Glomerular Filtration Flashcards
what are the names of the basic renal processes?
filtration
reabsorption
secretion
what is filtration?
Filtration: the formation at the glomerular capillaries of an essentially protein-free filtrate of plasma, (~20%)
What is glomerular filtration rate? is it high or low? and what does this result in?
Glomerular filtration rate describes the flow rate of filtered fluid through the kidney
The glomerular filtration rate (GFR) is very high = 180 l/day. This means that the kidney has ample opportunity to precisely regulate ECF volume and composition and eliminate “nasty” substances
what is reabsorption?
Reabsorption: Substances that the body wants are reabsorbed, those it doesn’t want stay in the tubule and are excreted
what is secretion?
Secretion: Substances may be specifically removed from the body in this way
- Process of Filtration:
How much blood flow do the kidneys recieve and what is you BF/g tissue ratio like?
Kidneys receive a blood flow of » 1200mls/min, ie 20-25% of total cardiac output
Kidneys weigh <1% of BW and therefore have almost the highest BF/g tissue of any tissue in the body
The fact the kidneys recieve such high BF is a risk factor for what?
The fact that the kidneys normally receive such a high BF explains their vulnerability to damage by vascular disease
In < 5mins, a volume of blood equal to the total BV passes through the renal circulation
Are red cells and plasma filtered through into the bowmans capsule?
None of the red cells and only a fraction of the plasma is filtered through into Bowman’s capsule
The remainder passes via the efferent arterioles into the peritubular capillaries and then to the renal vein
WHat percentage of blood does plasma make up?
Plasma constitutes around 55% of total BV
how do you calculate renal plasma flow?
Kidneys receive a blood flow of » 1200mls/min, ie 20-25% of total cardiac output
55% of 1200mls/min = 660mls/min = renal plasma flow
GFR is normally 125mls/min
therefore what is the filtration fraction?
Plasma constitutes » 55% of total BV
55% of 1200mls/min = 660mls/min = renal plasma flow
Therefore the filtration fraction = 125/660 x 100 = 19% ie 19% of the renal plasma becomes glomerular filtrate
Does Glomerular filtration occurs in the same way as what other process in the body?
Glomerular Filtration occurs in exactly the same way as fluid filters out of any capillary in the body
Glomerular filtration is dependent on a balance between what?
It is dependent on the balance between the hydrostatic forces favouring filtration and the oncotic pressure forces favouring reabsorption (Starling’s forces)
what factors are improtant in regards to determaning permselectivity of glomerular barrier?
Molecular size, electrical charge and shape determine the filterability of solutes across the glomerular filtration barrier
describe the structure of the filtration membrane
Glomerular capillary pressure PGC is higher than in most of the capillaries in the body, why is this?
Glomerular capillary pressure PGC is higher than in most of the capillaries in the body because the afferent arteriole is short and wide and offers little resistance to flow. So the blood arriving at the glomerulus still has a high hydrostatic pressure
In addition, the unique arrangement of the efferent arteriole which is long and narrow offers a high post-capillary resistance
Golden rule of the circulation:
If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased
Golden rule of the circulation:
If you have a high resistance, hydrostatic pressure upstream is _________, while the pressure downstream is ________
What contributes to the high glomerular capillary pressure?
Therefore both the afferent and efferent arterioles contribute to the very high PGC
At glomerular capillaries, how does hydrostatic pressure compare to oncotic pressure?
Therefore at the glomerular capillaries the hydrostatic P favouring filtration always exceeds the oncotic P.
PGC >> oncotic p
What is the only thing that happens at glomerular capillaries?
ONLY filtration occurs at the glomerular capillaries
how is net filtration pressure calculated?
what is the primary factor affecting GFR?
In normal physiology, 1° factor is PGC and this is dependent on the afferent and efferent arteriolar diameter and therefore the balance of resistance between them
afferent and efferent arteriolar diameter and therefore the balance of resistance between them is subject to extrinsic control via what?
a) Sympathetic VC nerves = afferent and efferent constriction, greater sensitivity of afferent arteriole
Sympathetic constriction means we can change the pressure in the glomerulus and therefore glomerulus filtration rate
b) Circulating catecholamines = constriction 1°ily afferent
c) Angiotensin II = constriction, of efferent at [low], both afferent and efferent at [high].
diagram showing how relative diameter of afferent and efferent arterioles is crucial in determining GFR
In d) GFR would increase as the PGC would increase and GFR would increase
how are BF and GFR kept constant?
Renal vasculature also exhibits a well developed intrinsic ability to adjust its resistance in response to changes in arterial BP
This keeps BF and GFR essentially constant = autoregulation
Over what range does autoregulation act?
In man, effective over a range of MBP from 60-130mmHg
Below 60mmHg, filtration falls and ceases altogether when MBP = 50mmHg
what happen if MAP increases? and what if it decreases?
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
Autoregulation is independent of ______ or ________, occurs in denervated and in isolated perfused kidneys
Autoregulation is independent of nerves or hormones, occurs in denervated and in isolated perfused kidneys
what else also plays a cruical role in glomerular filtration?
blood volume
There is interaction between intrinsic and extrinsic controls
In situations where blood volume/BP face serious compromise, eg in haemorrhage, what happens?
activation of sympathetic VC nerves and AII, can override autoregulation liberating blood for more immediately important organs.
how much blood can be provided to other organs at the expense of the kidneys?
As much as 800mls of blood per minute can thus be provided to perfuse these other organs at the expense of the kidney
this is particularly important in haemorrhage
what can prolonged reducion in renal BF lead to?
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 does endurance training affect renal blood flow?
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 responsible for reabsorption?
Peritubular capillaries are responsible for reabsorption
how much fluid each day is reabsorbed back into the peritubular capillaries?
Since 180 l/day are filtered through the glomerulus into the renal tubule, but only 1-2 l/day are excreted as urine an enormous amount of fluid must be reabsorbed back into peritubular capillaries
how does the efferent arteriole affect peritubular capillaries?
The unique efferent arteriole has important effects on Starling’s forces in the peritubular capillaries
Because it offers resistance along its entire length, there is a large P drop so that hydrostatic pressure is very low, ie PPC around 15mmHg
How does reabsorption occur in the peritubular capillaries?
In peritubular capillaries:
PPC very low because hydrostatic P overcoming frictional resistance in efferent arteriols
osmotic pressure high compared to normal, loss of 20% plasma concentrates plasma protein
osmotic pressure >> PPC only reabsorption
what is the only thing to happen in the glomerular capillaries?
PGC >> Osmotic Pressure
Only filtration occurs at glomerular capillaries
filtration is driven by……………
reabsorption is driven by……………
Filtration - hydrostatic pressure
Reabsorption – driven by osmotic forces (starlings forces)
Since aorund 20% of the plasma has filtered into Bowman’s capsule in the glomerulus, (filtration fraction), the blood remaining in the efferent arteriole and then the peritubular capillaries has what?
a higher concentration of plasma proteins and therefore increase osmotic pressure
As a consequence, the net result of the low PPC and the high oscmotic pressure causes what?
the net result of the low PPC and the high ismotic pressure is that the balance of Starling’s forces in the peritubular capillaries is entirely in favour of reabsorption
what substances are reabsorbed and where about are they mainly reabsormbed?
99% H2O, 100% glucose, 99.5% Na+, 50% urea filtered at the glomerulus are reabsorbed within the tubule, mainly at the proximal convoluted tubule
