Renal Plasma Clearance
Key Topics:
The definition of glomerular filtration rate (GFR)
The rate of urine formation and the processes that control it
The importance of GFR to proper functioning of the kidneys
Description of different methods to measure GFR in terms of excretion of
particular substance
The inulin method to measure GFR
Derivation of the formula to calculate GFR from inulin viz. GFR=urinary inulin
concentration (Uin) x urine flow (V)/plasma inulin concentration (Pin)
Definition of renal clearance
Derivation of formula to calculate clearance of any substance (S) = urinary
concentration of S (Us)x urine flow (V)/plasma concentration of S (Pin)
Why inulin clearance and GFR are identical
Graph showing relationship between inulin clearance and plasma concentration
for inulin and how this related to GFR
The creatinine method for calculation of GFR and why this is preferred clinically
Graph showing relationship between plasma creatinine concentration and GFR
and the relationship to clearance
Graph showing decline in GFR with age
Comparison of clearance rates for different substances compared to inulin and
what this tells us about how they are treated by the kidney
Substances with a clearance of less than GFR e.g. albumin, glucose and how
tubule handles them
Graphs of filtration rate at the glomerulus against excretion rate and
secretion/reabsorption rate against the plasma concentration for glucose
Using the graphs to explain the concepts of plasma excretion threshold and “Tm”
Graphs of the clearance rate against the plasma concentrations of inulin
Graphs of the clearance rate against the plasma concentration of glucose
Substances with a clearance greater than GFR e.g. penicillin, para-amino hippuric
acid (PAH) and how tubules handle them
What is PAH? How is it secreted in tubules? What is it used for?
How PAH clearance studies can be used to measure renal plasma flow and renal
blood flow
The term ‘renal filtration fraction’
How the ‘renal filtration fraction’ can be measured
Introduction
The first step in urine formation begins with glomerular filtration. Most substances
in plasma are freely filtered so the glomerular filtrate is the same composition as
plasma except it is devoid of cellular elements like RBCs and is essentially protein
free. Because the proteins cannot get through, substances bound to protein will also
not enter the filtrate, like calcium, fatty acids and hormones like thyroxine
For this reason urine is routinely tested on wards for protein (proteinuria)
Proteinuria is a sign of renal/urinary tract disease.
Remember that 20% of the blood that passes through the glomerulus is filtered.
, Glomerular filtration rate (GFR) is how much filtrate is removed from the blood each
minute, not how much blood passes through the glomerulus each minute.
The force favouring filtration at the glomerulus is the glomerular capillary pressure
(PGC), usually around 50-60mmHg
Opposing filtration are two forces; the hydrostatic pressure in the Bowman’s space
(PBS) around 15mmHg and the osmotic force (oncotic) of plasma proteins (ΠGC)
around 29mmHg.
Therefore, PGC – (PBS + ΠGC) = 16mmHg is the net filtration pressure
Rate of Urine Formation
The rate at which different substances are excreted into urine is the sum of three
renal processes: the rate at which it is filtered minus its reabsorption plus the rate of
secretion.
Remember that only 20% of blood flowing through the kidneys is filtered through
the glomerular capillaries. Hence every minute around 625ml/min of plasma goes to
the kidney, this is renal plasma flow. Of this 625ml/min of plasma that enters the
glomerulus, 125ml/min are filtered.
Of this, almost all the water is reabsorbed and put back into the blood. So for a
substance (S) that is in plasma and freely filtered in Bowman’s capsule and is not
subsequently reabsorbed/secreted, all the (S) that is in that filtered plasma will be
cleared into urine.
I.e. 125ml in 1min, the other 500ml which is not filtered keeps all its (S) as it cannot
get into the urine.
Urinary Excretion Rate = GFR – reabsorption rate + secretion rate
Key Topics:
The definition of glomerular filtration rate (GFR)
The rate of urine formation and the processes that control it
The importance of GFR to proper functioning of the kidneys
Description of different methods to measure GFR in terms of excretion of
particular substance
The inulin method to measure GFR
Derivation of the formula to calculate GFR from inulin viz. GFR=urinary inulin
concentration (Uin) x urine flow (V)/plasma inulin concentration (Pin)
Definition of renal clearance
Derivation of formula to calculate clearance of any substance (S) = urinary
concentration of S (Us)x urine flow (V)/plasma concentration of S (Pin)
Why inulin clearance and GFR are identical
Graph showing relationship between inulin clearance and plasma concentration
for inulin and how this related to GFR
The creatinine method for calculation of GFR and why this is preferred clinically
Graph showing relationship between plasma creatinine concentration and GFR
and the relationship to clearance
Graph showing decline in GFR with age
Comparison of clearance rates for different substances compared to inulin and
what this tells us about how they are treated by the kidney
Substances with a clearance of less than GFR e.g. albumin, glucose and how
tubule handles them
Graphs of filtration rate at the glomerulus against excretion rate and
secretion/reabsorption rate against the plasma concentration for glucose
Using the graphs to explain the concepts of plasma excretion threshold and “Tm”
Graphs of the clearance rate against the plasma concentrations of inulin
Graphs of the clearance rate against the plasma concentration of glucose
Substances with a clearance greater than GFR e.g. penicillin, para-amino hippuric
acid (PAH) and how tubules handle them
What is PAH? How is it secreted in tubules? What is it used for?
How PAH clearance studies can be used to measure renal plasma flow and renal
blood flow
The term ‘renal filtration fraction’
How the ‘renal filtration fraction’ can be measured
Introduction
The first step in urine formation begins with glomerular filtration. Most substances
in plasma are freely filtered so the glomerular filtrate is the same composition as
plasma except it is devoid of cellular elements like RBCs and is essentially protein
free. Because the proteins cannot get through, substances bound to protein will also
not enter the filtrate, like calcium, fatty acids and hormones like thyroxine
For this reason urine is routinely tested on wards for protein (proteinuria)
Proteinuria is a sign of renal/urinary tract disease.
Remember that 20% of the blood that passes through the glomerulus is filtered.
, Glomerular filtration rate (GFR) is how much filtrate is removed from the blood each
minute, not how much blood passes through the glomerulus each minute.
The force favouring filtration at the glomerulus is the glomerular capillary pressure
(PGC), usually around 50-60mmHg
Opposing filtration are two forces; the hydrostatic pressure in the Bowman’s space
(PBS) around 15mmHg and the osmotic force (oncotic) of plasma proteins (ΠGC)
around 29mmHg.
Therefore, PGC – (PBS + ΠGC) = 16mmHg is the net filtration pressure
Rate of Urine Formation
The rate at which different substances are excreted into urine is the sum of three
renal processes: the rate at which it is filtered minus its reabsorption plus the rate of
secretion.
Remember that only 20% of blood flowing through the kidneys is filtered through
the glomerular capillaries. Hence every minute around 625ml/min of plasma goes to
the kidney, this is renal plasma flow. Of this 625ml/min of plasma that enters the
glomerulus, 125ml/min are filtered.
Of this, almost all the water is reabsorbed and put back into the blood. So for a
substance (S) that is in plasma and freely filtered in Bowman’s capsule and is not
subsequently reabsorbed/secreted, all the (S) that is in that filtered plasma will be
cleared into urine.
I.e. 125ml in 1min, the other 500ml which is not filtered keeps all its (S) as it cannot
get into the urine.
Urinary Excretion Rate = GFR – reabsorption rate + secretion rate
