The quantity
of glomerular filtrate formed each minute in all the nephrons of both kidneys
125 ml/min
[normal] > 180 liters/day
over 99% of
filtrate is reabsorbed > approx 2 liters urine
GFR = Filtration
pressure x filtration constant
Filtration
pressure = Glomerular pressure
(Bowmans pressure + Glomerular COP)
Filtration
constant does not change except in diseased kidneys
Glomerular
filtration is dependent on:
i) Glomerular
blood pressure: average pressure in glomerular capillaries, tends to push
fluid out of the glomerulus into Bowmans capsule. Approx 60 mmHg.
ii) Bowmans
Capsule pressure: resists movement of fluid out of glomerulus into Bowmans
capsule. Approx 18 mmHg
iii) Glomerular
colloid osmotic pressure: tends to
increase as plasma protein free plasma is fitered into the capsule. Resists
movement of fluid into capsule. Approx 32 mmHg
a) effect of
renal blood flow on GFR
increased RBF
> reduce tendency of rising glomerular COP to resist filtration through
glomerulus > increased GFR
[is this the rationale for flow, not pressure being important in
maintaining urine production?]
b) effect of
afferent arteriolar constriction on GFR
afferent
arteriolar constriction > decrease RBF &
decrease glomerular pressure > decreased GFR
c) effect of
efferent arteriolar constriction on GFR
efferent
arteriolar constriction > increased resistance to outflow from glomeruli:
i) > increased glomerular pressure >
increased GFR
ii)> decreased RBF > increased
glomerular COP > decreased GFR
‑> net effect ?
e) effect of
sympathetic stimulation on GFR
sympathetic
stimulation > preferential afferent arteriolar constriction > decrease
RBF & decrease glomerular
pressure > decreased GFR
f) effect of
arterial BP on GFR
Increased BP
> automatic afferent arteriolar constriction [autoregulation]:
i) > decrease RBF > decreased GFR
ii)>
attenuated increase in glomerular pressure
θ
slight
increase in GFR > large increase in urine production
K. C. Potger
Copyright © 2001