Glomerular Filtration I Flashcards
What happens if the body does not filter enough plasma
Edema (fluid overload) and azotemic (too much urea), also electrolyte and acid-base disturbances
Glomerular filtration is governed by
Starling Forces
Glomerular Capillary Pressure (Pc)
largely determines the glomerular filtration rate (GFR)
Glomerular Capillary Pressure (Pc) is largely determined by
arterial BP and afferent and efferent arteriolar resistances
What regulates the afferent and efferent arteriolar resistances?
Hormones and SNS
Increased afferent arteriolar resistances will have what effect on Pc and GFR?
Vasoconstriction of the afferent arteriole causes decreased glomerular capillary pressure and decreased GFR
Increased efferent arteriolar resistances will have what effect on Pc and GFR?
Vasoconstriction of the efferent arteriole causes increased glomerular capillary pressure and increased GFR
Intrinsic permeability (Kf) of the glomerular capillary wall effects GFR, increased Kf causes
increased GFR
The thick ascending limb is _________ to water
impermeable
In the late distal tubule and the collecting duct water permeability depends on
presence of ADH
In the proximal tubule and descending limb of hence, water permeability is dependent on
waters concentration gradient
Na reabsorption drives the reabsorption of
H2O, HCO3, glucose, aa
Na reabsorption drives the secretion of
H+
Na/K ATPase drives the reabsorption of other solutes by
establishing a concentration gradient of Na, which will drive the reabsorption of Na into the cell which drives the transport of other solutes
If too little ATP is available for the Na/K pump
the pump will slow, the concentration gradient will decline and the reabsorption of solutes slows
If the plasma osmolality moves away from its set point then
the hypothalamus signals the release of ADH form the posterior pituitary, and ADH changes the permeability of the collecting duct to H2O
Dilution of the urine is dependent on 2 factors
ability of the thick ascending limb to transport Na w/o H2O and a low level of ADH
Concentration of the urine is dependent on 2 factors
ability of the thick ascending limb to set up a concentration gradient and a high level of ADH
What determines the osmolality of the ECF
Na, Cl, and HCO3
Eating salty food (increased Na intake)
results in reabsorption of H2O to correct the high osmolality (high BP)
ECF osmolality is corrected for at the expense of
ECF volume
How does the body correct for a low ECF? (low Na diet, diuretics)
Reabsorb Na, but the kidney’s cannot produce H2O or Na they must be consumed
What signals determine the kidneys Na and water reabsorption
BP, neural mechanisms, and hormonal mechanisms
Pressure-Naturiesis
Arterial pressure has a DIRECT effect on Na excretion; increased BP = increased excretion of Na via INTRARENAL MECHANISM
How does chronic BP effect pressure-naturiesis
Shifts the relationship to the right or has a depressed slope; higher BP at same steady-state Na intake
Na content regulated
ECF volume
Body water content regulates
plasma osmolarity
SNGFR
single nephron glomerular filtration rate
GFR
sum of all the SNGFR for the 2 kidneys
GFR =
Kf x [(Pgc-Pbs) - (Πgc - Πbs)]
Oncotic pressure of Bowman’s Space Πbs =
0, since there is virtually no proteins in Bowman’s Space
Permeability of the glomerular capillaries is
~100X greater than systemic and is the reason for a high filtration rate
Renal Clearance
amount of solute excreted/ concentration of solute in plasma
Input must equal output
substance IN = excretion (out1) + return to circulation (out2) *as long as it is not being metabolized
How to measure excretion of solute
urine concentration of solute x urine flow rate
Clearance of a solute (Cx) =
(Ux x V) / Px (urine concentration of solute X x urine flow rate) / plasma concentration of solute X
Definition of clearance
the mL of blood plasma completely cleared of a given substance in 1 minute
Clearance of a substance is typically ______ than renal plasma flow
LESS; because most solutes are reabsorbed
Clearance can estimate
GFR and renal plasma flow (RPF)
Clearance determines
net reabsorption or net or secretion of a filtered substance
GRF can be determined by clearance IF
the substance is not metabolized, neither secreted or reabsorbed, and freely filtered
Filtered load =
GFR x plasma concentration of solute
Excretion =
Urine conc of solute x urine flow rate
What substances can be used then to estimate GFR?
Inulin, I-iothalamate, creatinine, Iohexol, Iothalamate
Inulin for GFR estimation
inulin (fructose polymer) infused via IV, gold standard
I-iothalamate for GFR estimation
can predict GFR via plasma clearance alone
Creatinine for GFR estimation
ESTIMATES GFR (some is secreted)
Plasma clearance used for GFR measurement
Give IV bolus of known amount and take plasma samples over time to measure the clearance which = GFR (iohexol, iothalamate)
For chronic kidney disease, creatinine clearance _______ GFR
OVERESTIMATES; more is SECRETED due to higher serum levels
Stage 1 Chronic Kidney Disease, GFR =
> 90ml/min/1.73 elevated GFR
Stage 2 Chronic Kidney Disease, GFR =
60-89 mild decrease in GFR
Stage 3 Chronic Kidney Disease, GFR =
30-59 moderate decrease in GFR
Stage 4 Chronic Kidney Disease, GFR =
15-29 severe decrease in GFR
Stage 5 Chronic Kidney Disease, GFR =
Failure <15
With increasing age, GFR
decreases
Inverse relationship between plasma creatinine concentration and
GFR; if GFR decreases then plasma [creatinine] must increase * assuming production is constant 1.8
if Pcreatinine increases from 1-2mg then GFR
must have decreased
In mild to moderate renal insufficiency, creatinine
is a poor predictor of GFR because as plasma [creatinine] increases section of creatinine increases
plasma creatinine predicts GFR when?
when kidney’s are functioning or when there is severe renal insufficiency (saturated creatinine tubular transporters); for the mild and moderate insufficiency secretion of creatinine overestimates GFR
What substance is cleared by the kidneys in one pass through?
PAH; kidney has HUGE ability to SECRETE PAH without saturation
How can PAH be used to measure RPF
Since PAH is cleared in one pass through, a bolus of PAH will all be excretion (secretion) = entry of RPF
Filtration Fraction
GFR/RPF – the portion of plasma that is filtered
Higher filtration fraction =
greater fraction of plasma that is filtrated
If a solute has a clearance > GFR than
that substance is also secreted
If a solute has a GFR > clearance than
that substance must also be reabsorbed
A substance CANNOT have a clearance > than
RPF
How does protein-binding effect the filtration of a substance?
DECREASES FILTRATION; If a drug or hormone is protein bound than that substance CANNOT be filtered. If it is partially bound, than the fraction of bound to unbound will determine its filtration rate
