Filtration Flashcards
What are the 3 basic renal processes?
Filtration
Reabsorption
Secretion
What is filtration?
Formation at glomerular capillaries of an essentially protein free filtrate of plasma
Glomerular filtration rate is very high - 180l/day so kidneys regulate ECF volume and composition
Explain kidney blood flow
Receive 1200mls/min so 20-25% of total CO
Receive such high blood flow so can be vulnerable to damage by vascular disease
Where is plasma filtered?
No red blood cells and only a fraction of plasma is filtered through Bowman’s capsule
Remainder passes via efferent arterioles into peritubular capillaries then to renal vein
What is the renal plasma flow?
660mls/min
As 55% of total BV (1200mls/min)
What is the normal GFR?
120mls/min
What is the filtration fraction?
125/660mls/min x 100 = 19%
So 19% of renal plasma becomes glomerular filtrate
What is glomerular filtration dependant on the balance between?
Hydrostatic forces favouring filtration and oncotic pressure forces favouring reabsorption (Starling’s forces)
What contributes to the perm-selectivity of the glomerular barrier?
Molecular size, electrical charge and shape determine filterability
What are the 3 layers (filters) of the Bowman’s capsule?
1 - fenestration of glomerular endothelium (prevents blood cells)
2 - basal lamina of glomerulus (prevents large molecules)
3 - slit membrane between pedicles (prevents medium sized molecules)
Why is the Glomerular capillary pressure (Pgc) higher than in most capillaries in the body?
Because the afferent arteriole is short and wide so offers little resistance to flow
So blood arriving to glomerulus still has a high hydrostatic pressure
What is the unique arrangement of the efferent arteriole?
Long and narrow so offers a high post-capillary resistance
What is the golden rule of the circulation?
If you have high resistance then hydrostatic pressure upstream is increased and pressure downstream is decreased - so both contribute to high Pgc
What is pressure is higher in the glomerular capillaries?
Hydrostatic pressure favours filtration which exceed oncotic pressure
What only occurs in glomerulus capillaries?
Filtration
Explain the net filtration pressure
Ph - 55mmHg
Oncotic - 30 mmHg
P fluid - 15mmHg
55 - 30 -15 = 10mmHg which drives filtration
What is the primary factor affecting GFR in normal physiology?
Afferent and efferent arteriolar diameters and therefore the balance of resistance between them
What extrinsic control affects GFR?
Sympathetic VC nerves
Circulating catecholamines
Angiotensin II
How does sympathetic VC nerves affect GFR?
Afferent and efferent constriction and greater sensitivity if afferent arteriole
How does circulating catecholamines affect GFR?
Constriction primarily in afferent arterioles
How does angiotensin II affect GFR?
Constriction of efferent at low levels and both afferent and efferent at high levels
What happens if there is increased resistance in afferent arteriole?
Increased blood flow to other organs
Decreased GFR as decreased capillary BP
What happens if there is increased resistance in efferent arteriole?
Increased GFR as increased Ph
Decreased RBF
What is the intrinsic affect on GFR?
Adjusts resistance in response to changes in arterial BP
Keeps BF and GFR essentially constant - autoregulation
What is the effective range of MBP?
60-130mmHg
What happens if MBP falls below or increases?
Falls - filtration falls and ceases all together. Dilation occurs
Increase - increase in afferent arteriolar constriction so prevents rise in Pgc
What is autoregulation independent of?
Nerves or hormones
Occurs in denerved and in isolated perfused kidneys
What happens if blood volume/BP face serious compromise?
Activation of sympathetic VC nerves and AII
Can overrise autoregulation liberating blood for more important organs
800mls of blood can be given to perfuse other organs in expense of kidney
What does prolonged reduction in renal BF result in?
Irreparable damage which may lead to death because kidney’s role in homeostasis is disrupted
Explain marathons and kidneys
Endurance training events means adaption of skeletal muscle so requirement of blood flow is more efficient so less need to take from kidneys
How much volume filters at Bowman’s capsule?
20% and then more than 19% is reabsorbed
So only less than 1% of volume is excreted to external environment
What are responsible for reabsorption?
Peritubular capillaries as 180l/day is filtered but only 1-2l/day is excreted as urine
Efferent arterioles have important effects on Starling’s forces
Describe the pressure in efferent arterioles
Offers resistance along entire length - large drop in pressure so that hydrostatic pressure is very low - Ppc is 15mmHg
Oncotic pressure higher so favours reabsorption
How is the plasma protein conc. higher in peritubular capillaries?
As 20% of plasma filtered into Bowman’s capsule
Blood remaining in efferent and peritubular capillaries has higher conc. so higher oncotic pressure
What is reabsorbed in the proximal convoluted tubule?
99% H2O
100% glucose
99.5% Na
50% urea
