Glomerular Filtration Flashcards
What is filtration?
The formation at the glomerular capillaries of an essentially protein-free filtrate of plasma (~0%)
(allows regulation of ECF volume and control of blood composition)
The glomerular filtration rate (GFR) is very high, what is the value?
180 l/day
What is reabsorption?
Substances that the body wants are reabsorbed, those it doesn’t want stay in the tubule and are excreted
What is secretion?
Substances may be specifically removed from the body in this way
Point out the 3 basic renal processes on this diagram
Left - glomerular filtration
Bottom - reabsorption
Top - secretoin
How much blood flow do the kidneys receive? What does this mean about their BF/tissue g?
~1200mls/min (20-25% of total CO)
They weigh <1% of BW so they have almost the highest BF/g tissue of any tissue in body
What does te high BF to the kidneys explain?
Their vulnerability to damage by vascular disease (this is why kidneys are so protected - at back, a lot of fat etc)
How long does it take for a BV equal to the total BV to pass through the renal circulation?
<5 mins
How many RBCs and plasma is filtered through into Bowman’s capsule? What happens to the remainder?
No RBCs
Only a fraction of plasma
Remainder passes via efferent arterioles into the peritubular capillaries and then into renal vein
What percentage of total BV is plasma? So what is the value of renal plasma flow?
55%
55% of 1200mls/min = 660mls/min = renal plasma flow
What is the normal GFR in mls/min?
125mls/min
If renal plasma flow = 660mls/min and GFR = 125mls/mins, waht is the filtration fraction i.e. what percentage of renal plasma becomes glomerular filtrate?
125/660 x 100 = 19%
What forces is glomerular filtration dependent on? (GF occurs in exactly the same way as fluid enters out of any capillary in the body)
Starling’s
(dependent on balance between hydrostatic forces favouring filtration and oncotic pressure forces favouroing reabsorption)
What 3 factors deterine the filterability of solutes across the glomerular filtration barrier?
Molecular size, electrical charge and shape
In chart - low molecular weight molecules all have diameter less than a nanometre; polyethylene glycol is last substance that will be completely filtered out; filtration rate then drops
What is the structure of the glomerular endothelial cell? How is this useful for its function?
Fenestrated
Prevents filtration of BCs but allows all components of blood plasma to pass through
What does the basal lamina of the glomerulus prevent filtration of?
Larger proteins
What does the slit membrane between pedicels (of podocytes) prevent filtration of?
Medium-sized proteins
How are the layers of filtration membrane arranged? How does this suit its function?
Glomerular endothelial cells -> basal lamina -> slit membrane
Starts with bigger particles being excluded, then getting finer and finer to ultimately exclude everything that needs to be (if it was the other way round it would clog up quickly)
What about the afferent and efferent arterioles at the glomerulus means that glomerular capillary pressure PGC is higher than in most of the capillaries in the body?
Afferent arteriole is short and wide and offers little resistance to flow, so blood arriving at the glomerulus still has a high hydrostatic pressure
Unique arrangement of efferent arteriole is long and narrow and offers a high post-capillary resistance
Golden rule of circulation: if you have a high resistance…
hydrostatic pressure upstream is increased, while the pressure downstream is decreased
At glomerular capillaries what is the relationship between hydrostatic P and oncotic P?
Hydrostatic P favouring filtration always exceeds oncotic P
Arterioles at both ends (wide afferent = good pressure coming in; narrow efferent = increased pressure inside) create a pressure high enough to counteract the oncotic forces that are produced by the osmotic effects of all the particles in the plasma
How do we calcultate net filtration pressure (NFP)?
hydrostatic pressure (blood pressure) - colloid osmotic pressure gradient due to proteins in plasma but not in Bowman’s capsule - fluid pressure created by fluid in Bowman’s capsule
PH - π - Pfluid
What is the value of net filtration pressure?
PH - π - Pfluid = 55 - 30 - 15 = 10mmHg
In normal physiology, what is the primary factor affecting GFR and what is this dependent on?
Primary factor is PGC and this is dependent on afferent and efferent arteriolar diameter and therefore the balance of resistance between them (pressure in glomerulus increases, filtration increases and vice versa)
GFR is subject to extrinsic control via 3 factors - describe them:
a) Sympathetic VC nerves -> afferent and efferent constriction, greater sensitivity of afferent arteriole
b) Circulating catecholamines -> constriction primarily afferent
c) Angiotensin II ->constriction, of efferent at [low], both afferent and efferent at [high]
What is the effect of increasing resistance in the afferent and then the efferent arterioles? What would happen if resistance of afferent arterioles was decreased?
- Afferent increase = pressure after constriction decreases and so does intraglomerular pressure - results in increased blood flow to other organs
- Efferent increase - constricts outflow of blood from glomerulus resulting in increased intraglomerular pressure ultimately resulting in increased GFR
- Afferent decrease - PGC (intraglomerular pressure) would increase and ultimately so would GFR
What does renal vasculature adjust its resistance in response to? What does this then mean for BF and GFR?
Changes in arterial BP (intrinsic ability)
BF and GFR are essentially kept constant = autoregulation
What is the range of MBP over which instrinsic autoregulation of BF and GFR is essentially constant?
60-130mmHg
Below MBP of 60mmHg what happens to filtration?
Filtration falls and at 50mmHg it ceases altogether
Above range - there is an uncontrolled increase in filtration
If mean arterial pressure increases, what happens to afferent arteriolar constriction?
Automatically increases, preventing a rise in glomerular PGC
(opposite occurs with decreased mean arterial pressure - dilatation)
Is autoregulation of BF and GFR dependent on nerves or hormones?
NO; occurs even in denervated and in isolated perfused kidneys
In situations where BV/BP face serious compromise, e.g. haemhorrage, what occurs in the kidneys?
Activation of sympathetic VC nerves and A II, can override autoregulation liberating blood for more immediately important organs
Up to how much blood can be provided to perfuse other organs at the expense of the kidney in haemhorrage?
800mls/min
(kidney can still function at a v reduced volume - bleeding and losing volumes we dont need to filter too much volume out)
What happens in prolonged reduction of renal BF?
Can lead to irreparable damage which may lead to death cos of disruption of kidney’s role in homeostasis
Give a brief summary of the information in these cards (including percentages reabsorbed, filtered, and excreted into external environment)
What capillaries are responsible for reabsorption?
Peritubular
What effects does the unique narrow, high resistance efferent arteriole exhibit 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 v low i.e. PPC ~15mmHg
In terms of pressures and Starling forces, why does only reabsorption occur in peritubular capillaries while only filtration occurs in glomeruluar capillaries?
Glomerular capillaries PGC > πp
Peritubular capillaries - P is v low because hydrostatic P overcoming frictional resistance in efferent arterioles; πp high compared to normal, loss of 20% plasma concentrates plasma protein; so πp > PPC means only reabsorption
How much of H2O, glucose, Na+ and urea filtered at the glomerulus is reabsorbed within the tubule, and where is it mainly reabsorbed?
99% H2O, 100% glucose, 99.5% Na+ and 50% urea
Mainly at proximal convoluted tubule
