Renal Blood Flow & Glomerular Filtration Flashcards
What is a cardinal sign of renal failure?
Sharp drop in GFR
What is glomerular filtration?
The formation of an ultrafiltrate of plasma in the glomerulus of a kidney nephron
What does ‘freely filtered’ mean?
A substance that has the same concentration in the filtrate as in the plasma (e.g. small solutes)
What is the driving force for ultrafiltration?
It is a passive process driven by the hydrostatic pressure of the heart
What are the two forces opposing the hydrostatic pressure?
Hydrostatic pressure of the tubule
Osmotic pressure of the plasma proteins in glomerular capillaries
What is the net ultrafiltration pressure?
Puf = 10 - 20 mm Hg
Define Glomerular Filtration Rate.
The amount of fluid filtered from the glomerulus to the Bowman’s capsule per minute (ml/min)
What is the equation for glomerular filtration rate?
GFR = Puf x Kf Kf = ultrafiltration coefficient (based on membrane permeability)
Explain, using the equation, how renal disease can cause a decrease in GFR.
You can lose nephrons hence you lose surface area –> decrease in Kf –> decrease in GFR
What proportion of cardiac output goes to the kidneys?
20%
What is filtration fraction? Give an approximate value.
The ratio between the renal plasma flow and the amount of filtrate filtered by the glomerulus (usually around 20%)
State another equation for glomerular filtration rate using filtration fraction.
GFR = RPF x FF
State four factors that affect GFR?
Glomerular capillary pressure
Plasma oncotic pressure
Tissue pressure
Glomerular capillary surface area or permeability (Kf)
State the two mechanisms involved in autoregulation of GFR.
Myogenic Mechanism
Tubuloglomerular Feedback
Describe the myogenic mechanism.
Vascular smooth muscle constricts when it is stretched (e.g. due to an increase in blood pressure)
This keeps GFR constant when blood pressure rises
Describe the tubuloglomerular feedback mechanism.
NaCl concentration of the urine is detected by the macula densa. The macula densa sends signals to the afferent arterioles to change the resistance and hence change GFR.
Describe how changes in the afferent and efferent arterioles can affect ultrafiltration.
They can be constricted or dilated to affect the net ultrafiltration. (e.g. vasodilating the efferent means that more blood moves away from the glomerular capillaries so the GFR decreases)
What is the normal GFR?
120 ml/min
Define clearance.
The number of litres of plasma that are cleared of a substance per minute.
State the equation for clearance.
C = V x U/P V = rate of urine production U = concentration of substance in urine P = concentration of substance in plasma
What are the properties of a substance used to measure GFR? Give an example.
Inulin - is freely filtered and is neither reabsorbed nor secreted
What is used practically to measure GFR?
Creatinine - this is a naturally produced waste product of creatine in muscle metabolism
It is released at a fairly constant rate in the urine
If renal function is stable then the amount of creatinine in the urine is stable.
What is used to measure Renal Plasma Flow Rate?
PAH - para aminohippurate
What properties does this substance have that makes it appropriate for measuring RPF?
It is totally cleared from the plasma when it passes through the kidneys. The normal glomerular filtration removes around 20% of the total PAH and then secretion from the blood into the tubules removes the rest of the PAH. There is no PAH in the renal vein.
RPF = V x Upah/Ppah
What change in creatinine concentration indicates renal disease?
Increase = the cretinine is building up because the kidneys are unable to remove it
