Renal Physiology B&B Flashcards
how is fluid compartmentalized between extracellular, intracellular, plasma, and interstitium?
1/3 fluid is extracellular, 2/3 is intracellular
of extracellular fluid, 1/4 is in plasma, 3/4 is interstitial
A patient is administered 120mg of inulin. An hour later the patient has excreted 20mg of inulin in the urine. The plasma inulin concentration is 1 mg/100 mL. What is the extracellular fluid volume for the patient?
amount of inulin in the body = 120 - 20 = 100mg
concentration = 1mg/100mL
ECF = 100mg/0.01mg/mL = 10,000 mL or 10L
*recall inulin can be used to measure ECF volume because it cannot enter cells
what is the effect of 5% dextrose infusion versus mannitol infusion on fluids in the body?
5% dextrose: initially isotonic but becomes hypotonic once sugar is metabolized, raising ECF and ICF fluid volume (essentially free water, will be drawn into cells)
mannitol: raises plasma osmolarity but remains in vascular system because it is large, increasing ECF while decreasing ICF volume and volume in interstitial space (which is drawn into vasculature/ECF)
explain why mannitol infusion is useful in patients with brain swelling
mannitol: raises plasma osmolarity but remains in vascular system because it is large, increasing ECF while decreasing ICF volume and volume in interstitial space (which is drawn into vasculature/ECF)
fluid is drawn out of brain cells/interstitium and into vasculature, allowing it to be excreted by kidneys
how do the following conditions affect effective circulating volume (ECV) vs total body water (TBW)?
a. volume depletion
b. Heart failure.
c. Cirrhosis.
a. volume depletion: low ECV, low TBW
b. Heart failure (low CO): low ECV, high TBW
c. Cirrhosis (low SVR): low ECV, high TBW
how do the following changes affect glomerular filtration rate (GFR) renal filtration fraction (FF)?
a. dilate afferent arteriole
b. constrict efferent arteriole
c. increase protein levels in blood
d. ureter obstruction
a. dilate afferent (arriving) arteriole: increased renal plasma flow (RPF) and pressure in glomerular capsule (GC) —> increased GFR, no change in FF
b. constrict efferent (exiting) arteriole: decreased RPF and increased pressure in GC —> increased GFR, increased FF
c. increase protein levels in blood (higher oncotic pressure in arteriole prevents blood from leaving): no change in RPF, decreased GFR —> decreased FF
d. ureter obstruction: increased pressure in Bowman’s capsule, no change in RPF —> decreased GFR, decreased FF
what are the 2 autoregulation mechanisms in place to maintain a constant GFR/RBF over a range of blood pressures?
(glomerular filtration rate/ renal blood flow)
- myogenic: afferent arteriole constricts with high BP via stretch receptors
- tubuloglomerular feedback: increased urinary flow in tubule increases the amount of NaCl in the distal tubule, which is sensed by macula densa (part of JG apparatus) - induces vasoconstriction of afferent arteriole
how will vomiting, diarrhea, or hemorrhage affect the myogenic and tubuloglomerular feedback mechanisms of renal autoregulation?
these autoregulation mechanisms are meant to maintain a constant GFR/RBF over a range of normal blood pressures
in severe volume loss, renal plasma flow falls significantly and autoregulation is overwhelmed —> GFR decreases, BUN/Creatinine increases, and there is pre-renal failure
if the plasma [Na+] is 8 mg/L but the urine [Na+] is 1 mg/L, and the urine flow rate is 2 L/min, what is the renal clearance of Na+?
renal clearance (Cx) = (Ux * V) / Px
where Ux = urine concentration, V = urine flow rate, and Px = plasma concentration
C(Na) = (2 mg/L x 2 L/min) / (8mg/L)
C(Na) = 0.5 L/min
how can inulin be used to determine GFR?
inulin: neither secreted nor absorbed (all filtered inulin goes out) - amount of blood “cleared” of inulin is the amount of blood filtered by the glomerulus
therefore, inulin clearance (L/min) = GFR, or…
C(inulin) = U(inulin)*V / P(inulin) = GFR
where C = renal clearance, U = urine concentration, V = urine flow rate, and P = plasma concentration
how can creatinine be used to determine GFR?
creatinine: breakdown product of muscle metabolism, closest naturally occurring substance to inulin - all filtered creatinine goes out + a small amount is secreted
secreted Cr is counted as filtered, so GFR will be slightly overestimated: C = (U*V) / P
can also use Cockcroft-Gault formula to estimate GFR using Cr:
CrCl = (140-age)x(Wt in kg)x(0.85 if female) / (72 x Cr)
how does creatinine clearance vary with normal kidney function, chronic kidney disease, and end-stage renal disease?
worsening renal function = high blood Cr level (recall creatinine clearance ~ GFR)
normal: 0.8mg/dL
chronic kidney disease: 2mg/dL
end-stage renal disease (dialysis): 4mg/dL
*note GFR declines with age, but this is not always accompanied by rise in creatinine
what substance can be used to estimate renal plasma flow?
para-aminohippuric acid (PAH) - 100% that enters kidney leaves blood in urine
clearance of PAH (L/min) = plasma to kidney (L/min)
C(PAH) = U(PAH)*V / P(PAH) = RPF
*note PAH clearance underestimates RPF by 10% because not all renal plasma/blood goes to glomeruli
how is renal blood flow (RBF) calculated?
blood = plasma + cells/proteins, and % of cells/proteins ~ % Hct
RBF = RPF / (1 - Hct)
*note you can then also calculate RPF from RBF by: RPF = RBF (1 - Hct)
if renal plasma flow is 1L/min, and the hematocrit is 40%, what is the renal blood flow?
RBF = RPF / (1 - Hct)
RBF = 1 / (1 - 0.4) = 1/ 0.6 = 1.6 L/min
