Lect 6: Filtration & Clearance Flashcards Preview

Unit 7 - Repro & Renal Physiology > Lect 6: Filtration & Clearance > Flashcards

Flashcards in Lect 6: Filtration & Clearance Deck (51)
Loading flashcards...

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


Pgc--Pbs is

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
3. Podocytes


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


Filterability equals

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.


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


Fluid Reabsorption in the Post-glomerular capillaries

the fluid exiting the glomerulus that has been filtered will not have higher osmolarity because you've not lost volume. It will have a higher oncotic pressure though