Filtration and Clearance Flashcards
where does plasma filter in the kidney? what type of ultrafiltrate is produced?
filters in the glomerular capillaries into Bowman’s space
forms protein free ultrafiltrate with plasma ion levels
what causes the net efflux of ultrafiltrate?
differences in the oncotic and hydrostatic pressure across the glomerular capillaries
what is the normal GFR? how long does it take to filter out the ECF volume?
125 ml/min or 180L per day
2 hours
what is the filtration coefficient of the glomerular capillary?
Kf
the product of capillary hydraulic conductivity and the surface area of filtration
how does capillary hydrostatic pressure change from beginning to end of the glomerular capillary? what is this due to?
only by 3 mmHg (small proportion of the total)
this is due to post capillary efferent arteriole constriction
how does the oncotic pressure of the glomerular capillary change from beginning to end? how about the oncotic pressure of the bowman’s capsule?
glomerular capillary- from 25 mmHg to 35mmHg
bowman’s capsule- always 0 mmHg
what is the hydrostatic pressure in the bowman’s capsule?
10 mmHg
why does so much more plasma filter into the bowman’s capsule compared to other body tissues?
because the filtration coefficient is so large and the capillary hydrostatic pressure does not change much due to arteriole constriction
where does glomerular filtration cease?
where starling forces cancel each other out towrad the efferent end of the capillary
describe the basment membrane of the glomerular capillaries and how it effects filtration`
their basement membrane restrict filtration of solutes >1 KDa and its anionic charge favors cation and restricts anion filtration
what cells have foot processes covering glomerular capillaries? what are their purpose?
podocytes
their anionic charge further restricts anionic protein filtration
what do the fenestrations of the glomerular capillaries filter out?
prevent cellular components from being filtered towards the basement membrane
which molecules are freely filtered through the bowman’s capsule?
water and solutes with a diameter of <4nm
the greater the size, the less likely it is to get filtered out
out of the plasma proteins myoglobin, hemoglobin, albumin and insulin, which ones are most likely to be seen in the ultrafiltrate?
insulin (freely filtered) > myoglobin > hemoglobin > albumin
what would occur if charge was removed from the glomerular barrier? what disease is this associated with?
it would increase the passage of anions and overall proteins into the ultrafiltrate
associated with nephritis
what is the cardiac output at rest equivalent to? how much of this perfuses the kidney?
equivalent to blood volume
20% of CO perfuses kidney
how do renal blood flow and renal plasma flow relate? how much of this plasma is filtered at the glomeruli?
RPF is 55% of RBF
GFR is 20% of RPF
what is the filtration fraction?
FF=GFR/RPF (fraction of plasma that is filtered out of the blood and into the bowman’s capsule)
about .2 in a normal person
how many liters of plasma does the kidney filter per day? how does this compare to the ECF volume?
180L which is 10 times the total ECF volume
how does GFR and the rate of urine excretion change with fluid consumed?
GFR stays the same but the amount of urine excreted varies
what occurs to GFR and FF with increasing RPF? how does this occur?
GFR increrases and FF decreases
a greater surface area of the glomerular capillaries filter plasma until maximal rate is achieved
this is still a smaller fraction due to the increased flow
describe the relationship between FF and RPF. why does this occur?
greater fraction of the RPF is filtered at lower rates and a smaller fraction is filtered at high RPF
maintains GFR at levels necessary for renal function when RPF is compromised
what does afferent arteriolar constriction doe to glomerular capillary hydrostatic pressure, RPF and GFR?
decreases capillary hydrostatic pressure
RPF decreases
GFR decreases
what does efferent arteriolar constriction doe to glomerular capillary hydrostatic pressure, RPF and GFR?
increases capillary hydrostatic pressure
RPF decreases
GFR increases
what does efferent and afferent arteriolar constriction doe to glomerular capillary hydrostatic pressure, RPF and GFR?
no change on capillary hydrostatic pressure
RPF decreases
GFR stays the same
what effects does increasing or decreasing plasma protein levels have on GFR?
increasing- decreases GFR
decreasing- increases GFR
what effect does ureter obstruction have on RPF and GFR?
no change on RPF
decreases GFR by increasing glomerular hydrostatic pressure
what causes fluid reabsorption in the post glomerular peritubular capillaries?
starling forces
the peritubular capillary oncotic pressure difference exceeds hydrostatic pressure difference that opposes absorption
describe the relationship of the glomerular and peritubular capillaries
the peritubular capillary is a continuation of the glomerular efferent capillary
describe the net absorptive pressure along the peritubular capillaries?
it decreases from 17mmHg to 12 mmHg
absorption is greater at the beginning of the peritubular capillary
what measurement provides an index of the number of functioning nephrons in kidney disease? when does renal failure begin?
