What is the normal GFR?
of which only ~2l ends up in urine so almost all of it is reabsorped too
What is the plasma filtration fraction at the glomerulus?>
1200ml/min of blood passes through
Of which 55% = 660ml/min is plasma
GFR of plasma is 125ml/min
Therefore 125/660 = ~19%
Explain the forces behind filtration?
Hydrostatic pressure favours filtration
Oncotic pressure favours reabsorption back into glomerular capillaries
For filtration to work capillary hydrostatic pressure (Pgc) must > fluid oncotic pressure
What factors influence filtration rate of a specific particle?
Small particles like Na+/K+/Water are almost completely filtered out.
Describe the physical structure of the capillary walls that allows filtration?
1) Fenestrated endothelial cells allow plasma to pass but not RBC
2) Then basal lamina prevents large proteins passing through
3) Then slit membrane between pedicals under the basal lamina prevent medium proteins passing
How is a high capillary Hydrostatic pressure maintained?
Afferent arterioles are wide and short to put up little resistance
Efferent arterioles are long and narrow to put up high resistance
Describe extrinsic control of Glomerular Capillary Pressure
Sympathetic vasoconstrictive effect
How do circulating catecholamines affect glomerular Capillary Pressure (Pgc)
Cause vasoconstriction primarily in the afferent arteriole. Thus lowering renal blood flow and Pgc to provide more blood to other tissues e.g. heart and muscle
How does angiotensin II affect Pgc?
Causes constriction of the efferent at low conc and both efferent/afferent at high concs
Renal vessels also have an intrinsic ability to control Pgc. whats the purpose of this?
To allow the vessels to react to changes in MAP and so maintain Pgc
Aka autoregulation of the GFR
How do renal vessels autoregulate GFR?
The afferent arterioles constrict and dilate in response to changes in blood volume in order to main Pgc.
When would autoregulation of the GFR be overridden?
When blood volume faces serious problems such as a large haemorrhage in order to liberate blood for more immediately essential tissues
(Symp nerves and angiotensin II)
What controls autoregulation of the GFR?
The renal vessels themselves.
Its independant of nerves and hormones, occuring even in denervated and isolated kidneys
How are the peritubular capillaries capable of resorping almost 180l a day?
1) Extremely low hydrostatic pressure thanks to the high resistance efferent arteriole upstream
2) High oncotic pressure in peritubular capillaries thanks to losing 20% of the plasma but still containing 100% of normal blood protein
Net starling forces are much more in favour of reabsorption leading to reabsorption of 50% of urea, 99% H2O, 100% Glc and 99.5% of Na.