Renal Plasma Clearance Flashcards
What is Glomerular Filtration Rate (GFR)?
It is how much filtrate is removed from blood each minute NOT how much blood passes through glomerulus each minute.
What are the differences between glomerular filtrate and
Devoid of cellular elements like RBC
Essentially protein-free
Very similar to plasma except for substances bound to plasma proteins e.g. Ca, fatty acids & hormones like thyroxine
What are the forces that control GFR?
Favouring filtration:
Glomerular capillary pressure (PGC) = 60mmHg
Opposing filtration:
Hydrostatic pressure in Bowman’s space (PBS) = 15mmHg
Osmotic force of plasma proteins (ΠGC) = 29mmHg
PGC – PBS – ΠGC = 16mmHg (net filtration pressure)
What is the rate of urine formation?
GFR – reabsorption rate + secretion rate
What can you use to measure GFR and how?
Inulin as
The rate of filtration through glomerular membrane per minute = the rate of entry into bladder per minute as
Inulin is not absorbed
Inulin is not secreted
note: Since we do not make inulin – have to infuse it in steady iv till its plasma level (Pin) becomes stable. Whilst this continues then timed urine sample is collected, analysis of this gives Uin and V
The plasma inulin concentration becomes stable when the inulin infusion rate equals the inulin excretion rate. When plasma inulin concentration has become stable urine is collected over timed intervals and a blood sample is obtained at the mid-point of each urine collection period. The plasma and urine inulin concentrations and the urine volume collected for each period is measured.
Inulin has same conc in glomerular filtrate as in plasma (experimentally check this in animals by directly sampling from Bowmans capsule with micropipette) and because NOT reabsorbed/secreted it has the same conc in urine
What is the equation for Inulin?
Pin x GFR = Uin x V*
GFR = glomerular filtration rate; ml.min-1 Pin = plasma inulin concentration; mg.ml-1 Uin = urine inulin concentration; mg.ml-1 V* = urine flow rate; ml.min-1
What is the definition of renal clearance?
Renal clearance of a substance is the volume of plasma that is completely cleared of the substance by the kidney per unit of time, (in other words excreted in urine each minute) (expressed in ml/min)
What are the negatives of inulin method?
prolonged infusion
repeated plasma samples
difficult routine clinical use
Advantages of using creatinine to measure GFR?
An intrinsic inert substance Released at ~steady level in plasma from skeletal muscle No infusion needed Freely filtered Not reabsorbed in the tubule
Disadvantages of using creatinine to measure GFR?
Some secreted into the tubule so really the amount excreted slightly exceeds the amount filtered, but because there’s usually a slight error in estimation of plasma [creatinine] these 2 errors cancel each other out and creatinine clearance provides a reasonable estimate of GFR
What is trimethoprim?
trimethoprim competitively inhibits renal tubular creatinine secretion and may cause an artificial increase in serum creatinine, particularly in patients with a pre-existing renal insufficiency; - however, GFR is unchanged;
Trimethoprim competes with Crn for same transporters that secrete Crn from the tubular blood into urine» increase in serum levels of Crn
What is the transport maximum?
The theoretical maximum amount of glucose that can be reabsorbed (20mM.ml-1) before glucose starts appearing in the urine.
What substances have a clearance lower than GFR?
Albumin, Glucose
Actively Reabsorbed (display transport maximum)
All amino acids
clearance = 0ml.min-1 unless excess filtered
pathological conditions (e.g. myeloma) - production of Bence-Jones protein in plasma
Ca2+, Na+, PO42-, Mg2+
Water-soluble vitamins
Passively Reabsorbed
Cl–, urea, some imp. drugs
What substances have clearance higher than GFR?
Substance that is secreted
Filters freely
Secreted actively against an electrochemical gradient
Endogenous substances e.g. weak organic acids & bases, adrenaline, dopamine, steroids.
Exogenous substances e.g. penicillin, probenecid, para-amino hippuric acid (PAH).
How to measure RPF (Renal Plasma Flow)?
Determined from PAH clearance
