Glomerular filtration Flashcards
List the three basic renal processes.
Filtration
Reabsorption
Secretion
What % of the plasma volume passing through the glomerulus at any given time is filtered
20%
What is the blood filtering component of a nephron
Renal corpuscle
20% of the plasma volume passing through the glomerulus at any given time is filtered, what happens to the other 80%
passes via the efferent arterioles into the peritubular capillaries and then to the renal vein
Plasma makes up what % of blood volume
55%
Glomerular filtration rate > ?mls/min
> 90
Glomerular filtration is dependant on the balance between what forces
Hydrostatic forces (pushing fluid out capillary, i.e. favouring filtration)
and
Oncotic forces (favouring fluid into capillary, i.e. favouring reabsorption)
What is starling’s hypothesis
that the fluid movement due to filtration across the wall of a capillary is dependent on the balance between the hydrostatic pressure gradient and the oncotic pressure gradient across the capillary
Glomeruli are permeably selective on what solutes they allow cross the glomerular filtration barrier - what are some of the things considered
Molecular size
Electrical charge
Molecular shape
The fenestrated capillaries making up each glomerulus prevents filtration of what cells into the glomerulus
Blood cells - RBC/WBC etc
only allows plasma to get through
Why is glomerular capillary pressure higher than most other capillaries in the body
because the afferent arterioles making up the glomerulus is short and wide and so little resistance to flow so blood arriving has high hydrostatic pressure
Afferent arterioles are one of the final divisions of each segmental artery supplying the kidney, what do the afferent arterioles form
capillary network, i.e. the glomerulus
Efferent arterioles of the glomerulus are unlike the afferent arterioles - how?
long and narrow so offers a high post-capillary resistance
At the glomerular capillaries, the high capillary pressure means what starling’s force is favoured
hydrostatic pressure favouring filtration exceeds the oncotic pressure
Explain why normally only filtration occurs at the glomerular capillaries
because glomerular capillary pressure always»_space; colloid osmotic pressure gradient due to proteins in plasma (i.e. oncotic pressure)
so filtration always favoured
Factors affeting GFR
Glomerular capillary pressure
Glomerular capillary pressure is dependent on
afferent and efferent arteriolar diameter and therefore the balance of resistance between them
Afferent and efferent arteriolar diameter is extrinsically controlled by what 3 things
Sympathetic nerves –> constrict them
Catecholamines –> constrict them
Angiotensin II –> constrict efferent arterioles at low conc of angiotensin II at both at high conc.
If resistance is increased in afferent arteriole of the glomerulus, what does this mean for blood flow to other organs
increased blood flow to other organs
If renal blood flow is decreased, what do the efferent arterioles do in response to this to maintain GFR
increase their resistance so decreasing their diameter therefore increasing hydrostatic pressure to favour filtration and increase GFR
If resistance of afferent arterioles were decreased further (they already are low resistance), what does this mean for
- glomerular capillary pressure
- GFR
Glomerular capillary pressure increases so GFR increases
GFR can be maintained over what range of mean arterial BP
60-160mmHg
Below 60mmHg, what happens to GFR
Filtration rate decreases
If mean arterial BP increased, what would be the intrinsic response of the glomerulus to maintain GFR
afferent arteriolar constriction, preventing pressure rise from being transmitted to glomerular capillaries
Renal vasculature has an intrinsic ability to adjust its resistance (i.e. change its diameter) in response to changes in what in order to keep BF and GFR essentially constant, i.e, renal autoregulation
arterial BP
If a severe haemorrhage were to happen, what implication does this have on GFR
systemic vasoconstriction would override autoregulation of renal blood flow and therefore GFR in order to liberate blood for more important organs in that moment
100% plasma volume entering the afferent arteriole of the glomerulus, 20% of this filters into the PCT, how much of this is reabsorbed/excreted
19% reabsorbed into peritubular capillaries and returns to systemic circulation
180 litres of plasma is filtered through the kidneys a day, how much of this is actually excreted as urine
1-2l
Glomerular capillaries lie between what 2 arterioles to form a vascular network
afferent and efferent
Explain why normally only reabsorption occurs at the peritubular capillaries
Because oncotic pressure exceeds hydrostatic pressure in the peritubular capillaries so always favours reabsorption
-since the other 80% blood not filtered into the tubule remains in the afferent arteriole and subsequently the peritubular capillary so it will have high conc. of plasma proteins than in the tubule so will exert increased oncotic pressure
Why is the peritubular capillary pressure very low compared to capillary pressure in the glomerulus
(peritubular capillaries carry blood away from glomerulus)
Because there’s a large frictional resistance along the length of the peritubular capillary (ie. efferent arteriole) while glomerular capillaries always favour filtration into the tubule
Function and afferent and efferent arterioles
Carry blood to and away from the glomerulus respectively
Constriction of the afferent arterioles has two effects:
increases the vascular resistance which reduces renal blood flow (RBF),
and it decreases the pressure downstream from the constriction, which reduces the GFR
Constriction of the efferent arterioles has 2 effects:
also increases the vascular resistance so it reduces RBF.
The pressure within the glomerular capillaries may increase so the net result is an increased filtration fraction (ratio of GFR to RBF) but GFR overall still decreases as RBF is decreased
Filtration fraction is the ratio of
GFR to RBF
If you have high resistance, does hydrostatic pressure upstream/downstream increase or decrease
Upstream - increase
Downstream - decrease
