Renal plasma clearance Flashcards
What is renal plasma clearance?
The volume of plasma from which a substance is completely removed
by the kidney per unit time (minute)
What does it mean by metabolic clearance of a substance?
Volume of plasma cleared in unit time
by any mechanism
What percentage of cardiac output does the kidney receive?
20%
What does renal plasma clearance depend on?
- Glomerular filtration hence variables which affect GFR
- Tubular reabsorption and secretion
- Tubular catabolism
What is the mass of insulin?
Insulin is a small peptide with a mass of 5.8 kDa
Where is insulin filtered and what happens to it in tubular cells?
Filtered in the glomerulus, reabsorbed
and broken down in the amino acid
In tubular cells
What are factors affecting GFR?
- Changes in renal blood flow
- Changes in systemic blood pressure
- Afferent and efferent arteriolar constriction
- Changes in hydrostatic pressure in the Bowman’s space
- Ureteral obstruction
- Oedema of kidney in the tight capsule
- Changes in concentration of plasma proteins (dehydration, hypoalbuminemia)
- Changes in filtration barrier
- Changes in effective filtration surface area
What are the 3 layers that make up the glomerular filtration barrier?
- Endothelium
- Basement membrane
- Podocytes
What is the size of the endothelium in the glomerular filtration barrier?
Endothelial fenestration size (70-90 nm)
What is the size of the podocytes in the glomerular filtration barrier?
Filtration slit (podocytes) (25 nm)
What does glomerular filtration depend on?
Hence the filtration of a substance depends on size, charge and plasma free concentration
What is the equation for urinary excretion rate?
Urinary excretion rate = GFR – reabsorption rate + secretion rate
What is the rate of urine clearance of creatine?
- Filtration only
What is the rate of urine clearance of electrolytes?
- Filtration
- Partial reabsorption
What is the rate of urine clearance for Glucose and amino acids?
- Filtration
- Complete reabsorption
What is the rate of urine clearance for organic acids?
- Filtration
- Secretion
What is inulin, not insulin?
An inert polysaccharide, MW ~5,000
What is inulin’s filtration properties in the glomerular membrane?
Filters freely through the glomerular membrane
Does anything else happen to Inulin during its clearance, apart from filtration?
No, not absorbed, secreted or metabolised
What is the rate of filtration of inulin through glomerular membrane per minute equal to?
Rate of filtration through glomerular membrane per minute = Rate of entry into bladder per minute
therefore
Pin x GFR = Uin x V*
What is the equation for rate of filtration for inulin?
Rate of filtration = [Pin] x GFR
What is the equation for rate of entry into bladder?
[Uin] x urine flow rate (V*)
What is clearance of a substance which is not secreted or reabsorbed in the tubule equal to and why?
Clearance of a substance which is not secreted or reabsorbed in the tubule is equal to glomerular filtration rate; its because the volume of plasma cleared is the volume filtered
What are exogenous agents used to measure GFR?
A. 125I-iothalamate
B. 59Cr-EDTA
C. 99mTc-DTPA
What are endogenous agents used to measure GFR?
A. Creatinine
B. Cystatin C
