Flashcards in Lect 6: Filtration & Clearance Deck (51)
What is glomerular filtration?
process of filtering plasma across the glomerular capillaries to form a protein-free ultrafiltrate. The filtrate is not urine:
all solutes in plasma exist in the glomerular filtrate at the same conc as they are in the plasma
NOT REABSORPTION, only filtration
GFR is normal
125 mL/min or 180L/day
Starling Forces drive Glomerular Filtration
Filtration rate is Kf (Pgc-Pbs)--(Pogc-Pobs)
Pobs is usually 0
difference in hydrostatic pressure inside the glomerular capillary and Bowman's space
difference in oncotic pressure inside the glomerular capillary and bowman's space
What is the Pgc?
it is around 45-50mmHg at the beginning of the glomerular capillary and decreases to 41-47mmHg at the end of glomerular capillary.
The glomerulus has the capacity to increase or decrease the surface area available for filtration.
as you decrease the flow of blood thru the glomerular capillary, the surface available for filtration decreases and GFR decreases
not an arteriole-venous; it is ARTERIAL Blood
Glomerular Barrier to Filtration
1. Endothelial cells
2. Capillary Basement Membrane
Endothelial cells of glomerular capillarier
sieve the passage of cellular elements into Bowman's space;
Basement Membrane on which the endothelial cells sit
negatively charged and repels anionic charges that are very large (such as albumin); PREVENTS filtration of plasma mambrane
cover the outer surface of glomerulus; where two of them meet there is slit diaphragm where anionic charge further restricts filtration of anionic proteins, but not smaller organic and inorganic anions
Where does the ultrafiltrate collect?
in Bowman's capsule. Then it exits via the efferent arteriole
Size dependence of solute permselectiveity at the glomerulus
size vs. filterability
filterability is measured as the ratio of solute conc in BS/solute conc in plasma
substaces such as water, NaCl, Inulin are freely filtered and very small in size when compared to Hemoglobin or myoglobin or Albumin
If something is freely filtered it
exists at the same conc in BS as it does in plasma solute conc BS/solute conc in plasma=1
==>such as water and inulin
The larger the molecule
the less it will be filtered; so none of the molecule is in BS. They are all tied up in plasma
the negative charge on the foot processes and basement membrane impedes the passage of negative charged solutes.
--so lots of cations of a given size would be able to pass
--Removing the negative charge from the glomerular barrier increases the passage of anions such as occurs with increased filtration of plasma proteins in nephrotic serum nephritis
clearance ratio (Cx/Cinulin); inulin is freely filtered
Why is regulation of cardiac output important?
P= Q x R
Total Peripheral Resistance (TPR) and CO determine BP. If TPR decreases, CO increases to maintain BP and vice versa. BP gradient highest in aortaBP lowest right as it enters heart.
BP= CO x TPR
CO=Total Blood Volume (5L)
20% of this goes to the kidney to become ultrafiltrate... bc it manages the volume and osmolarity of ECF. ECF broken down into plasma flow. 20% of this RPF is GFR. This is the filtration fraction
125mL/min/600mL/min = 0.2
When your urine output is 1mL/min
you are dehydrated; urine trying to retain water as uch as possible. When you are overhydrated you pee out a lot. Urine output changes based on your fluid balance
(FF-GFR/RPF) GFR increases with increasing RPF
and FF decreases with increasing RPF (bc it has less time with the stuff to filter???)
As RPF increases GFR increases until it reaches a plateau (bc all the surface area is maximally filtering).
So slope (GFR/RPF, which is equal to FF) is greater than 1.
FF is the fraction of plasma flow filtered at the glomeruli. Accordingly, the FF is higher at low RPF (bc you don't want it to saturate?) and lower at high RPF. This maintains the GFR at levels necessary for renal function when RPF is compromised due to dz.
Effect of Changing Starling forces on RPF and GFR: Effect of AFFERENT arteriolar constriction on these two
it decreases both GFR & RPF -
this will decrease the hydrostatic pressure (necessary for squeezing the filtrate out) and depending on the magnitude of the decrease, it may even decrease the capillary surface area. When the RPF rate is less than normal the filtration surface area is reduced!
Effect of Changing Starling forces on RPF and GFR: Effect of EFFERENT arteriolar constriction on these two
Effect on RPF: decreases
Effect on GRF: increased bc you increased gcHydrostatic pressure (upstream) - so you will squeeze more filtrate out of the glomerulus into Bowman's space. So this will happen at a faster rate
Effect of Changing Starling forces on RPF and GFR: When both afferent and efferent arterioles are constricted
So long as flow is sufficient to allow filtration across the entire capillary bed, RPF will decrease but there will be NO CHANGE in GFR because the hydrosatic pressure didn't change. As long as the starling force is sufficient to drive filtration you will not get any change in GFR.
Effect of Changing Starling forces on RPF and GFR:
increasing plasma oncotic pressure
Always think in terms of Starling forces. What is the relationship of oncotic pressure to hydrostatic pressure if you increase oncotic pressure?
This increased oncotic pressure resists filtration. If it is increased it will resist hydrostatic pressure causing it to decrease. This decreases GFR but has NO EFFECT on renal plasma flow
If you decrease plasma protein (oncotic pressure)
you will be reducing RPF which increases GFR (No effect on RPF)
If you obstruct the ureter this increases the Hydrostatic pressure in Bowman's capsule
sending stuff up back into the glomerulus. This decreases the filtration rate (GFR) but it will not affect RPF but it will decrease