Lect 6: Filtration & Clearance 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
Oncotic
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
- Endothelial cells
- Capillary Basement Membrane
- 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
Podocytes
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
Charge
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.
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
FF= GFR/RPF
125mL/min/600mL/min = 0.2