Filtration and Renal Blood Flow Flashcards
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
What is are the units of clearance?
ml/min
What are the equations for clearance?
Cl = rate of excretion/plasma conc Cl = X urine x V urine / X plasma
Why is inulin good for calculating GFR?
freely filtered neither absorbed or secreted not metabolised not toxic easy to measure
What can be used to measure clearance instead of inulin?
Creatinine
What does it mean when a substance has a clearance of O?
completely reabsorbed and not secreted - glucose
not filtered and not secreted
What does it mean when the clearance is less than GFR?
partly reabsorbed, not secreted i.e. urea
What does it mean when clearance is greater than GFR?
substance is filtered, secreted and not reabsorbed
What does it mean if clearance is equal to GFR?
the substance is neither reabsorbed or secreted
What is used to measure renal plasma flow?
para-amino hippuric acid - PAH
What is renal plasma flow normally about?
650ml/min
What makes PAH suitable for calculating RPF?
freely filtered
secreted and not reabsorbed
completely cleared from plasma
What is creatinine?
a muscle metabolite produced at a near constant rate
What are the properties of a clearance marker?
non-toxic
inert - not metabolised
easy to measure
What are the properties of a GFR marker?
freely filtered
not secreted or absorbed
What are the properties of an RPF marker?
should be filtered and completed secreted
What is the filtration fraction?
the fraction of plasma flowing through the glomeruli that is filtered into the tubules
What is the equation for filtration fraction?
filtration fraction = GFR/RPF
i.e. 20% is filtered, the rest is secreted in peritubular capillaries
What is the calculation for RBF?
RBF = RPF x 1/1-Hct
What is Hct?
haematocrit - 1.85
