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