Renal Blood Flow & Glomerular Filtration Flashcards
Define glomerula filtration?
What does an abrupt fall in glomerula filtration indicate?
What is primary urine?
What is the amount excreted of a substance equal to?
Write an equation showing the net ultrafiltration pressure?
Define glomerular filtration rate (GFR)
List 4 factors that gfr depends on?
- Glomerular capillary pressure (Pgc)
- Plasma oncotic pressure (πgc)
- Tubular pressure (Pt)
- Glomerular capillary surface area or permeability (Kf).
GFR is not a fixed value but is subject to physiological regulation. This is achieved by neural or hormonal input to the afferent/efferent arteriole resulting in changes in Puf.
What is the myogenic mechanism of autoregulation in the kidney?
- Myogenic mechanism:
Vascular smooth muscle constricts when stretched.
Keeps GFR constant when blood pressure rises.
Arterial pressure rises → afferent arteriole stretches →
arteriole contracts → (vessel resistance increases)→ blood flow reduces and GFR remains constant:
What is the normal gfr value and state 4 things that effect gfr?
Severe haemorrhage
Obstruction in nephron tubule
Reduced plasma protein concentration
Small increase in blood pressure
In a normal individual carrying out a daily routine, GFR will be maintained at 120ml/min.
Define renal clearence and state the equation
Kidneys receive …% of the cardiac output
Kidneys receive 20% of the cardiac output
What is the tubulogomerular feedback mechanism of autoregulation?
o Tubuloglomerular feedback – macula densa response to NaCl increase
NaCl conc in tubular fluid sensed by macula densa in juxtaglomerular apparatus – release ATP as signaling molecule which signals afferent arteriole to vasoconstrict
reduced filtration (with associated negative feedback loop)
Regulation of GFR is achieved by …………… or …………… input to the afferent/efferent artriole resulting in changes to the glomerular capillary pressure
What happens to decrease gfr and what happens to increase GFR?
When is autoregulation required?
Regulation of GFR is achieved by neural or hormonal input to the afferent/efferent artriole resulting in changes to the glomerular capillary pressure
Autoregulation ensures fluid + solute excretion remain reasonably constant (without which urine production + ion loss will vary)
o To decrease GFR, constrict the afferent arteriole or dilate the efferent arteriole
o To increase CFR, construct the efferent arteriole or dilate the afferent arteriole
Autoregulation is required e.g. during exercise, as bp increases but you don’t want to be producing large amounts of urine
Pressure is not the only thing that influences the glomerular filtration rate. Other factors include:
Name them?
Glomerular filtration rate (GFR)
Pressure is not the only thing that influences the glomerular filtration rate. Other factors include:
o Permeability of tubule membrane
Clinical correlation: kidney disease may result in a change in membrane permeability
o Surface area of membrane available for filtration
Clinical correlation: kidney disease may damage nephrons, thus reducing the number of functioning glomeruli
o reducing SA available for ultrafiltration
These factors are considered in the ultrafiltration coefficient – written as Kf
Clinical relevance of GFR
In a normal individual carrying out a daily routine, GFR will be maintained at 120ml/min
Severe haemorrhage decreased GFR (result of decreased blood pressure
Obstruction in nephron tubule decreased GFR (due to increased hydrostatic pressure in tubule = opposing pressure)
Reduced plasma protein concentration increase GFR (due to increased oncotic pressure = opposing pressure
) Small increase in blood pressure, e.g. during exercise GFR remains constant (result of autoregulation)
However, if a molecule is freely filtered (i.e. neither reabsorbed nor secreted following its ultrafiltration), clearance = ……
What might a low creatine clearence indicate
What might a high creatine clearence indicate
However, if a molecule is freely filtered (i.e. neither reabsorbed nor secreted following its ultrafiltration), clearance = GFR (i.e. amount excreted = amount filtered)
o Example of this is inulin; a plant polysaccharide that is measurable in both urine + plasma and gives a clearance value of 120ml/min = GFR
o However in humans, GFR is estimated from creatinine clearance Creatinine is a waste product from creatine in muscle metabolism
The amount of creatinine is fairly constant, thus is renal function is stable, the amount of creatinine in urine is stable (120ml/min)
A low creatinine clearance may indicate renal failure
A high plasma creatinine may also indicate renal failure
What is used to measure renal plasma flow?
Define renal plasma flow?
Renal plasma flow (RPF)
The volume of plasma (component of blood) reaching the kidney per unit time
PAH (para aminohippurate) is completely removed from the plasma passing through the kidney, therefore its clearance = renal plasma flow = 625ml/min
With other substances, the amount of the substance appearing in the urine reflects the combined effects of filtration, reabsorption + secretion, so that:
o Amount excreted = amount filtered – amount reabsorbed + amount secreted o Thus most solutes have aclearance of < 120ml/min (GFR) = controlled excretion
Amount excreted =
finish the equation
RENAL DIAGNOSTICS
What is the cardinal feature of renal disease?
A fall in GFR is the cardinal feature of renal disease
If GFR falls, excretory products will build up in the plasma.
A raised plasma concentration of creatinine is diagnostic of renal disease.
Excretion of many other substances will also be impaired in renal failure - including some drugs. This needs to be taken into account when calculating drug doses - PHARMACOKINETICS.
