R l Ph i lRenal PhysiologyRenal filtration and clearanceRenal filtration and clearance

D K t D tDr Kate DentonKate.Denton@monash.edu

MEDICAL SCIENCESNaish Revest Court Chapter 14Naish, Revest, Court – Chapter 14

TEXTBOOK OF MEDICAL PHYSIOLOGYGuyton & Hall Chapters 25 29Guyton & Hall - Chapters 25-29

LECTURE 2: Processes involved in the formation of urine

Glomerular filtration

ObjectivesObjectivesAfter this lecture you should be able to:

• Define the basic processes involved in the formation of urineD ib th t f th l l• Describe the anatomy of the glomerulus– Including the structure of the filtration barrier

• Understand the forces driving glomerular filtration• Define glomerular filtration rate (GFR)• Understand how renal autoregulation maintains a constant

GFRGFR• Describe clearance techniques for the measurement of

renal blood flow and GFR

Processes involved in urine formationProcesses involved in urine formation

• Glomerular filtration• Glomerular filtration • Tubular reabsorption

T b l ti• Tubular secretion

• It is the balance between all components pthat determines the volume and composition of the urine excreted

Overview of Urine Formation in the NephronO e e o U e o at o t e ep o

Afferent arterioleAfferent arterioleGlomerulus

Efferentarteriole

Glomerularcapsule

Nephrontubule

Peritubularcapillary

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Excretion = Filtration - Reabsorption + Secretionc et o t at o eabso pt o Sec et o

Gl l filt ti• Glomerular filtration– the movement of fluid and

solutes from the glomerular capillaries into Bowman’s space.p p

• Tubular reabsorption – the movement of materials from

Filtrationthe filtrate in the tubules into the peritubular capillaries.

– Taking fluid back into the body!

Reabsorption

Secretion

• Tubular secretion– the movement of solutes from

the peritubular capillaries into Secretionthe peritubular capillaries into the tubules

– Removing fluid from the body

Urinary Excretion

Sites of filtration, reabsorption & excretionS tes o t at o , eabso pt o & e c et o

Filtration, reabsorption & excretionFiltration, reabsorption & excretion

Glomerular FiltrationGlomerular Filtration

The Renal CortexThe Renal Cortex

Afferentt i larteriole

Efferent arteriole120 m

Glomerular Filtration Barrier

Glomerular Filtration Barrier–Endothelial fenestrations–Basement Membrane–Podocytes & slit diaphragm

Fenestrated capillaryFenestrated capillary

Glomerular Filtration Fluid forced through filtration barrier by hydrostatic pressure

Gl l Filt ti B i

Fluid forced through filtration barrier by hydrostatic pressureGlomeruli Mechanical Filters

• Glomerular Filtration Barrier– Endothelial fenestrations– Basement Membrane (-ve charged)– Podocytes & slit diaphragm

• Glomerular capillaries more efficient filters than other capillaries

– very large fenestrations– high hydrostatic pressures driving

filtration (55 vs 18 mmHg)

Filterability of solutesySize & Charge

small molecules (<3nm or 7000MW) filtered freely>7-9nm or 70000MW essentially blockedMost proteins prevented due to negative charge they carryMost proteins prevented due to negative charge they carry

Filtrate inside BC is virtually identical to plasma but essentially free of protein (0.02%)

Glomerular filtration barrierGlomerular filtration barrier

• Filtration barrier consists of 3 layers:• Filtration barrier consists of 3 layers: – Single celled capillary endothelium (fenestrated),– Non-cellular basement membrane (-ve charge), – Single celled epithelial lining of Bowmans capsule (podocytes, slit

diaphram)

• Rate of filtration– Due to the hydrostatic pressure of the cardiac pump– fluid is filtered from the blood through fenestra in the glomerular

capillaries into slit pores between the foot processes of the podocytes.

Glomerular filtration ratenet filtration pressure X Kf

• SNGFR = single nephron glomerular filtration rate• SNGFR = single nephron glomerular filtration rate= NFP x Kf

• Net filtration pressure = (NFP) = PGC - PBS - GC

• Kf = glomerular filtration coefficient= k . S

= hydraulic conductivity X glomerular capillary surface area

Wh l kid T t l GFR th f th SNGFR f• Whole kidney or Total GFR = the sum of the SNGFR for each individual nephron for both kidneys.

