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Renal physiology

Renal physiology. Dr Alison Chalmers Consultant Anaesthetist Queen Victoria Hospital. What you need to know. Blood flow, glomerular filtration and plasma clearance Tubular function and urine formation Assessment of renal function Regulation of fluid and electrolyte balance

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Renal physiology

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  1. Renal physiology Dr Alison Chalmers Consultant Anaesthetist Queen Victoria Hospital

  2. What you need to know • Blood flow, glomerular filtration and plasma clearance • Tubular function and urine formation • Assessment of renal function • Regulation of fluid and electrolyte balance • Regulation of acid base balance • Micturition • Pathophysiology of acute renal failure

  3. Example questions from SOE 1 • Buffers: what are they, how are they classified, formation of bicarbonate, how is H+ eliminated • Fluid balance: regulation of fluid balance, what happens when you infuse saline • Renal: renal blood flow, factors affecting GFR, autoregulation, tuberoglomerular feedback, measurement of GFR/RBF

  4. Aims of this session • Key facts about the kidney – function, anatomy • Renal blood flow • Glomerular filtration and tubular function • Regulation of body fluid volume and electrolyte balance • Renal regulation of acid base balance • Micturition

  5. Key facts – function of the kidney • Electrolyte and water homeostasis • Acid-base homeostasis • Excretion of waste products and toxins • Calcium and phosphate homeostasis • Endocrine functions – erythropoietin • Retention of vital substances – glucose, proteins

  6. Key facts - anatomy

  7. Receives 20% cardiac output Cortex > outer medulla > inner medulla 2 capillary beds in series RBF = RPF x 100/55 PAH clearance used to measure RPF Renal blood flow

  8. Autoregulation Renin-angiotensin system Macula densa cells Juxtaglomerular complex Regulation of renal blood flow

  9. GFR = 125ml/min Approx 20% renal blood flow is filtered GFR ~ forces favouring filtration – forces opposing filtration Glomerular filtration

  10. Factors affecting GFR • Forces favouring filtration • PG = hydrostatic pressure in glomerular capillary • ΠB = colloid osmotic pressure in Bowman’s capsule • Forces opposing filtration • PB = hydrostatic pressure in Bowman’s capsule • ΠG = colloid osmotic pressure in glomerular capillary • GFR ~ PG – (PB + ΠG) • Molecular size – charge and size • Inulin clearance used to measure GFR (or Cr cl ~ GFR)

  11. Tubular function

  12. Passive diffusion Across membrane Through channels/pores Facilitated diffusion Active transport Secretary processes Organic acids/bases Proximal tubule

  13. Passive diffusion Na+ linked symport and antiport Na+K+ATPase into LIS Sodium reabsorption

  14. Reabsorption occurs as a result of H+ secretion into the tubular lumen Bicarbonate reabsorption

  15. Loop of Henle

  16. Distal tubule and collecting duct • Distal tubule • Final reabsorption of ions • Collecting duct • Variable permeability to water depending on ADH present • Urea diffuses from collecting duct to aid in the concentrating of urine under influence of ADH

  17. Fluid and electrolyte balance Angiotensinogen Na depletion Renin Decrease blood volume Angiotensin I ACE Increase BP Angiotensin II Decrease blood pressure Na retention Increase blood vol Aldosterone Increase ADH Decrease ANP

  18. Renal control of acid base balance • Key reaction: • CO2 + H2O <=> H2CO3 <=> H+ + HCO3- • Rate of H+ secretion inversely proportional with pH • Respiratory acidosis • ↑CO2 → ↑ rate of H+ secretion in kidney → ↑ HCO3- reabsorption = renal compensation • Metabolic acidosis • Change in pH detected by peripheral chemoreceptors • ↑ ventilation rate – respiratory compensation • ↑ H+ secretion in kidney → ↑ HCO3- reabsorption – renal correction

  19. Detrusor muscle – smooth muscle 2 sphincters Internal – smooth muscle External – skeletal muscle Spinal reflex with learned voluntary control Micturition

  20. Any questions?

  21. Summary • Key facts about the kidney – function, anatomy • Renal blood flow • Glomerular filtration and tubular function • Regulation of body fluid volume and electrolyte balance • Renal regulation of acid base balance • Micturition

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