Glomerular Filtration Rate and its Regulation Flashcards
Glomerular Filtration Rate (GFR)
The amount of glomerular ultrafiltrate formed each minute and an indicator of kidney function
How is GFR expressed in cats?
ml/min/kg
What may be the first and only clinical sign of kidney disease?
A fall in the GFR
What can GFR measurement tell?
A key significance in patients that are in early stage of renal failure that non-azotemic (no significant increase in BUN or creatinine)
Nephron and GFR reduction
Functioning nephrons reduced to half, but GFR reduces by 25% because the remaining nephrons compensate
How does a substance have to be to measure GFR?
Must be freely filtered through the glomeruli
Neither secreted nor reabsorbed by the tubules
Nontoxic
Not metabolized by the body
Renal plasma clearance
The volume of plasma from which a substance is completely removed by the kidney in a given amount of time (usually minutes)
What is the result of substance that appears in urine per unit of time result?
The the renal filtering of a certain number of ml of plasma that contained this amount
mL/ min
How do you measure the plasma level of a substance?
By measuring the amount of that substance that’s secreted
What substance meets the GFR criteria?
Inulin- a polymer of fructose, molecular weight of 5200, injected into the body
GFR calculation for inulin
GFR= Urine inulin x volume / plasma inulin
What happens if a substance is filtered and reabsorbed but not secreted and what is the formula?
The plasma clearance rate is always less than the GFR
This cannot be used because it calculate the GFR incorrectly
Ex; glucose
What happens if a substance is filtered, not reabsorbed but secreted?
The plasma clearance rate is always greater than the GFR (hydrogen ion)– also can’t be used
How is kidney function usually assessed in a clinical setting?
By measuring the plasma creatinine concentration
Creatinine
A breakdown product of creatine phosphate in the muscle and produced at a fairly constant rate by the body
How is creatinine processed?
Freely filterable
No reabsorbed
Small amount secreted into the tubular lumen
Why aren’t blood creatinine levels a good indicator of early stages of chronic kidney disease?
Because significant increase in blood creatinine only occurs following significant nephron loss
What gives you a better GFR estimation for renal function?
Blood creatinine, urine creatinine and urine flow rate does
SDMA
Serum levels of Symmetrc dimethylarginine
Correlates well in cats and dogs (detects kidney problems)
How is SDMA better than Creatinine?
An increase of SDMA levels occur earlier than an increase in creatinine in kidney disease
Increase has been detected when as little as 25% loss of renal function has occurred
How would a blockage in the nephron affect the Bowman’s capsule hydrostatic pressure and the GFR?
Tubular obstruction –> increased Bowman’s capsule hydrostatic pressure –> decease in GFR
How would loss of water in patient suffering from diarrhea affect the plasma-colloidal oncotic pressure and GFR?
Loss of water due to diarrhea–> increase in plasma colloid osmotic pressure –> decrease in GFR
How do the changes in glomerular hydrostatic pressure affect regulation of GFR?
Primary means for physiological regulation of GFR
Increase = increase in GFR
Decrease = decrease in GFR
Intrinsic myogenic mechanism to maintain constant GFR rate (increased blood pressure)
1/ physiology increases blood pressure
2/ myogenic constrictor response in afferent arterioles
3/ contraction of the vascular smooth muscle within seconds
4/ attenuation of transmission of increased arterial pressure to the glomerular capillaries
5/ helps prevent excessive increases in renal blood flow and GFR when arterial pressure increases
Intrinsic myogenic mechanism to maintain constant GFR rate (decreased blood pressure)
1/ physiology fall in blood pressure
2/ reduced blood flow to the glomerulus
3/ myogenic vasodilator response in afferent arterioles
4/ transient increase in the blood flow to the glomerular capillaries despite the fall in blood pressure
Intrinsic tubuloglomerular feedback mechanism to maintain a constant GFR rate
1/ increased blood pressure
2/ increase in GFR
3/ increased flow rate in the tubule
4/ decreased absorption of NaCl
5/ increased concentration of Na and Cl in the mecula densa
6/ increased ATP and adenosine release
7/ vasoconstriction of afferent arteriole and decreased blood pressure
What do the mechanisms do?
Keep the GFR constant under physiological conditions
