Glomerulus, RBF, GFR Flashcards
ultrafiltrate
fluid that passes from capillaries into pores; contains nutrients, waste, small peptides, unbound/partially bound ions
layers of selective permeability
- glomerular pores
- basement membrane
- slit diaphragms
glomerular pores
openings in the endothelium that restrict molecule filtration by size and charge
negatively charged - prevent filtration of negatively charged ions/proteins
slit diaphragms
filtered openings between podocyte foot processes
renal blood flow (RBF)
the volume of blood delivered to the kidneys per unit time
glomerular filtration rate (GFR)
rate at which blood is filtered each minute
depends on hydraulic permeability + pore surface area (Kf), and net filtration pressure (NFP)
GFR = Kf x NFP
net filtration pressure (NFP)
forces favoring filtration - forces opposing filtration
NFP = P(GC) - [pi(GC) + P(BC)]
forces favoring filtration
P(GC): hydrostatic pressure in capillary
pi(BC): oncotic pressure in Bowman’s space – equals ZERO in health
forces opposing filtration
P(BC): hydrostatic pressure in bowman’s space
pi(GC): oncotic pressure in capillary
what does it mean if NFP = 0
only filtration is occurring (forces favoring filtration = forces opposing filtration)
NO reabsorption or secretion
what does it mean if NFP < 0
filtration AND reabsorption
(forces opposing filtration > favoring filtration)
do NOT want this to occur in the glomerulus
does NFP change along the length of the capillary
yes - NFP decreases along the length of the capillary
as solutes exit the capillary, P(GC) decreases –> LESS FAVORING FORCES
as solutes exit, proteins in the capillary become more concentrated causing pi(GC) to increase –> MORE OPPOSING FORCES
renal clearance (Cx)
volume of plasma that is cleared of a substance in a unit time (mL/min)
Cx = (Ux x V) / Px
Ux: urine concentration of X
V: urine flow
Px: plasma concentration of X
what does it mean if Cx = GFR
filtration only
(clearance from plasma = clearance from urine)
what does it mean if Cx > GFR
filtration and secretion
(more is cleared from the urine than is filtered at the glomerulus, must be coming from secretion)
what does it mean if Cx < GFR
filtration and reabsorption
(less is cleared from the urine than is filtered at the glomerulus, must be getting reabsorbed into the blood)
filtration fraction
proportion of plasma that reaches the kidneys that gets filtered by the glomerulus (ratio of GFR to RPF)
FF = 0.2 = 20% in healthy animals
relationship between GFR and FF
Increase GFR = Increase FF
Decrease GFR = Decrease FF
**ONLY if RPF stays the same
relationship between RPF and FF
Increase RPF = Decrease FF
Decrease FF = Increase RPF
OVERRIDES GFR because changes in radius are to the 4th power
renal plasma flow (RPF)
volume of plasma flowing through the kidneys per unit time (mL/min)
RPF = RBF x (1 - HCT)
what does RPF depend on
capillary resistance
Resistance = 1/r^4
RPF = delta P / R
SMALL changes in radius = LARGE changes in resistance = LARGE changes in RPF
vasoconstriction = decrease radius = increase resistance = decrease RPF = increase FF
vasodilation = increase radius = decrease resistance = increase RPF = decrease FF
what are the main sites of resistance in the kidneys
afferent and efferent arterioles
effect of vasoconstriction: afferent arteriole
DECREASE:
- RBF
- P(GC)
- GFR - minor
INCREASE:
- FF - minor
effect of vasoconstriction:
efferent arteriole
DECREASE:
- RBF
INCREASE:
- P(GC)
- GFR - minor
- FF
effect of vasoconstriction: both arterioles
DECREASE:
- RBF
- P(GC) - minor
- GFR - minor
regulatory mechanisms of renal blood flow
- intrinsic: myogenic, tubuloglomerular feedback
- extrinsic: neural, hormonal
intrinsic regulation of renal blood flow
autoregulatory
maintains constant RBF and GFR even when BP changes within a certain range (80-180 mmHg)
occurs mainly at AFFERENT arterioles
myogenic regulation
smooth muscles in capillary walls respond to changes in BP
Inc. BP –> stretches smooth muscle –> Ca signaling –> vasoconstriction –> decreases RBF
Dec. BP –> no SM stretch –> vasodilation –> increases RBF
tubuloglomerular feedback
juxtaglomerular apparatus uses paracrine signaling to respond to changes in NaCl concentration in distal convoluted tubule
High NaCl –> JG cells release vasoactive substance –> vasoconstriction –> decrease RBF
Low NaCl –> JG cells release vasoactive substance –> vasodilation –> increase RBF
neural regulation
sympathetic nervous system responds to drops in BP
Dec. BP –> inc. sympathetic tone –> releases NE –> binds to a1 receptors on afferent > efferent arterioles –> vasoconstriction –> dec. RBF
hormonal regulation
JG cells release renin in response to low BP sensed by macula densa cells –> becomes angiotensin I/II –> binds to efferent then afferent arterioles –> vasoconstriction –> dec. RBF
ways of measuring GFR
- inulin
- creatine
- p-Aminohippurate
inulin
exogenous substance that has constant free plasma concentration, is freely filtered by kidneys, and is not reabsorbed or secreted
allows for direct measuring of GFR b/c rate of appearance in kidney = rate of filtration
how to calculate inulin clearance
V(plasma) = volume of plasma cleared of inulin per min
inulin is filtered only so clearance (V(plasma)) = GFR
V(plasma) = [urine concentration x urine flow] / plasma concentration
creatine
endogenous metabolite that is not reabsorbed, secreted, or metabolized by the kidney
GFR = [urine concentration x urine flow] / plasma concentration
p-Aminohippurate
completely cleared from plasma through filtration and secretion from peritubular capillaries into proximal tubules
how to calculate pAH clearance
filtration and secretion –> clearance > GFR
Clearance = urine concentration x urine flow / plasma concentration
Clearance = RPF
