Glomerular filtration Flashcards
What are the basic renal processes?
Filtration
Reabsorption
Secretion
What is filtration?
The formation of an essentially protein-free filtrate of plasma - which takes place at the glomerular capillaries
What is the Glomerular filtration rate?
What are its normal values?
GFR describes the amount of fluid (the filtrate of plasma) that flows through the kidneys each minute - ie how well the kidneys are working
125 mls/min
180 L/day
What is the purpose of reabsorption in the kidneys?
Where does it take place?
Substances that the body wants are reabsorbed, those it doesn’t want stay in the tubule and are excreted
Takes place in both the proximal and distal tubule
What is the purpose of the secretion process in the kidneys?
Where does it take place?
What is secreted?
Provides a way to remove specific substances from the body
Both Proximal and distal tubules
Proximal - organic ions, some drugs
Distal - K+ and H+
How much blood flows through the kidneys?
Does this predispose it to any pathologies?
1200 mls/min
This is 20-25% of our total cardiac output
Total blood volume passes through in about 5 mins
The fact that the kidneys normally receive such a high BF explains their vulnerability to damage by vascular disease
What components of the blood are not allowed to enter the kidneys?
None of the red cells and only a fraction of the plasma proteins are filtered through into Bowman’s capsule
All these components just pass through efferent arterioles into the peritubular capillaries and then into the renal vein
What is renal plasma flow?
How do you calculate it?
What is its normal value?
Plasma constitutes about 55% of our total BV
0.55 x 1200 ml/min = 660 ml/min = renal plasma flow
Renal plasma flow is just how much PLASMA the kidneys receive through the renal arteries
Normal value is 660 ml/min
What is meant by the filtration fraction?
Figure out its normal value
normal renal plasma flow = 660 ml/min
normal GFR = a number that you should know
Kidneys as a whole receive 660 ml of plasma each minute
GFR = 125 ml/min - so the glomerular stuff removes 125ml’s of the 660ml’s each minute
Filtration fraction so for some stupid fkn reason its a percentage
125/660 x 100 = 19%
What are the forces that drive plasma through the glomerular capillaries?
Glomerular Filtration occurs in exactly the same way as fluid filters out of any capillary in the body
It is a balance between hydrostatic forces (favouring filtration) and oncotic pressure forces (favouring reabsorption)
These are called the Starling forces
What features are present in the glomerular capillaries that affect its permeability?
Fenestrations (pores):
- fenestrations of glomerular endothelial cells do not allow RBC’s through but allow all other components of the plasma through
Basal lamina:
- Does not allow passage of larger proteins
Slit membranes between Pedicels:
- Does not allow passage of medium-sized proteins
Describe the differences in pressure between glomerular capillaries and other capillaries
Why is this?
PGC is higher than most of the capillaries of the body
Afferent arterioles are short and wide - so offer little resistance to flow
Efferent arterioles are long and thin - so there is high post-GC resistance
Creates a sort of traffic jam - creating really high pressure in the glomerular capillaries
In terms of the starling forces - describe the net flow in the glomerular capillaries
Glomerular capillaries ALWAYS have higher hydrostatic pressure than oncotic pressure
PGC >> πp
Only filtration occurs at the glomerular capillaries
Give an equation for the Net filtration pressure into Bowman’s capsule
What is the normal value in humans?
NFP = PH - π - Pfluid
10 mmHg
What factors affect GFR?
Subject to both extrinsic and intrinsic control
Extrinsically:
- Sympathetic vasoconstriction
- Catecholamines
- Angiotensin II
Intrinsically:
- Arterial BP autoregulation
Describe the extrinsic control of GFR
Sympathetic vasoconstriction:
- Sympathetic nerve stim causes vasoconstriction in both
- afferent arteriole has greater sensitivity
Circulating catecholamines:
- Primarily causes afferent vasoconstriction
Angiotensin II:
- constriction, of efferent at [low], both afferent and efferent at [high]
Describe the effect of sympathetic stimulation on Glomerular Pressure and thus GFR
Sympathetic stimulation would reduce GFR
Fight or flight response reduces kidney filtration function to allow more blood to pump the muscles eh
What is the effect of Angiotensin II on the GFR?
a) at high [Ang]
b) at low [Ang]
[Low] - efferent constriction = Increased GFR
[High] - afferent and efferent constriction = Decreased GFR
Describe the intrinsic control of GFR (autoregulation)
What range of BP does it work over?
Describe the intrinsic response to increased BP
BF and GFR kept relatively constant due to autoregulation
Works within the range of 60-130 mmHg
If mean arterial pressure increases - there is an automatic constriction of afferent arterioles
This prevents a rise in PGC and thus prevents an unwanted rise in GFR
How does intrinsic and extrinsic control of GFR work?
In situations where Blood volume or blood pressure are compromised (eg haemorrhage) - then extrinsic controls can OVERRIDE autoregulation
This provides blood for usage in other tissues which need it more urgently
What is the reabsorptive role of the peritubular capillaries?
Since 180 l/day are filtered through the glomerulus into the renal tubule, but only 1-2 l/day are excreted as urine an enormous amount of fluid must be reabsorbed back into peritubular capillaries
They are very long and narrow - it offers resistance along its entire length, there is a large P drop so that hydrostatic pressure is very low
In terms of the starling forces - explain why there is reabsorption in the peritubular capillaries
Ppc - Hydrostatic pressure in peritubular capillaries is low
πp - oncotic pressure high compared to normal (loss of plasma earlier means its way more concentrated)
πp >> Ppc