PGC = Glomerular capillary pressure PBS = Bowman’s Space pressureGC = Glomerular capillary oncotic pressure

Forces driving glomerular filtrationForces driving glomerular filtration

Starlings Law

F i f h l l fil i B ’ lFormation of the glomerular filtrate in Bowman’s capsule is the outcome of opposing pressures: - Hydrostatic pressure from the heart favors filtration,y p ,- Plasma osmotic pressure and hydrostatic pressure of the filtrate oppose it.

Glomerular Filtration Rate: Kff

Kf = glomerular filtration coefficient= k . S

= hydraulic conductivity X glomerular capillary surface area

The glomerulusThe glomerulus

• AA afferent arteriole• EA efferent arteriole

MD

DT

EA efferent arteriole• G renin containing cells• M mesangial cell• B Bowman’s capsule• B Bowman s capsule• BS Bowman’s space• EN endothelial cell

EP ith li l ll• EP epithelial cell• F foot processes• BM basement membrane• PT proximal tubule• DT distal tubule• MD macula densa

Summary: Glomerular filtrationSummary: Glomerular filtration

• Passive non-selective process• Passive, non-selective process

• Small molecules – water, glucose, amino acids pass freely

• Larger moleculesg– cannot freely cross the glomerular filtration barrier.– Proteins. Thus protein in the urine indicates a renal problem!

• Glomerular filtration rate (GFR)– Equals the volume of filtrate formed each minute.

• GFR is directly proportional to the net filtration pressure

Glomerular Filtration Rate

Aff EffCap

Glomerular Filtration Rate

Aff

BC

Eff

SNGFR = Kf x NFPSNGFR= Kf x (PGC + BC) - (PBC + G) BCSNGFR Kf x (PGC BC) (PBC G)

Kf: In disease basement membrane can thicken GFRor glomerular capillaries can be destroyedor glomerular capillaries can be destroyed

G: Liver disease plasma proteins G NFP GFRDehydration G NFP GFR

PBC: Kidney stones PBC NFP GFR

BC: Normally low but if NFP GFRBC y

PGC: Primary means of changing GFR PGC GFR PGC GFR

Filtration fraction - 20% plasma filtered

Glomerular capillariesAfferent arteriole Efferent arteriole

Peritubular 1. Filtration

Peritubular capillaries

2 Reabsorption2. Reabsorption

3. Secretion

Excretion

Renal vein

Excretion = Filtration - Reabsorption + Secretion

E

The glomerulus is uniqueThe glomerulus is uniquehaving two resistance beds that control the pressure within it

Th t f hi h t i t (th i t f t t d liThe two areas of highest resistance (the points of greatest decline in blood pressure) are the afferent and efferent arterioles.

Control of glomerular capillary pressure

Aff EffCap

Arterial Pressure and Relative resistance of Afferent vs Efferent Determine PG

Aff EffAP PG GFR RBF1. Arterial Pressure

AAR PG GFR RBF2. Afferent Arteriolar Resistance

AAR PG GFR RBF

Resistance

EAR GFR RBFPG

3. Efferent Arteriolar Resistance G

EAR PG GFR RBF

Resistance

Control of GFR

• GFR needs to be relatively constant: reabsorption of H2O & other substances from filtrate partly reabsorption of H2O & other substances from filtrate partly

dependent on rate of flow through tubules

GFRinadequate reabsorption substances lost in urine GFRinadequate reabsorption substances lost in urine GFR reabsorption increasedwastes not excreted

• Small changes in GFR equal large changes in the volume of filtrate that must be processed.

10% i i GFR l 18L filt t 10% increase in GFR equals 18L more filtrate to be processed

RBF and GFR are maintained at l ti l t t l lrelatively constant levels

A toreg lation• Autoregulation -– The ability of an organ to

maintain its blood flow nearly constant despite changes in

RBF(ml/min)

constant despite changes in arterial pressure.

RBF and GFR do not change GFR• RBF and GFR do not change when arterial pressure is between 70-150 mmHg

GFR(ml/min)

• Urine and sodium outputIS NOT autoregulated.

