Renal physio 3

• Filtration fraction


  • The fraction of RPF filtered across the glomerulus
  • GFR divided by RPF
  • ~20%
• Filtered Load/Tubular Load


  • The amount of a specific solute (electrolyte, glucose, or amino acid) dissolved in the fluid that enters Bowman's capsule
  • There is an equation for this, but you have to know: Patient's GFR and plasma concentration of that solute (to determine this, draw blood, place in spectrophotometer)

  
• Micturition


  1. Autonomic reflex activity

  2. Skeletal muscle

  3. Some voluntary regulation

  
• Micturition (Innervation)


  Pelvic sensory/afferent fibres sense stretch from stretch receptors in the bladder wall, and urethra, when it distends

  
• Micturition (Emptying of the bladder)


  Pelvic parasympathetic efferent contraction of the bladder wall (detrusor muscle)

  
• Micturition (Urine storage)


  1. Sympathetic efferent via hypogastric nerve relaxes the detrusor muscle

  2. Contracts the internal sphincter (neck of the bladder; smooth muscle)

  
• Micturition (Voluntary control)


  1. Pudendal nerve somatic motor neuron; efferent

  2. Controls external sphincter (urethralis muscle; skeletal muscle)

  3. Contraction - aids in urine storage

  4. Relaxation – aids in urine voiding

  
• Fick Principle


  • Renal plasma flow is the volume of plasma that reaches the kidneys per unit time
  • Renal plasma flow is given by the Fick principle
  • States that the amount of a substance in the blood that flows into an organ is the amount that must flow out of that organ, assuming that the organ doesn't produce or degrade the substance

  
• Substances that can be used to measure renal plasma flow

  • Should be freely filtered
  • Should not be metabolized by the kidney
  • Should not be stored or produced by the kidneys
  • Should be completely secreted by the renal tubules

  
• To measure effective renal plasma flow p-aminohippuric acid (PAH) is used

  
• Renal Clearance


  The volume of plasma cleared of a substance per unit time

  
• Substances used to measure GFR

  • Inulin
  • Creatinine
• Inulin


  • Polymer of fructose
  • Molecular mass: 5,200 Da
  • Not bound to plasma proteins
  • Freely filtered by the glomeruli
  • Neither reabsorbed nor secreted by the tubules
  • Not metabolized by the kidneys or any other organ

  
• Inulin clearance is the laboratory standard for GFR determination

  
• Normal values for GFR as measured by inulin clearance are 3-5 mL/min/kg in the dog and 2.5-3.5 mL/min/kg in the cat
• Creatinine


  • Produced endogenously
  • Excreted primarily by glomerular filtration, so its clearance can be used to estimate GFR
  • Neither reabsorbed nor secreted by the tubules

  
• Endogenous creatinine clearance (dog and cat) is approximately 2-5 mL/min/kg

  
• To determine GFR from creatinine

  1. Collect urine for 24-hours and draw blood before and after the 24-hour period

  2. Measure the amount of creatinine that was removed from the blood during that time

  3. Apply the results to a formula to determine GFR

  
• Limitations of using creatinine to estimate GFR:
• The use of the serum level of creatinine as an index of GFR rests on three important assumptions:
• 1. aminohippuric acid (PAH) clearance

  • Used to measure renal plasma flow because PAH is filtered, not reabsorbed but secreted from all the plasma that flows through the kidneys
  • Theoretically, if a substance is completely cleared from the plasma, its clearance rate would equal renal plasma flow

  
• Limitations of using creatinine to estimate GFR:

  patients with decreased
• Limitations of using creatinine to estimate GFR:

  15-20% of creatine in the bloodstreamisnotfilteredbyglomerulusbut secretedbyrenal tubules