Physiology 3: glomerular filtration and renal blood flow Flashcards
what are the 3 glomerular filtration barriers
glomerular capillary endothelium –> basement membrane –> podocyte slit process
what is the the glomerular filtration rate (GFR)
rate at which protein-free plasma is filtered from the glomerulus into Bowman’s capsule
what is the filtration coefficient Kf
holey glomerular membrane
how do you calculate the GFR in terms of the filtration coefficient and net filtration pressure
GFR = Kf x net filtration pressure
what is the normal GFR
125ml/min
what is the major determinant of GFR and what other factor promotes filtration
glomerular capillary fluid pressure (glomerular BP)
what forces oppose filtration and why
glomerular capillary oncotic pressure - concentrated blood does not want to lose water via filtration /// (bowman’s hydrostatic pressure - self explanatory)
what is the net pressure across the glomerular capillaries and bowman’s capsule
10mmHg
describe the process in how the GFR would adjust to help compensate a fall in blood volume eg haemorrhage
decreased arterial BP –> detected by aortic and carotid baroreceptors –> increased sympathetics –> arteriole constriction –> afferent arteriole constricted –> decreased glomerular capillary pressure (BP) –> decreased GFR –> decreased urine
what extrinsic factor can regulate GFR
sympathetic control via baroreceptors
how does sympathetic activity regulate GFR
causes vasoconstriction of arteriole and lowers GFR (raised BP = lower GFR, lowered BP = raised GFR)
what 2 intrinsic factors regulate GFR
myogenic mechanism and tubuloglomerular feedback
how does the myogenic mechanism work
if vascular smooth muscle is stretched (increased BP) it contracts the arteriole to keep the BP constant
how does the tubuloglomerular apparatus work
juxtaglomerular apparatus where macula densa senses high NaCl in tubule and constricts afferent arteriole - this maintains the GFR despite a short term increased BP
what is the purpose of intrinsic regulation
to prevent short term BP changes affecting GFR
when would extrinsic control override intrinsic control
when a large haemorrhage occurs
what effect does kidney stones have on GFR and why
decreased - causes a block downstream which increased bowman’s capsule fluid pressure so no pressure gradient for blood to enter the capsule
what effect does diarrhoea have on GFR and why
decreased GFR - patient is dehydrated and plasma proteins within the blood will become more concentrated and increase the osmotic pressure of the blood which opposes filtration
what effect does extreme burns have on GFR and why
increased GFR - plasma proteins are lost in the burn so concentration within blood is less and a weaker capillary oncotic pressure promotes filtration
what does plasma clearance indicate
how effectively the kidneys can ‘clean’ the blood if a substance - varies for each substance
how is plasma clearance measures
volume of plasma cleared of a substance per minute
what is the equation for plasma clearance
rate of excretion of X/ plasma [X] = ([X]urine x Vurine) / [X]plasma
what X clearance is a perfect marker for GFR
inulin
why is creatine used over inulin as a marker of GFR
it occurs naturally in the body