Urineflowrate(ml/min)

• Urine flow rate is directly proportional to arterial pressure

(ml/min)

Arterial pressure(mmHg)

0 200

p opo t o a to a te a p essu e“Pressure-Natriuriesis”

(mmHg)

ThThe Juxta-Gl lGlomerularApparatus

Autoregulation occurs via two mechanismsuto egu at o occu s a t o ec a s s

1. Myogenic Mechanism (stretch response)i i bl d ff i l i– increases in blood pressure causes afferent arterioles to constrict

– causing decreased blood flow to glomerulus– vice versa

Vascular smooth muscle cells contract to “counter” pressure

in the vessel lumen

P = Intraluminal PressureP

in the vessel lumen

P

L l ’ LLaplace’s Law

Tension = Pressure x radiusAfferent arteriole

The Juxta-Glomerular Apparatusmacula densa

Macula Densa:specialised cells in the distal tubule

2. Tubuloglomerular Feedback (TGF)Arterial Pressure Aff Eff

Cap

AP PG

Glomerular Hydrostatic Pressure

(-) (-) GFR

macula densa responds to changes in flow (and ion content).

GFR Proximal NaCl

reabsorption

Macula Densa NaCl

p

Renin

AAR EAR

PG

G

Angiotensin II

Eff t A t i lAff t A t i l GFR(back to normal)

Efferent Arteriole Resistance

Afferent Arteriole Resistance

Autoregulation is not perfectAutoregulation is not perfect

• Autoregulation doesn’t occur at arterial pressures below 70 mmHg and above p g150 mmHg. Therefore at pressures above or below these values, changes in RBF and GFR occur.

• Despite autoregulation, RBF and GFR can be altered considerably, even within the autoregulatory range.

Why Test Renal Function?Why Test Renal Function?

T id tif l d f ti• To identify renal dysfunction.• To diagnose renal disease.• To monitor disease progress.• To monitor response to treatment.To monitor response to treatment.• To assess changes in function that may

impact on therapy (e g Digoxinimpact on therapy (e.g.Digoxin, chemotherapy).

Biochemical Tests of Renal Function

• Urinalysis• Urinalysis– Appearance– Specific gravity and osmolalityp g y y– pH– Glucose– Protein– Urinary sediments?

• Measurement of GFR• Measurement of GFR– Clearance tests– Plasma creatinine

• Tubular function tests

Renal Clearance - definition

•The volume of plasma that is completely cleared ofThe volume of plasma that is completely cleared of a particular substance by the kidney per unit of time

12

3 1 hour500ml

4

3 1 hour

Pl U i TiPlasma Urine Time

In this case, the clearance of blue molecules is 500ml/hour

For each substance in plasma, a particular combination of filtration reabsorption & secretion occursfiltration, reabsorption & secretion occurs.

Substance A: Not filtered (eg large proteins)

Substance B: Filtered but not reabsorbed or secreted

Substance C: Filtered, completely reabsorbed & not ( g g p )

(eg inulin)p y

secreted. (eg glucose)

Substance D: Filtered, some Substance E: Filtered, not Substance F: Filtered, not reabsorbed, not secreted (eg. many electrolytes)

reabsorbed, some secreted reabsorbed, completely secreted (eg PAH)

Renal Clearance

The clearance of substance sThe clearance of substance s– filtered, maybe reabsorbed and/or secreted

• Ps: concentration of s in plasma (mg/ml)• Us: concentration of s in urine (mg/ml)• V: urine flow rate (ml/min)

amount of s removed from plasma = amount of s excretedamount of s removed from plasma = amount of s excreted amount of s removed from plasma: clearance (ml/min) x Ps (mg/ml)amount of s excreted: V (ml/min) x Us (mg/ml) =mg/min

clearance (ml/min) x Ps (mg/ml) = V (ml/min) x Us (mg/ml)

Clearance (ml/min) = V (ml/min) x Us (mg/ml) Ps (mg/ml)Ps (mg/ml)

Renal ClearanceM t f Gl l Filt ti R t (GFR)Measurement of Glomerular Filtration Rate (GFR)

• Substance that is• Substance that is– freely filtered (small)– Not reabsorbed, nor secreted, nor metabolised– amount filtered = amount excreted

Inulinl h id (5000MW)– polysaccharide (5000MW)

– derived from roots of certain plants (ie not naturally occurring) in body)

– non toxic, able to be measured in plasma & urine using simple techniques

– Clearance of inulin = GFR

GFR (ml/min) = V (ml/min) x Uinulin (mg/ml)Pinulin (mg/ml)Pinulin (mg/ml)

