View
54
Download
1
Category
Tags:
Preview:
DESCRIPTION
The Renal System. 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. Measurement of G lomerular Filtration R ate (GFR). Clearance. - PowerPoint PPT Presentation
Citation preview
Dr. Amal Mahmoud SaeedDept. Of Physiology, Faculty of MedicineUniversity of Khartoum Lecture 4
ObjectivesTo define clearance To describe how to measure GFR using the
clearance of Inulin and CreatinineTo describe how to measure renal blood flow
using the clearance of Para-aminohippurate
Dr. Amal Mahmoud SaeedDept. Of Physiology, Faculty of MedicineUniversity of Khartoum Lecture 4
ClearanceClearance 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 = 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
Cx = Ux X Vml/min
px
Clearance
Measure glomerular filtration rate (GFR)
Measure Renal Plasma Flow (RPF)
Assess tubular transport mechanism
Assess GFR & RPF in patients
Clearance is used to:
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 tubulesNot secreted by tubulesNot metabolised Not stored Not toxicHas no effect on filtration ratePreferably easy to measure in plasma and
urine
Inulin ClearanceAmount filtered Amount filtered
Amount excretedAmount excreted
cort
exco
rtex
med
ulla
med
ulla
Amount filtered = GFR x Plasma concentration
Amount excreted = Urinary concentration x Urine volume
Amount filtered = GFR x Px
Amount excreted = Ux x V
Measurement of GFR
amount filtered = amount excreted GFR x Pc = Uc x Vml/min
GFR = Uc x VPc
ml/ min
Measurement of GFR
• 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.
1.Inulin:Procedure:
Substances used to measure GFR:
Fructose polymer, molecular weight 5200.
Measurement of GFR product of metabolism of creatine
phosphate in muscles. plasma concentration =< 1 mg/dL.
2.Creatinine
urine is collected for 24 hours.
a sample of plasma to measure creatinine is obtained at mid-time collection of urine.
Procedure:
Renal Blood Flow Measurement(RBF)
The substance used is Para amino hippurate (PAH).
Characteristics: easily filtered not reabsorbed highly secretednot metabolizednot synthesisednot stored
Renal Blood Flow Measurement RBF
RPF = Qx
RBF = Qx
Fick’s equation:
RAx-RVxQx = rate at which substance x
consumedRAx concentration of x in renal arteryRVx = concentration of x in renal veinIf we apply this equation to plasma then
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
(100-Hct)
RBF = RPF X 100
Then
Renal vein plasma concentration of PAH is not zero as 10-15% is not cleared
PAH clearance=effective renal plasma flow ( ERPF)
RPF = Ux X V
Px
RPF = ERPF X 100 90
Hct = heamatocrit
RPF = 625 ml/min
To describe the factors that modify GFR
Factors Affecting GFR1. changes in renal blood flow
Factors Affecting GFR2. changes in glomerular capillary
hydrostatic pressurea. changes in systemic blood pressureb. afferent or efferent arteriolar
constriction
Factors Affecting GFR3. changes in hydrostatic pressure in
Bowman’s capsule
a. ureteric obstructionb. Edema of kidney inside tight renal
capsule
Factors Affecting GFR4. Changes in capillaries oncotic
pressure.
Factors Affecting GFR5. increased permeability of
glomerular filtration membrane:
Various diseases
Factors Affecting GFR6. decrease in total area of glomerular
capillary bed
a. Diseases that destroy glomeruli with or without destruction of tubules
b. Partial nephrectomy.
Factors Affecting GFR1. changes in renal blood flow2. changes in glomerular capillary
hydrostatic pressurea. changes in systemic blood pressureb. afferent or efferent arteriolar constriction
3. changes in hydrostatic pressure in Bowman’s capsule
a. ureteral obstruction b. Edema of kidney inside tight renal capsule
Summary
Factors Affecting GFRSummary
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.
4. Changes In capillaries Oncotic pressure.
Recommended