GFR measurement
renal failuer when GFR decreases to below 20 ml/min or a loss of function of 85% of nephrons
what is renal clearance?
the virtual volume of plasma from which a solute is completely removed from by the kidney per unit of time (rate of ml/min)
under what conditions can renal clearance of a solute be used to measure GFR?
if the solute is freely filtered at the glomerulus, if it is not reabsorbed or secreted in the nephron, and if it is not synthesized or metabolized in the kidney
what is inferred about GFR by the renal clearance of an appropriate solute? (equation included)
plasma solute concentration x GFR = urine solute concentration x rate of urine flow
can use this equation to measure GFR
how does the amount of solute filtered in a given time compare to the amount of solute excreted at a given time? what is this fact used for?
they are equal
provides the derivation for finding the GFR
what exogenous solute is used to measure GFR? how is this done?
inulin- a fructose polymer (measured very accurately)
given by IV infusion to maintain constant plasma levels and measured in plasma and urine
what endogenous solute is used to measure GFR? how is this done?
creatinine in the absence of strenuous exercise and disease a constant rate of creatinine diffuses from skeletal muscle to plasma
what does increased plasma creatinine indicate?
decreased clearance from the plasma (major way to detect GFR)
decreased GFR resulting from decreased number of functioning nephrons
what is the normal amount of plasma creatinine? how does the kidney metabolism of creatinine impact GFR estimation?
1mg/100ml
minor secretion by the kidney results in 10% over estimation of GFR
what is the equation for clearance of any solute? what does clearance depend on?
clearance= (urine concentration of the solute x rate of urine flow)/plasma concentration of solute
depends on amount that is reabsorbed in the nephron
what are the possibilities of renal handling of a solute?
just resorption, just secretion or a combination of resorption and secretion in the same or different segments of a nephron
what do clearance ratios use for comparison? what do they indicate?
compare clearance of a solute to inulin or creatinine.
1 indicates net solute secretion into tubular fluid
what is the fractional excretion of water?
the fraction of the glomerular filtrate not reabsorbed from the nepron (excreted)
how does fractional excretion of water compare to the GFR?
FE H2O= rate of urine excretion/GFR
how does fractional excretion of water relate to inulin concentrations? can this property be transferred to creatinine?
FE H2O= plasma concentration of inulin/ urine concentration of inulin
this also can be used for creatinine clearance
what is the fractional excretion of solute? how is it estimated?
the fraction of filtered solute which appears in urine
estimated as the ratio of solute clearance to GFR
what are the equations for fractional excretion of solute?
=clearance of solute/clearance of creatine
=(urine concentration of solute x plasma concentration of creatine)/ (urine concentration of creatine x plasma concentration of solute)
when water and Na balance, what is the fractional excretion of water and Na? what happens in dehydration and positive water balance?
both are 1%
dehydration: FE H2O < 1% and FE Na=1%
positive: FE H2O > 5% and FE Na= 1%
what is fractional reabsorption?
fraction of filtered water or solute which is reabsorbed
1-FE
how does the kidney modulate renal blood flow?
renal vascular resistance increases and decreases with increased and decreased MAP keeping blood flow fairly constant
what is the equation for renal blood flow?
RBF= (renal A-V blood pressure) / renal vascular resistance
what causes autoregulation of RBF?
myogenic response of renal vasculature to pressure changes
what senses the increase or decrease in GFR? what is the result?
tubuloglomerular feedback at macula densa cells sense increase or decrease in GFR
cause increased or decreased resistance of afferent arteriole by contraction or relaxation
