1. Glomerular filtration Flashcards
what are the three basic renal processes
- filtration
- reabsorption
- secretion
what is filtration
the formation of an essentially protein-free filtrate of plasma at the glomerular capillaries
what percentage of plasma is filtered out at one time
~20%
what is the glomerular filtration rate (GFR)
around 180 l/day - very high so allows opportunity to precisely regulate ECF volume and composition and eliminate “nasty” substances
what happens during reabsorption
substances the body wants are retaken up and those it doesnt want stay in the tubule and are excreted
where does reabsorption take place
proximal or distal tube
what/what percentage of plasma is reabsorbed/filtered back
sugars, amino acids, Na, water, etc are reabsorbed
around 99% filtered back
what happens in secretion/excretion
substances may be specifically removed from the body
what 2 things is excretion particularly important for
drug metabolites
proton excretion for acid base balance
in basic terms how are drug metabolites excreted in the kidneys
drug metabolites broken down and processed by the liver to convert to water soluble substances – can then be actively excreted by the kidney
what percentage of total cardiac output do the kidneys receive
20-25% (~1200mls/min)
how long does it take for a volume of blood equal to total BV to pass through the renal circulation
<5 mins
as they get 20-25% of blood flow per minute
why is kidney haemorrhage particularly dangerous
due to high BV passing though - can bleed out in less than 5 mins
what components of plasma are excluded from filtration through the bowmens capsule
all cells - RBC< WBC, platelets, etc
*also only a tiny fraction of plasma gets filtered thorough the bowmens capsule
what percentage of total blood volume is plasma
~55%
what is the renal plasma row
around 660mls/min
what is the Glomerular Filtration Rate
around 125mls/min
what is the filtration fraction
GFR/renal plasma flow x100
= 19%
ie ~20% of the renal plasma become glomerular filtrate
what 2 forces is glomerular filtration dependent on
- hydrostatic forces from blood stream - pressure on top of fluids to drive through filter - favour filtration
- oncotic pressure forces (starlings forces) - osmotic effects - favour reabsorption
what is the main criteria for how effectively substances are filtered out
size - if small the filtrate ratio is 1 = concentration infiltrate is the same as plasma concentration
= get completely filtered out
apart from size what can also have an effect on how effectively substances are filtered out
charge of molecule and basement membrane that mediates the filtration process
how are different substances filtered out
by a “sandwich” system of different membranes
what are the three membranes that are part of the filtration process and what passes through them
- fenestrated glomerular endothelium - lets everything pass through except cells
- Basal lamina of glomerulus - prevents filtration of larger proteins
- slit membrane - prevents filtration of medium sized proteins
why is glomerular capillary pressure higher than most other capillaries in the body
because afferent arterioles are short and wide - little resistance to flow - lots of blood - blood arriving still has high hydrostatic pressure
how do the efferent arterioles allow for a build up of pressure in the glomerulus and thus aid filtration
they are long and narrow - increase friction and resistance - build up glomerular pressure - inhibit free outflow from glomerulus - push fluid through
what is the golden rule off circulation
high resistance =
upstream pressure INCREASED
downstream pressure DECREASED
what is the relation between hydrostatic pressure and oncotic pressure at the glomerular capillaries
hydrostatic pressure always exceeds oncotic pressure at glomerular capillaries
BUT only at glomerular capillaries
what happens to the plasma concentration as it gets filtered
become more concentrated
what would happen if oncotic pressure exceeded hydrostatic pressure
filtration would reverse
what happens when the tubes start to fill with filtered fluid
start to exhibit hydrostatic pressure themselves - makes water want to go back from tubes to bloodstream
why does the fluid NOT go back to the blood stream despite increased hydrostatic pressure and oncosmotic pressure from the tubes
NET filtration is 10mmHg - still in favour of filtration
15 mmHg out
30 mmHg out
BUT
55mmHg in
what is the primary factor affecting glomerular filtration rate
glomerular capillary pressure (PGC)
what affects PGC
afferent and efferent arteriolar diameter and therefore balance of resistance between them
what are three EXTRINSIC controls that can affect GFR
- sympathetic vasoconstrictory nerves - afferent and efferent constriction, greater sensitivity of afferent arteriole
- circulating catecholamines - constriction primarily afferent
- angiotensin II - constriction of efferent at [low]
constriction of both afferent and efferent at [high]
what happens when you constrict afferent arterioles
reduce blood flow to kidney and glomerulus – pressure in glomerulus drops – glomerular filtration goes down – as a result increased blood flow to other organs - can be important in preserving organ function if blood volume drops
what happens when you constrict efferent arterioles
overall bloodflow decreased – but pressure in glomerulus goes up – upstream constriction = increased pressure – despite reduced blood flow to kidney
what happens when you dilate afferent arterioles
decrease resistance in afferent – blood flow increased – efferent unchanged - pressure in glomerulus goes up – GFR goes up also
how is BF and GFR kept constant in the kidneys
auto regulation of renal vasculature from intrinsic ability to adjust resistance in response to changes i arterial BP
what is the range of mean blood pressure in man that the kidneys can auto regulate BF and GFR over
when mean blood pressure is from 60-130mmHg
how is a rise in glomerular capillary pressure prevented
If mean arterial P increases, there is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular PGC . Dilatation occurs if P falls.
what happens if mean blood pressure falls below the effective range for self regulation
If goes below effective range – filtration stops – pressure not big enough to drive filtration – movement of fluid can even reverse
*filtration ceases when MBP = 50mmHg
how can glomerular filtration rate be maintained if blood volume if low
constrict afferent arterioles
in what situations can autoregulation be overridden
in situations when BV/BP face serious compromise e.g. haemorrhage
how can auto regulation be overridden
activation of sympathetic vasocontrictory nerves and angiotensin II can override to liberate more blood for immediately important organs e.g. brain
what happens if there is prolonged reduction in renal BF
can lead to irreparable damage which may then lead to death because of disruption of the kidney’s role in homeostasis.
how do marathons affect the kidneys
Endurance training for very long events involves adaptation of skeletal muscle so that its requirement for blood flow becomes more efficient and therefore there is less need to take it from the kidneys
what is the % plasma volume entering the afferent arteriole
100%
what is the % plasma volume filtered through the bowman capsule
20%
what is the % plasma volume reabsorbed along remainder of nephron
> 19%
what is the % plasma volume that enters back into circulation after entering the kidney
> 99% goes into systemic circulation
what is the % plasma volume that is excreted to the external environment
<1%
what capillaries are responsible for reabsorption
peritubular capillaries
what does resistance along the entire length of peritubular capillaries mean pressure wise
causes a large drop in pressure so that hydrostatic pressure is very low - ~15mmHg
how do pressure changes allow for reabsorption in the peritubular capillaries
Very low hydrostatic pressure in peritubular capillaries but high osmolarity (due to high concentrations due to fluid having been take out) – allows for reabsorption
ie balance of starling forces in favour of reabsorption
what % of water is reabsorbed within the tubule
99%
what % of glucose is reabsorbed within the tubule
100%
what % of Na+ is reabsorbed within the tubule
99.5%
what % of urea is reabsorbed within the tubule
50%
