R l Ph i lRenal PhysiologyRenal filtration and clearanceRenal filtration and clearance
D K t D tDr Kate [email protected]
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.