Renal physiologyRenal physiology

Dr Alison ChalmersDr Alison Chalmers

Consultant AnaesthetistConsultant Anaesthetist

Queen Victoria HospitalQueen Victoria Hospital

What you need to knowWhat you need to know

Blood flow, glomerular filtration and Blood flow, glomerular filtration and plasma clearanceplasma clearance

Tubular function and urine formationTubular function and urine formation Assessment of renal functionAssessment of renal function Regulation of fluid and electrolyte balanceRegulation of fluid and electrolyte balance Regulation of acid base balanceRegulation of acid base balance MicturitionMicturition Pathophysiology of acute renal failurePathophysiology of acute renal failure

Example questions from Example questions from SOE 1SOE 1

Buffers: what are they, how are they Buffers: what are they, how are they classified, formation of bicarbonate, classified, formation of bicarbonate, how is Hhow is H++ eliminated eliminated

Fluid balance: regulation of fluid Fluid balance: regulation of fluid balance, what happens when you infuse balance, what happens when you infuse salinesaline

Renal: renal blood flow, factors affecting Renal: renal blood flow, factors affecting GFR, autoregulation, tuberoglomerular GFR, autoregulation, tuberoglomerular feedback, measurement of GFR/RBFfeedback, measurement of GFR/RBF

Aims of this sessionAims of this session

Key facts about the kidney – function, Key facts about the kidney – function, anatomyanatomy

Renal blood flowRenal blood flow Glomerular filtration and tubular Glomerular filtration and tubular

functionfunction Regulation of body fluid volume and Regulation of body fluid volume and

electrolyte balanceelectrolyte balance Renal regulation of acid base balanceRenal regulation of acid base balance MicturitionMicturition

Key facts – function of the Key facts – function of the kidneykidney

Electrolyte and water homeostasisElectrolyte and water homeostasis Acid-base homeostasisAcid-base homeostasis Excretion of waste products and toxinsExcretion of waste products and toxins Calcium and phosphate homeostasisCalcium and phosphate homeostasis Endocrine functions – erythropoietinEndocrine functions – erythropoietin Retention of vital substances – Retention of vital substances –

glucose, proteinsglucose, proteins

Key facts - anatomyKey facts - anatomy

Renal blood flowRenal blood flow

Receives 20% cardiac Receives 20% cardiac outputoutput

Cortex > outer Cortex > outer medulla > inner medulla > inner medullamedulla

2 capillary beds in 2 capillary beds in seriesseries

RBF = RPF x 100/55RBF = RPF x 100/55 PAH clearance used to PAH clearance used to

measure RPFmeasure RPF

Regulation of renal blood Regulation of renal blood flowflow

AutoregulationAutoregulation

Renin-angiotensin Renin-angiotensin systemsystem Macula densa cellsMacula densa cells Juxtaglomerular Juxtaglomerular

complexcomplex

Glomerular filtrationGlomerular filtration

GFR = 125ml/minGFR = 125ml/min Approx 20% renal Approx 20% renal

blood flow is blood flow is filteredfiltered

GFR GFR ~ forces ~ forces favouring filtration favouring filtration – forces opposing – forces opposing filtrationfiltration

Factors affecting GFRFactors affecting GFR

Forces favouring filtrationForces favouring filtration PPGG = hydrostatic pressure in glomerular capillary = hydrostatic pressure in glomerular capillary ΠΠBB = colloid osmotic pressure in Bowman’s capsule = colloid osmotic pressure in Bowman’s capsule

Forces opposing filtrationForces opposing filtration PPBB = hydrostatic pressure in Bowman’s capsule = hydrostatic pressure in Bowman’s capsule ΠΠGG = colloid osmotic pressure in = colloid osmotic pressure in glomerular capillaryglomerular capillary

GFR GFR ~ P~ PGG – (P – (PBB + + ΠΠGG))

Molecular size – charge and sizeMolecular size – charge and size Inulin clearance used to measure GFR (or Cr cl ~ GFR)Inulin clearance used to measure GFR (or Cr cl ~ GFR)

Tubular functionTubular function

Proximal tubuleProximal tubule

Passive diffusionPassive diffusion Across membraneAcross membrane Through Through

channels/poreschannels/pores Facilitated diffusionFacilitated diffusion Active transportActive transport

Secretary Secretary processesprocesses Organic acids/basesOrganic acids/bases

Sodium reabsorptionSodium reabsorption

Passive diffusionPassive diffusion NaNa++ linked symport linked symport

and antiportand antiport NaNa++KK++ATPase into ATPase into

LISLIS

Bicarbonate reabsorptionBicarbonate reabsorption

Reabsorption Reabsorption occurs as a result occurs as a result of Hof H++ secretion into secretion into the tubular lumenthe tubular lumen

Loop of HenleLoop of Henle

Distal tubule and collecting Distal tubule and collecting ductduct

Distal tubuleDistal tubule Final reabsorption of ionsFinal reabsorption of ions

Collecting ductCollecting duct Variable permeability to water Variable permeability to water

depending on ADH presentdepending on ADH present Urea diffuses from collecting duct to aid Urea diffuses from collecting duct to aid

in the concentrating of urine under in the concentrating of urine under influence of ADHinfluence of ADH

Fluid and electrolyte Fluid and electrolyte balancebalance

Na depletion

Decrease blood volume

Decrease blood pressure

Angiotensinogen

Angiotensin I

Renin

Angiotensin II

ACE

Aldosterone

Increase BP

Na retentionIncrease blood volIncrease ADH

Decrease ANP

Renal control of acid base Renal control of acid base balancebalance

Key reaction: Key reaction: COCO22 + H + H22O O <=> H<=> H22COCO3 3 <=> H<=> H+ + + HCO+ HCO33

--

Rate of HRate of H++ secretion inversely proportional with pH secretion inversely proportional with pH Respiratory acidosisRespiratory acidosis

↑↑COCO2 2 → ↑ rate of H→ ↑ rate of H++ secretion in kidney → ↑ secretion in kidney → ↑ HCOHCO33- - reabsorption = reabsorption =

renal compensationrenal compensation Metabolic acidosisMetabolic acidosis

Change in pH detected by peripheral chemoreceptorsChange in pH detected by peripheral chemoreceptors ↑ ↑ ventilation rate – respiratory compensationventilation rate – respiratory compensation ↑ ↑ HH++ secretion in kidney → ↑ secretion in kidney → ↑ HCOHCO33

- - reabsorption – renal reabsorption – renal correctioncorrection

MicturitionMicturition

Detrusor muscle – Detrusor muscle – smooth muscle smooth muscle

2 sphincters 2 sphincters Internal – smooth Internal – smooth

musclemuscle External – skeletal External – skeletal

musclemuscle Spinal reflex with Spinal reflex with

learned voluntary learned voluntary controlcontrol

Any questions?Any questions?

SummarySummary

Key facts about the kidney – function, Key facts about the kidney – function, anatomyanatomy

Renal blood flowRenal blood flow Glomerular filtration and tubular Glomerular filtration and tubular

functionfunction Regulation of body fluid volume and Regulation of body fluid volume and

electrolyte balanceelectrolyte balance Renal regulation of acid base balanceRenal regulation of acid base balance MicturitionMicturition