Renal ClearanceM t f Gl l Filt ti R t (GFR)Measurement of Glomerular Filtration Rate (GFR)

Inulin solution is infused intravenously• constant plasma [inulin]

– Timed urine collection taken• urine flow rate (urine volume/time)

– Blood sample taken in middle of urine collection periodp p– Plasma & urine samples analysed for inulin

GFR (ml/min) = V (ml/min) x Uinulin (mg/ml)Pinulin (mg/ml)

– Eg. V = 1 ml/minUinulin = 125 mg/mlPi li 1 / lPinulin = 1 mg/ml

GFR (ml/min) = 1 ml/min x 125 mg/ml1 mg/mlg

= 125ml/min

Renal Clearance

Measurement of GFRMeasurement of GFR– Inulin

• Best method for measurement of GFR– used a lot in laboratoryused a lot in laboratory

• Inconvenient for clinical use– doesn’t occur naturally & must be infused at a continuous &

constant rate for hrs (invasive)Creatinine– Creatinine

• Endogenous– by-product of skeletal muscle metabolism

• plasma levels quite constantp q• most widely used method clinically

– single blood sample & 24hr urine sample• BUT small amount of creatinine is secreted

– amount excreted > amount filteredamount excreted > amount filtered– overestimation– reliable marker

Plasma creatininea marker of renal function

• Single blood sample• Single blood sample– Simple

• SenstivityP b t ti t– Poor between patients

– Great across time

PCr not a sensitive marker

Properties of Agents used to Determine GFRope t es o ge ts used to ete e G

Property Urea Creatinine Inulin 99mTcDTPA

Not Protein Yes Yes Yes YesNot ProteinBound

Yes Yes Yes Yes

FreelyFiltered

Yes Yes Yes YesFilteredNo secretionor absorbtion

Flow relatedreabsorption

Somesecretion

Yes Yes

Constantendogenousproduction

t

No Yes No No

rateEasilyAssayed

Yes Yes No No

Renal ClearanceMeasurement of Renal Plasma Flow (RPF)

– substance that is• freely filtered• Completely secreted

Para-aminohippurate (PAH)• organic anion• not naturally occuringinfused• freely filtered• almost all secreted at low [PAH]

Why should [PAH] be kept low?Why should [PAH] be kept low?

Renal ClearanceMeasurement of Renal Plasma Flow (RPF)

– ~10% of total renal plasma flow supplies non filtering & non secreting portions of kidneys (peripelvic fat)

Cl f PAH Eff ti R l Pl Fl (ERPF)– Clearance of PAH = Effective Renal Plasma Flow (ERPF)ERPF (ml/min) = V x UPAH

PPAH– Eg. V = 1 ml/min

UPAH= 5.85 mg/mlPPAH = 0.01 mg/L

ERPF = 1 ml/min x 5.85 mg/ml0 01 mg/L0.01 mg/L

= 585 ml/min

Renal ClearanceMeasurement of Renal Plasma Flow (RPF)

– Extraction ratio of PAH is 0.9 (ie PAH is 90% cleared from lplasma

RPF = ERPF/ Extraction ratio of PAH= 585 ml/min / 0.9= 650 ml/min

Measurement of Renal Blood Flow (RBF)( )RBF = RPF (ml/min)

(1-Haematocrit)= 650 ml/min= 650 ml/min

(1-0.45)= 1.2 L/min

(Haematocrit = volume of red blood cells in blood, expressed as percentage, usually equals approx 45%)

Revision questionsRevision questions

Correctly label this diagram of the glomerulus• Correctly label this diagram of the glomerulus

Revision questionsRevision questions

• Describe the pathway of urine formation (from when blood• Describe the pathway of urine formation (from when blood enters the kidney to urine arrives at the bladder)

D ib th f d i i l l filt ti t• Describe the forces driving glomerular filtration rate– What is the primary determinate of changes in GFR?– How do changes in afferent and efferent arteriole resistance effect this?

• Describe the glomerular filtration barrier.– What are the specialised featuresp

• Describe the two intrinsic mechanisms by which the kidney keeps GFR constantkeeps GFR constant.

• Explain the concept of renal clearance of a substance.– Give examples of substances which can be used to measure GFR and

renal blood flow.