Transcript Renal system1

Dr. Amal Mahmoud Saeed
Dept. Of Physiology, Faculty of Medicine
University of Khartoum
Lecture 4
Objectives
 To define clearance
 To describe how to measure GFR using the clearance
of Inulin and Creatinine
 To describe how to measure renal blood flow using the
clearance of Para-aminohippurate
Dr. Amal Mahmoud Saeed
Dept. Of Physiology, Faculty of Medicine
University of Khartoum
Lecture 4
Clearance
 Clearance is the volume of plasma from which a
substance is completely cleared to be excreted in
urine per unit time.
 Clearance of substance x is calculated as follows:
Cx = Ux X Vml/min
px
Cx = clearance of substance x
Ux = concentration of substance x in urine
V = urine volume per unit time (urine flow rate)
Px = concentration of substance x in plasma
Clearance
Clearance is used to:
 Measure glomerular filtration rate
(GFR)
 Measure Renal Plasma Flow (RPF)
 Assess tubular transport mechanism
 Assess GFR & RPF in patients
Characteristics of a substance suitable
for measuring the GFR by determining
its clearance:
Freely filtered (i.e. not bound to protein in plasma).
Not reabsorbed by tubules
Not secreted by tubules
Not metabolised
Not stored
Not toxic
Has no effect on filtration rate
Preferably easy to measure in plasma and urine
Inulin Clearance
Amount filtered
Amount filtered = GFR x Plasma
concentration
Amount filtered = GFR x Px
Amount excreted = Urinary concentration x
Urine volume
Amount excreted = Ux x V
Amount
excreted
Measurement of GFR
amount filtered
= amount excreted
GFR x Pc = Uc x Vml/min
GFR = Uc x V
Pc
ml/ min
Measurement of GFR
Substances used to measure GFR:
1. Inulin:
Fructose polymer, molecular weight 5200.
Procedure:
•
•
an intravenous load of inulin is administered
followed by sustained infusion of inulin until
plasma level is constant.
urine is collected for a timed period to get
urine flow rate and urinary concentration of
inulin.
Measurement of GFR
2. Creatinine

product of metabolism of creatine
phosphate in muscles.
 plasma concentration =< 1 mg/dL.
Procedure:
urine is collected for 24 hours.
a sample of plasma to measure creatinine is
obtained at mid-time collection of urine.
Renal Blood Flow Measurement
(RBF)
The substance used is Para amino hippurate
(PAH).
Characteristics:
easily filtered
not reabsorbed
highly secreted
not metabolized
not synthesised
not stored
Renal Blood Flow Measurement
RBF
Fick’s equation:
RBF =
Qx
RAx-RVx
Qx = rate at which substance x consumed
RAx concentration of x in renal artery
RVx = concentration of x in renal vein
If we apply this equation to plasma then
RPF =
Qx
PRAX -PRVX
Renal Blood Flow Measurement
RBF
 If substance x is not metabolised or synthesised
or stored, then rate of consumption by the kidney
can be equated to the rate of its excretion.
(Ux X V)
If substance x is not metabolized by other
tissues, then
PRAx=Px
If substance x is highly secreted, then
PRVx =0
Renal Blood Flow Measurement
RBF
Then
RPF = Ux X V
Px
 Renal vein plasma concentration of PAH is not zero
as 10-15% is not cleared
 PAH clearance=effective renal plasma flow ( ERPF)
RPF = ERPF X 100
90
RBF = RPF X
100
(100-Hct)
Hct =
heamatocrit
RPF = 625 ml/min
To describe the factors that modify GFR
Factors Affecting GFR
1. changes in renal blood flow
Factors Affecting GFR
2. changes in glomerular capillary hydrostatic
pressure
a. changes in systemic blood pressure
b. afferent or efferent arteriolar constriction
Factors Affecting GFR
3. changes in hydrostatic pressure in Bowman’s capsule
a. ureteric obstruction
b. Edema of kidney inside tight renal capsule
Factors Affecting GFR
 4. Changes in capillaries oncotic pressure.
Factors Affecting GFR
 5. increased permeability of glomerular
filtration membrane:
Various diseases
Factors Affecting GFR
6. decrease in total area of glomerular capillary
bed
a. Diseases that destroy glomeruli with or
without destruction of tubules
b. Partial nephrectomy.
Factors Affecting GFR
Summary
1. changes in renal blood flow
2. changes in glomerular capillary hydrostatic
pressure
a. changes in systemic blood pressure
b. afferent or efferent arteriolar constriction
3. changes in hydrostatic pressure in
Bowman’s capsule
a.
b.
ureteral obstruction
Edema of kidney inside tight renal capsule
Factors Affecting GFR
Summary
4. Changes In capillaries Oncotic pressure.
5. increased permeability of Glomerular.
filtration membrane: various diseases.
6. decrease in total area of Glomerular
capillary bed.
a. Diseases that destroy Glomeruli with or without destruction of
tubules
b. Partial nephrectomy.