glomerular filtration Flashcards
the rate of ultrafiltration in all glomeruli of both kidneys
GFR mL/min
In a healthy person, what is the osmotic pressure in Bowman’s space?
zero. (should not be proteins or osmotically active particles in there)
If the osmotic pressure in Bowman’s space is zero, what keeps fluid in there at all?
Hydrostatic pressure
In systemic capillaries, what is the net effect of fluid movement? In glomerular capillaries, what is the net effect of fluid movement?
Systemic: fluid out = fluid in
Glomerular: fluid out only
The rise of renal/glomerular plasma flow causes a ________ in GFR.
rise
What 3 elements make up the filtration barrier in the glomerulus?
1) Fenestrae of the endothelial capillary cells (filter based on size (70nm) and charge of particles)
2) Basement membrane (filters based on charge of particles)
3) Filtration slits of the podocytes (filter based on size–25-60nm)
How can we measure GFR?
We need to look at a substance that is filtered freely, like water (Inulin). The amount of this substance cannot be affected by the kidneys (it can’t be reabsorbed, secreted, etc) If this is true, we can measure the substance excreted in the urine and determine GFR. So we Infuse Inulin IV, wait for it to diffuse, collect urine over time and draw blood. We know urine flow is 1mL/min. So say we see 60mg/mL of Inulin in the urine. We also see 0.5mg/mL in the plasma. We can multiply the 60mg (urine) x 1.0 (filter rate) and divide by 0.5mg (plasma) = 120mL Inulin filtered by kidneys per minute.
Clearance = urine concentration x filter rate / plasma concentration
Again, what is the formula for clearance of any substance?
Clearance = Urine concentration x flow rate / plasma concentration
Besides Inulin, what is another substance that is mostly not reabsorbed, secreted or metabolized by the renal tubules? (thereby making it useful to measure GFR)
creatinine
______________is a breakdown product of muscle
creatinine
About how much of the plasma in the glomerular capillaries is actually filtered?
20%
If you measure the clearance of some random substance and find that its clearance is less than that of Inulin, what can you assume?
(ClearanceX < Clearance Inulin)
Kidneys are REABSORBING the substance
If the clearance of a substance is MORE than Inulin clearance, what can you assume?
The kidneys are SECRETING the substance.
In early stages of renal disease (aka slightly decreased GFR), will you detect a marked elevation in your plasma creatinine levels?
No. Hypertrophy of nephrons will compensate until about 50% are damaged. Then you will see dramatic increase in creatinine.
You notice that your patient’s creatinine level has increased to slightly above normal. Is this significant? Or is this a normal variation based on various circumstances?
Significant! Small increases in creatinine denote a dramatic decline in GFR in early renal disease
