lecture 3: renal blood flow and glomerular filtration Flashcards
what is renal/kidney failure?
an abrupt fall in glomerular filtration
define glomerular filtration?
passive process with fluid ‘driven’ through the
fenestrated walls of the glomerular capillaries by heart hydrostatic
pressure.
what does the passive process refer to?
- no ATP pumping
what is the filtration barrier ( endothelium of capillaries) permeable to and impermeable to?
permeable to
- fluids, small solutes
impermeable to
- cells
- proteins
- drugs bound to proteins
how does the concentration of the solute change?
- the concentration remains the same
what is the primary urine?
A clear fluid (ultrafiltrate), completely free from blood and proteins, is produced containing electrolytes and small solutes
In urine?
concentration of important things?
concentration of less important things?
- low concentration as they are reabsorbed
- high as they are actively secreted
where does the filtrate move down?
Filtrate moves down fenestra in podocytes foot processes.
how to work out the net glomerular filtration pressure
Puf = Pgc- Pt- πgc
Puf= net ultrafiltration pressure
Pgc = osmotic pressure of plasma proteins in glomerular capillaries (πgc)
Pt = hydrostatic pressure of tubule (Pt)
how to work at the glomerular filtration rate?
GFR = Puf x Kf
Puf= net ultrafiltration pressure
Kf is an ultrafiltration coefficient
what might kidney diseases result in?
Kidney diseases may reduce number of functioning glomeruli = reduced surface area = decrease Kf = decrease GFR
what might Dilation of glomerular arterioles by drugs/hormones result in?
more surface area = increase Kf = increase GFR.
what is the GFR?
The amount of fluid filtered from the glomeruli into the Bowmans capsule per unit of time (ml/min).
it acts as an index of kidney function
how to work at the GFR
from
renal blood flow, renal plasma flow and filtration factor ?
Glomerular Filtration Rate (GFR) = RPF x FF
Renal plasma flow
Filtration fraction
what are the mechanisms of auto regulation?
Myogenic Mechanism:
Tuboglomerular Feedback:
how does the myogenic mechanism work?
Intrinsic vasoconstriction in response to stretch – keeps the GFR constant when BP rises.
how does Tuboglomerular Feedback?
NaCl concentration in distal convoluted tubule sensed by macula densa in juxta-glomerular apparatus.
- so the macula densa releases ATP which signals to vasoconstriction
what would a severe haemorrhage do to the GFR?
GFR down – lower hydrostatic pressure.
what would an Obstruction in nephron tubule do to the GFR?
GFR down – more tubular pressure.
what would Reduced plasma protein concentration do to the GFR?
GFR up – less osmotic pressure.
what would Small increase in BP do to the GFR?
GFR unchanged – compensated for.
what are the main factors which GFR depends on?
- Glomerular capillary pressure Pgc.
▪ Plasma oncotic pressure - πgc
▪ Tubularpressure–Pt.
▪ Glomerular capillary surface area or permeability–Kf.
what is renal clearance?
number of litres of plasma that is completely cleared of substance - x per unit time
(Substances are filtered to different degrees,but the extent to which they are removed from the blood is called clearance.)
what is the equation for renal clearance?
𝐶 =( 𝑈 𝑥 𝑉 ) / P 𝑚𝑙/𝑚𝑖𝑛.
U = concentration of substance in urine.
P = concentration of substance in plasma.
V = rate of urine production.
give examples of substances freely filtered AND not reabsorbed?
- inulin
Not found in mammals so is transfused if we use this method. - creatine
amount that is released is constant - so measure creatine to look at low clearance or high plasma
what is renal plasma flow measured by?
Measured by PAH
- it is essentially filtered and actively secreted in one pass of the kidney
- so it can be used to measure RPF as it is all cleared
what is the amount excreted?
amount excreted = amount filtered - amount reabsorbed + amount secreted
what does a fall in GFR show us?
- kidney disease
- if the GFR falls excretory products will build up in the plasma and raised plasma concentration of
creatine is diagnostic of renal disease.
what does pharmacogenetics show?
excretion of many drugs is impaired in renal failure so the drug doses need to be monitored in these patients
