Filtration and renal blood flow Flashcards
what are the 3 filtration barriers to glomerular filtration?
(1) Glomerular Capillary Endothelium (barrier to RBC)
(2) Basement Membrane (basal lamina) (plasma protein barrier)
(3) Slit processes of podocytes (plasma protein barrier)(Glomerular epithelium)
what is the basement membrane made of?
Doesn’t contain cells, but is made of collagen and glycoprotein
why is the basement membrane a barrier to plasma proteins?
it has a net negative charge and so it repels negatively charged plasma proteins
what 4 forces compromise net filtration pressure?
-Glomerular capillary blood pressure (BPGC)
-Bowman’s Capsule hydrostatic (fluid) pressure
(HPBC)
- Capillary oncotic pressure
(COPGC)
-Bowman’s Capsule oncotic pressure
(COPBC)
what 2 forces favour filtration?
- glomerular capillary blood pressure (55 mm Hg)
- Bowmans capsule oncotic pressure (0mm Hg)
what forces oppose filtration?
- bowmans capsule hydrostatic (fluid pressure) (15mm Hg)
- capillary oncotic pressure (30mm Hg)
what is GFR (glomerular filtration rate)?
rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time
How can you calculate GFR?
GFR = Kf x net filtration pressure
where Kf is the filtration coefficient
what is the major determinant of GFR?
Glomerular capillary fluid (blood) pressure
what is the extrinsic regulation of renal blood flow and GFR?
sympathetic control via baroreceptor reflex
what is the intrinsic regulation of renal blood flow and GFR?
(a) Myogenic mechanism
(b) Tubuloglomerular feedback mechanism
what happens during vasoconstriction of the afferent arteriole?
decreases blood flow into the glomerulus - decreases glomerular capillary blood pressure -decrease net filtration pressure- GFR
what does autoregulation of mean arterial blood pressure do?
prevents short term changes in systemic arterial pressure affecting GFR
how can you calculate rate of secretion?
Rate of secretion of X = rate of excretion of X – rate of filtration of X
what happens in the myogenic response?
Smooth muscles surrounding arterioles stretch or contract to enlarge or constrict the arteriole
what happens in the tubuloglomerular feedback?
- Involves the juxtaglomerular apparatus
- If GFR rises, more NaCl flows through the tubule leading to constriction of afferent arterioles
what are forms of pathophysiological conditions that effect GFR?
- kidney stones and and diarrhoea have decreased GFR
- severly burned patients have increased GFR
what is plasma clearance?
A measure of how effectively the kidneys can ‘clean’ the blood of a substance,
Equals the volume of plasma completely cleared of a particular substance per minute
what is the equation for calculating the clearance of a substance?
Clearance of a substance (X)= [X]urine x Vurine/ [X]plasma
what are the units of clearance?
ml/min
what substance has a clean race that equals GFR?
inulin
How does inulin have equal clearance to GFR?
Inulin enters the urine via filtration alone (it is not secreted or reabsorbed)
what substance can be used instead of inulin to clinically determine GFR?
Creatinine
when is clearance 0?
For substances which are filtered, completely reabsorbed and not secreted (e.g. glucose)
when is clearance smaller than GFR?
For substances which are filtered, partly reabsorbed and not secreted (e.g. urea)
when is clearance larger than GFR?
For substances which are filtered, secreted but not reabsorbed (e.g. H+)
what is used to calculate the renal plasma flow (RPF)?
para-amino hippuric acid (PAH)
why is para-amino hippuric used to measure renal plasma flow?
1) freely filtered at glomerulus,
2) secreted into the tubule (not reabsorbed)
3) completely cleared from the plasma
So all the PAH in the plasma that escapes filtration is secreted from peritubular capillaries
what are the average values for renal plasma flow and GFR?
Renal plasma flow- 650ml/min
GFR- 125ml/min
what properties are needed for a clearance marker?
(1) Non-toxic
(2) Inert (i.e. not metabolised)
(3) Easy to measure
what marker should be filtered freely, not secreted or reabsorbed?
GFR marker
what marker should be filtered and completely secreted?
RPF marker
what is filtration fraction?
The fraction of plasma flowing through the glomeruli that is filtered into the tubules
how do you calculate filtration fraction ?
GFR/renal plasma flow
