renal plasma clearance Flashcards
Glomerular Filtration Rate (GFR)
how much filtrate is removed from the blood each minute
pressure that favours filtration
hydrostatic pressure in the glomerular capillaries
pressures that don’t favour filtration
- oncotic pressure/colloid osmotic pressure generated by the plasma proteins (mainly albumin) in the blood
- hydrostatic pressure of the Bowman’s space
Urinary excretion rate
GFR - reabsorption + secretion
how much of the plasma that flows through the glomerular capillaries is filtered?
20% (other capillary beds = 1%)
means that 80% doesn’t go through the nephron, just remains in the CVS
why is GFR important:
- contributes to rapid removal of waste products
- important clinical indicator of renal function, as it is affected by the pressures mentioned above, surface area and capillary permeability - if any of these change the GFR will also change
2 substances which have been used to measure GFR, and why are they used?
inulin and creatinine
used because because nothing is secreted, reabsorbed or metabolised so the concentration of the substances in the plasma will equal the concentration of these substances in the urine
Rate of filtration through glomerular membrane per minute = Rate of entry into bladder per minute
what is inulin?
an inert polysaccharide that filters freely through the glomerular membrane and is not absorbed, secreted or metabolised
(for inulin and creatinine) the rate of filtration through glomerular membrane per minute equals….
the rate of entry into bladder per minute
since we do not make inulin how does it get into the body?
have to infuse it in steady iv until its plasma level becomes stable
when does the inulin plasma level become stable?
when the inulin infusion rate equals the inulin excretion rate
when plasma inulin concentration has become stable what happens?
1) at timed intervals, plasma and urine inulin concentrations and the urine volume is collected and measured using a micropipette
2) a blood sample is obtained at the mid-point of each urine collection period
GFR equation
Pin
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 V* and how is it calculated?
the urine flow rate
calculated by dividing the urine volume collected by the duration in minutes of the collection period
renal clearance
the volume of plasma that is completely cleared of the substance by the kidney per unit of time, ml/min (ie. excreted in urine)
where does all the inulin that is filtered into the bowman’s capsule end up?
ends up in urine
formula of renal clearance
Cs x Ps = Us x V*
where Cs = clearance rate of substance (s) Ps = plasma concentration of s Us = urinary concentration of s V* = urine flow rate
drawbacks to the inulin method?
- prolonged infusion
- repeated plasma samples
- difficult routine clinical use
advantages of the creatinine method?
- inert substance
- Released at a steady level in plasma from skeletal muscle
- No infusion needed (because it is intrinsic ie. made by the body in the skeletal muscle)
disadvantages of the creatinine method?
some secreted into the tubule so more goes into urine
where is the majority of creatine found?
95% found in muscular tissues
how does Creatine (Cr) get into the muscle?
-muscle has virtually no Cr-synthesizing capacity so Cr has to be taken up from the blood against a large concentration gradient by a saturable, Na+- and Cl−-dependent Cr transporter that spans the plasma membrane
how is the daily demand for Cr met?
by intestinal absorption of dietary Cr or by de novo Cr biosynthesis
how is creatine converted to creatinine?
creatine (Cr) is metabolised to phosphocreatine (PCr) in the skeletal muscle by creatine kinase - this process generates a lot of energy, and it’s a reversible reaction
creatine and phosphocreatine can be non-enzymatically converted to creatinine which is released from the muscle (diffuses out of the cells) and is excreted by the kidneys into urine.
