Filtration and Renal Blood Flow
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What are the barriers to glomerular filtration?
glomerular capillary Endothelium - RBC Basement Membrane - plasma protein barrier Slit processes of podocytes - plasma protein barrier
What are the forces that comprise net filtration pressure?
Glomerular capillary blood pressure Bowmans capsule hydrostatic pressure Capillary Oncotic Pressure Bowman’s capsule oncotic pressure
Which forces favour filtration?
Glomerular capillary pressure Bowmans Capsule Oncotic pressure
Which forces oppose filtration?
Bowmans Capsule Hydrostatic Pressure Capillary Oncotic Pressure
What is oncotic pressure?
the pressure that tends to pull water back into the blood
What are the typical values of the forces favouring filtration?
GCBP - 55mmHg BCOP - 0mmHG
What are the typical values of the forces opposing filtration?
BCHP - 15mmHg COP - 30mmHg
What is the typical net filtration pressure?
10mmHg towards the bowmans
What is the GFR?
the rate at which protein free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time
What is the equation for GFR?
GFR = Kf x net filtration pressure
What is a normal GFR?
125ml/min
What is the major determinant of GFR?
glomerular capillary blood pressure
What are the extrinsic controls of GFR?
sympathetic control via baroreceptor reflex
What are the intrinsic or autoregulatory controls of GFR?
myogenic mechanism tubuloglomerular feedback mechanism
What is the effect of an increase in arterial blood pressure?
increased GCBP increases net filtration and GFR
What causes the GCBP to fall?
constriction of the afferent arteriole
What does increased sympathetic activity do to the GFR?
it decreases it in order to compensate for a fall in blood volume
What does autoregulation prevent?
short term changes in systemic arterial pressure affecting GFR
What is protected from changes in MABP over a wide range of MABP?
RBF and GFR
What is myogenic control?
if vascular smooth muscle is stretched, it contracts thus constricting the arteriol
What is tubuloglomerular feedback?
involves juxtaglomerular apparatus If GFR rises, more NaCl flows through the tubule leading to constriction of afferent arterioles
What senses the NaCl content of tubular fluid?
macula densa
What may also determine GFR?
kidney stone - increase BCHP - decrease GFR diarrhoea - increase COP - decrease GFR burns patients - decrease COP - increase GFR decrease in Kf - surface area change - decrease in GFR
What is plasma clearance?
the volume of plasma completely cleared of a particular substance per minute