Glomerular Filtration L04 Flashcards
List the parts of a nephron.
- afferent arteriole
- efferent arteriole
- glomerulus
- Bowman’s capsule
- Proximal tubule
- Loop of Henle
- Distal tubule
- Collecting duct
what is the renal plasma flow rate?
600ml/min
define bulk flow.
movement of water and solutes together due to a pressure gradient.
*Glomerular filtration involves bulk flow as well as diffusion of molecules.
which two ‘forces’ drive the flow of filtration across the glomerulus?
hydrostatic pressure - pressure generated by the heart, drives fluid out of capillaries.
osmotic/oncotic pressure - impedes flow, created by plasma proteins
does the osmotic/oncotic force increase or decrease across the capillary?
increase - losing more water, increase in concentration of plasma proteins, greater oncotic/osmotic pressure (greater pressure to stop movement of water by osmosis)
technically osmotic force decreases as osmotic is opposing osmosis and water potential gradient decreases along capillary as solutions start to equilibrate but oncotic increases as more force from plasma proteins as more concentrated at end of capillary
what effect will CONSTRICTION of the AFFERENT arteriole (proximal) have on glomerular hydrostatic pressure and flow of substances from capillary into glomerular space?
- decrease pressure in capillary
- decrease flow into glomerulus
what effect will CONSTRICTION of the EFFERENT arteriole (distal) have on glomerular hydrostatic pressure and flow of substances from capillary into glomerular space?
- increase pressure in capillary
- increase flow into glomerulus
what effect will DILATION of the AFFERENT arteriole (proximal) have on glomerular hydrostatic pressure and flow of substances from capillary into glomerular space?
- increase pressure in capillary
- increase flow into glomerulus
what effect will DILATION of the EFFERENT arteriole (distal) have on glomerular hydrostatic pressure and flow of substances from capillary into glomerular space?
- decrease pressure in capillary
- decrease flow into glomerulus
Why is filtration pressure along the glomerular capillary always outwards i.e. why is fluid always moving out of the capillary?
because the glomerular hydrostatic pressure is greater than the Bowman’s space hydrostatic pressure and the oncotic pressure. Therefore the net perfusion pressure is always out of the capillary (although this perfusion pressure decreases along the length of the capillary - pressure gradient decreases)
Describe the course of the efferent arteriole once it has left the glomerulus.
> portal vein > second capillary bed surrounding loop of Henle (peritubular capillaries) > interlobular vein > arcuate vein > interlobar vein > renal vein
How is the hydrostatic pressure in peritubular capillaries different to that of the glomerular capillaries?
much more similar to systemic capillary pressure *oncotic pressure is much higher
What are the three layers separating the blood in capillary from the lumen of the Bowman’s capsule?
- capillary endothelium
- glomerular basement membrane
- epithelial cells/podocytes of Bowman’s capsule
What is special about the endothelial cells of the glomerular capillaries?
they have fenestrations
Describe the structure of podocytes with regards to the filtration barrier.
podocytes have small foot processes (pedicles). pedicles interdigitate to form the barrier with filtration slits between them to allow the passage of substances.
Apart from size of molecules, how else can the filtration barrier restrict the passage of molecules into the capsule?
Charge - basement membrane (collage) and endothelium (glyococalyx) are both negatively-charged.
This prevents the passage of negatively charged proteins into the filtrate but does allow the passage of positively charged molecules and generally neutral molecules too (but less).
List the molecules which pass into the filtrate.
Na+, K+, Cl-, HCO3-, Mg2+/Ca2+, glucose, urea, water
roughly, what is the glomerular filtration rate (GFR)?
120ml/min or 180L/day
What is the filtration fraction?
the proportion of renal plasma flow which is filtered by the glomerulus
OR
GFR/ renal plasma flow
= 120/ 600
= 0.2
Define GFR.
the amount of blood filtered in the glomerulus per unit time
Why is creatinine used to measure GFR?
It is freely filtered into the filtrate but it is not secreted (adding more creatinine into filtrate) nor reabsorbed (losing creatinine) over the length of the tubules.
Therefore the concentration of creatinine in urine is same as concentration of creatinine in plasma.
- creatinine is a substance derived from creatine and creatine phosphate in muscle. It is produced by the body at a steady rate.
- can also use inulin (injected intravascularly) based on same concept
How is the GFR measured using creatinine?
- can calculate rate of excretion of creatinine:
creatinine excretion rate = (Ccr,u x V)/t = Ccr,u x V(dot)
where Ccr,u - urine creatinine concn
V - volume of urine produced
t - collection time
V(dot) - rate of production of urine
- rate of creatinine excretion = rate of creatinine filtration (as freely filtered and not reabsorbed)
- rate of creatinine filtration = GFR x Ccr,p
where Ccr,p = concentration of creatinine in plasma
- Ccr,u x V(dot) = GFR x Ccr,p
- rearrange to get FORMULA:
GFR = (Ccr,u x Vdot)/Ccr,p
What is the formula for estimated GFR (eGFR)?
eGFR = k/Ccr,p
where k is a constant correlating with age, mass and sex.
*in Cockcroft-Gault formula:
eGFR = ((140-Age) x mass xC)/Ccr,p
where C=1.23 for men and 1.04 for women
What does the eGFR formula tell you about the relationship between GFR and the rate of creatinine production/filtration?
they are inversely proportional.
