Glomerular Filtration Flashcards by Sophia Boyce

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)

How well did you know this?

Not at all

Perfectly

The glomerular filtration rate (GFR) is very high, what is the value?

180 l/day

How well did you know this?

Not at all

Perfectly

What is reabsorption?

Substances that the body wants are reabsorbed, those it doesn’t want stay in the tubule and are excreted

How well did you know this?

Not at all

Perfectly

What is secretion?

Substances may be specifically removed from the body in this way

How well did you know this?

Not at all

Perfectly

Point out the 3 basic renal processes on this diagram

Left - glomerular filtration

Bottom - reabsorption

Top - secretoin

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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 well did you know this?

Not at all

Perfectly

How long does it take for a BV equal to the total BV to pass through the renal circulation?

<5 mins

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

What is the normal GFR in mls/min?

125mls/min

How well did you know this?

Not at all

Perfectly

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%

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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 P_GC 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?

P_H - π - P_fluid = 55 - 30 - 15 = 10mmHg

In normal physiology, what is the primary factor affecting GFR and what is this dependent on?

Primary factor is P_GC 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 - P_GC(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. P_PC ~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 P_GC \> π_p Peritubular capillaries - P is v low because hydrostatic P overcoming frictional resistance in efferent arterioles; π_p high compared to normal, loss of 20% plasma concentrates plasma protein; so π_p \> P_PC 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