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