Glomerular Filtration Flashcards
What are the 3 basic renal processes?
Filtration
Reabsorption
Secretion
Explain filtration.
Filtration occurs to form a protein free filtrate of plasma at the glomerular capillaries
What is the level of the GFR per day?
180 l/day
What kind of things are reabsorbed in the kidneys?
NaCl
Water
Amino acids
Sugars
Why is secretion by the kidneys clinically important?
Important for drug secretion
Why are kidneys so susceptible to damage by vascular disease?
It is a small organ that deals with very high volumes of blood at a time
Is all this high volume of blood filtered?
No, none of the big cells (RBC, WBC, platelets) are filtered in the bowman’s capsule
Only a small fraction of the plasma is actually filtered
What happens to the rest of the blood not filtered by the kidneys?
Passes via efferent arterioles into the peritubular capillaries and into the renal vein
How much blood is filtered by the kidney’s per min and how much is just plasma?
1200 mls/min
~55% - 660 mls/min
What is the normal GFR per min?
125mls/min
What is the filtration fraction?
GFR/Total renal plasma flow x 100
125/660 x 100 = 19%
What is glomerular filtration dependant on?
Hydrostatic forces favouring filtration
Oncotic pressure forces (starlings) favouring reabsorption
What affects the filterability of solutes across the glomerular filtration barrier?
Molecular size
Electrical charge
Shape
What prevents filtration of blood cells but allows all components of blood plasma to pass through the glomerulus?
Fenestration of glomerular endothelial cell
What does the basal lamina of the glomerulus do?
Prevents filtration of larger proteins
What prevents filtration of medium sized proteins?
Slit membrane between pedicles
What is Pgc?
Glomerular capillary pressure
Why is Pgc higher than most capillaries?
The afferent arteriole is short and wide and offers little resistance to flow meaning the blood arriving at the glomerulus still has a high hydrostatic pressure
Equally the efferent capillaries are long and narrow offering high post-capillary resistance
What is the golden rule of circulation?
If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased
Why is this high pressure needed in the glomerulus?
Hydrostatic pressure favours filtration while oncotic pressure favour reabsorption
So at the glomerular capillaries since the hydrostatic pressure is higher filtration will be favoured
What is the pressure of the hydrostatic forces?
55 mmHg
What is the opposing osmotic pressure and fluid pressure?
30 mmHG osmotic and 15mmHg fluid pressure
So what is the net filtration pressure?
55 - (30+15) = 10 mmHG
What is the 1y factor in normal physiology affecting GFR?
Glomerular capillary pressure which depends on the afferent and efferent arteriolar diameter’s and therefore resistance
In what way are the glomerular capillaries subject to extrinsic control?
Sympathetic VC nerves = afferent and efferent vasoconstriction–> greater sensitivity of afferent arteriole
Circulating catecholamines - constriction of afferent
Angiotensin 2 - constriction of only efferent at low volumes and both at high volumes
What happens to GFR in resistance is increased in afferent vessel? What happens when there is little resistance ?
Decreases as blood can’t get through
GFR increases with little resistance as more blood can get through which increases pressure
What happens to GFR if resistance is increased in efferent vessel?
Increases even though less blood gets through as pressure is increased
Renal vasculature has a BLANK ability to adjust its resistance in response to changes in BLANK
What does this help do?
Intrinsic
Arterial BP
Helps keep blood flow and GFR constant = autoregulation
What range of mean BP is the autoregulation effective over?
60-130 mmHg
What happens below 60 mmHg mean BP?
Filtration starts to fall
What happens below 50 mmHg mean BP?
Filtration stops
What happens if mean arterial pressure increases? Why does this happen?
Autonomic increase in afferent arteriolar constriction - if this did not happen glomerular capillary pressure would increase and so would filtration
What happens if mean arterial pressure decreases?
Afferent vessel dilates to restore Pgc
What is autoregulation independent of?
Nerves or hormones as it happens in denervated and isolated perfused kidneys
Is there any interaction between intrinsic and extrinsic controls?
Yes - in situations when blood volume drops very low (haemorrhage)
The activation of sympathetic VC nerves and Angiotensin 2 overrides autoregulation (which would have sought to dilate afferent vessel to raise GFR) and instead constricts the vessels
This stops filtration allowing blood to be redirected to important organs
What does prolonged reduction in renal blood filtration cause?
Irreparable damage which may then lead to death due to disruption of the kidney’s role in homeostasis.
What % of blood plasma volume filters in the glomerulus/bowmans capsule?
20% the rest returns to systemic circulation
What % of fluid is re-absorbed?
19%
Peritubular capillaries are responsible for ?
Reabsorption
How do we know reabsorption of fluid occurs?
Since 180 l/day are filtered through the glomerulus into the renal tubule, but only 1-2 l/day are excreted as urine
Why is the efferent arteriole good for reabsorption
Because it offers resistance along its entire length, there is a large P drop so that hydrostatic pressure is very low - 15mmHg
So starlings forces are not opposed and actually increase and can cause reabsorption
What causes Starling’s forces to increase in peritubular capillaries?
Since 20% of the plasma has been filtered. The blood remaining in the efferent arteriole and then the peritubular capillaries has a higher concentration of plasma proteins and therefore a higher con. of osmotic pressure
