141 - 2 - Kidney Function I Flashcards
What does kidney autoregulation do?
Alters resistance of the afferent arteriole to maintain GFR.
Implication of a resistance vessel
Pressure will drop across the length of vessel
Role of the afferent arteriole
Maintains pressure gradient of glomerular capillary at 50mmHg
Pressure drop between afferent and efferent arteriole
From 100mmHg to 10mmHg
Pressure in peritubular capillaries
~5-10mmHg
Effect on GFR of constricting afferent arteriole
Decrease GFR (decreased hydrostatic pressure)
Effect on GFR of constricting efferent arteriole
Increase GFR (increased hydrostatic pressure)
Blood pressures between which autoregulation can maintain a constant GFR
Mean arterial blood pressure between 80 and 180mmHg
Average GFR
180L/day
Mechanisms leading to autoregulation
1
2
1) Myogenic response (contraction to resist distension)
2) Tubuloglomerular feedback (TGF)
Part of Bowman’s capsule that is responsible for myogenic response
Afferent arteriole
Part of nephron responsible for tubuloglomerular response
Macula densa (where junction between ascending limb and distal tubule are in contact with afferent and efferent arterioles of the same nephron)
*Location of juxtaglomerular apparatus
JUXTAGLOMERULAR
Unusual cell type in afferent arteriole
Layer of granular cells
Role of juxtaglomerular apparatus
Monitors amount of glomerular flow (by measuring chloride concentration).
If there is too much, makes glomerulus filter less
Role of macula densa cells in GFR regulation
1
2
3
1) If there is a greater flow rate from increased GFR, there will be more Cl- detected by macula densa cells
2) Ca2+ channels will open on macula densa cells, triggering a paracrine signal which stimulates paracrine factor release (most importantly adenosine).
3) Adenosine acts on granular cells (which have adenosine type 1 receptors), which makes them vasoconstrict. This reduces GFR.
Part of kidneys from which renin is released
Granular cells of the afferent arteriole
Amount of filtered water that is excreted
1%
Amount of filtered creatinine that is excreted?
100%
Why is GFR high, if most of what is filtered is reabsorbed?
To filter toxic substances that are present in small amounts (EG: bilirubin, creatinine)
Renal clearance
Excretion rate of a substance relative to plasma concentration.
Excretion/plasma concentration of substance
What does renal clearance tell you?
The volume of plasma cleared of particular substance per time
How is GFR estimated?
1
2
3
1) Use inulin (a substance that is freely filtered, not reabsorbed or secreted).
2) Establish stable plasma concentration of inulin
3) Amount of inulin is measured, which is directly proportional to GFR, as inulin isn’t reabsorbed or secreted.
Good approximation of GFR 1 2 3 4
1) Measure creatinine clearance per unit time.
2) Measure creatinine concentration in urine
3) Creatinine is at a constant level in blood normally.
4) Creatinine is filtered, not reabsorbed. Secreted at a low rate (so not a perfect measure).
What might it mean if there is glucose in urine?
Too much glucose in blood being filtered, and is overloading the kidney’s ability to reabsorb glucose.
Amount of filtered glucose that is reabsorbed
100%
Rate of penicillin clearance
150mL minute
What is strange about rate of penicillin clearance?
It is greater than GFR
How is the rate of penicillin clearance greater than GFR?
100% of filtered penicillin is excreted.
Some additional penicillin is secreted into the proximal/distal tubule from the interstitial fluid.
How can penicillin have an active transporter into the proximal and distal tubules?
Transporter is not specific to penicillin.
Transports toxic acidic molecules (EG: probenecid)
Why could it be beneficial to co-prescribe penicillin and probenecid?
Both compete for binding to active transporter that secretes these substances into the proximal/distal tubules.
Slows clearance of penicillin.
