the loop of Henle

  • Maintains medullary hypertonicity
  • Reabsorbs H2O, *NaCl, *urea
• Ascending/thick limb of the loop of Henle


  • Reabsorbs Na+, K+ and Cl-
  • Dilutes tubule fluid
  • Generates medullary hypertonicity
• Distal convoluted tubule


  • Reabsorbs Na+, Ca2+, Mg2+, Cl-
  • Dilutes tubule fluid
• Connecting segment


  Regulates acid, HCO-3, ammonia, Na+, Ca2+, K+ and water excretion
• Cortical collecting duct


  Regulates acid, HCO-3, ammonia, Na+, K+ and water excretion
• Medullary collecting duct


  Regulates acid, ammonia, urea, Na+, K+ and water excretion
• Renal blood flow pathway


  1.Renal artery 2. segmental arteries 3.interlobar arteries arcuate arteries interlobular afferent arterioles glomerulus efferent arterioles peritubular capillaries vasa recta interlobular veins arcuate vein interlobar vein renal vein

  
• Total renal plasma flow


  Volume of plasma delivered to the kidneys per unit time

  
• Effective renal plasma flow


  • A measure used in renal physiology to calculate renal plasma flow and hence estimate renal function
  • Describes a fraction of plasma flow through a nephron; the level in renal venous plasma is not measured

  
• Glomerular filtrate/ultra-filtrate


  The fluid formed via filtration of the blood by the glomerulus, which is nearly identical to plasma in its electrolyte and water

  
• Glomerular filtration rate


  The quantity of glomerular filtrate formed each minute in all the nephrons of both the kidneys/kg body weight

  
• A 10 kg dog, with a typical GFR of 3.7 mL/min/kg would produce approximately 37 mL of glomerular filtrate per min
• Three factors that determine the filtration pressure

  • Glomerular pressure
  • Plasma colloidal osmotic pressure (COP)
  • Bowman's capsular pressure
• Greater the glomerular pressure


  Greater the filtration
• Greater the plasma COP and Bowman's capsular pressure

  Less filtration
• Main force favoring filtration

  Hydrostatic pressure of the blood within the capillary
• Forces opposing filtration


  Plasma oncotic pressure within the glomerular capillary and the hydrostatic pressure in Bowman's space
• Greater the blood flow


  Greater the filtration rate
• Afferent arteriolar constriction


  Decreases the rate of blood flow into the glomerulus and decreases GFR = decreased filtration rate
• Efferent arteriolar constriction


  Increases the resistance and outflow from the glomeruli increases the glomerular pressure and GFR initially but when blood remains in the glomerulus for a prolonged period, increase in plasma COP occurs which causes a fall in GFR = net effect is slight increase in GFR
• Mild to moderate sympathetic stimulation

  Causes afferent arteriolar constriction and decreases GFR
• Strong sympathetic stimulation


  Causes great reduction in the glomerular blood flow and glomerular pressure resulting in decrease of GFR
• Myogenic response


  • Immediate afferent arteriolar constriction after an increase in arteriolar wall tension = increase resistance to blood flow in response to increased perfusion pressure
  • Arteriolar dilation occurs almost immediately after a decrease in arteriolar wall tension = reduce resistance to flow when vascular perfusion pressure decreases
• Tubuloglomerular feedback


  • Mediated by the macula densa cells of the JG apparatus
  • These cells sense changes in the Na+ and CL-
  • If GFR is increased, due to increased glomerular HP there will be increase in macula densa flow and Na+ and CL- delivery initiates a response that returns GFR and macula densa flow towards the normal due to afferent arteriole constriction (which lowers glomerular HP)
  • If renal blood flow falls too low it decreases the GFR decreases Na+ and CL- delivery to the distal tubule which initiate a signal from macula densa and dilates the afferent arterioles and it increases glomerular blood flow and glomerular pressure = GFR increase to normal