Glomerular Filtration

Question 1

What are the 3 basic renal processes?

Answer 1

Filtration
Reabsorption
Secretion

Question 2

Explain filtration.

Answer 2

Filtration occurs to form a protein free filtrate of plasma at the glomerular capillaries

Question 3

What is the level of the GFR per day?

Answer 3

180 l/day

Question 4

What kind of things are reabsorbed in the kidneys?

Answer 4

NaCl
Water
Amino acids
Sugars

Question 5

Why is secretion by the kidneys clinically important?

Answer 5

Important for drug secretion

Question 6

Why are kidneys so susceptible to damage by vascular disease?

Answer 6

It is a small organ that deals with very high volumes of blood at a time

Question 7

Is all this high volume of blood filtered?

Answer 7

No, none of the big cells (RBC, WBC, platelets) are filtered in the bowman’s capsule

Only a small fraction of the plasma is actually filtered

Question 8

What happens to the rest of the blood not filtered by the kidneys?

Answer 8

Passes via efferent arterioles into the peritubular capillaries and into the renal vein

Question 9

How much blood is filtered by the kidney’s per min and how much is just plasma?

Answer 9

1200 mls/min

~55% - 660 mls/min

Question 10

What is the normal GFR per min?

Answer 10

125mls/min

Question 11

What is the filtration fraction?

Answer 11

GFR/Total renal plasma flow x 100

125/660 x 100 = 19%

Question 12

What is glomerular filtration dependant on?

Answer 12

Hydrostatic forces favouring filtration

Oncotic pressure forces (starlings) favouring reabsorption

Question 13

What affects the filterability of solutes across the glomerular filtration barrier?

Answer 13

Molecular size
Electrical charge
Shape

Question 14

What prevents filtration of blood cells but allows all components of blood plasma to pass through the glomerulus?

Answer 14

Fenestration of glomerular endothelial cell

Question 15

What does the basal lamina of the glomerulus do?

Answer 15

Prevents filtration of larger proteins

Question 16

What prevents filtration of medium sized proteins?

Answer 16

Slit membrane between pedicles

Question 17

What is Pgc?

Answer 17

Glomerular capillary pressure

Question 18

Why is Pgc higher than most capillaries?

Answer 18

The afferent arteriole is short and wide and offers little resistance to flow meaning the blood arriving at the glomerulus still has a high hydrostatic pressure

Equally the efferent capillaries are long and narrow offering high post-capillary resistance

Question 19

What is the golden rule of circulation?

Answer 19

If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased

Question 20

Why is this high pressure needed in the glomerulus?

Answer 20

Hydrostatic pressure favours filtration while oncotic pressure favour reabsorption

So at the glomerular capillaries since the hydrostatic pressure is higher filtration will be favoured

Question 21

What is the pressure of the hydrostatic forces?

Answer 21

55 mmHg

Question 22

What is the opposing osmotic pressure and fluid pressure?

Answer 22

30 mmHG osmotic and 15mmHg fluid pressure

Question 23

So what is the net filtration pressure?

Answer 23

55 - (30+15) = 10 mmHG

Question 24

What is the 1y factor in normal physiology affecting GFR?

Answer 24

Glomerular capillary pressure which depends on the afferent and efferent arteriolar diameter’s and therefore resistance

Question 25

In what way are the glomerular capillaries subject to extrinsic control?

Answer 25

Sympathetic VC nerves = afferent and efferent vasoconstriction--> greater sensitivity of afferent arteriole Circulating catecholamines - constriction of afferent Angiotensin 2 - constriction of only efferent at low volumes and both at high volumes

Question 26

What happens to GFR in resistance is increased in afferent vessel? What happens when there is little resistance ?

Answer 26

Decreases as blood can't get through GFR increases with little resistance as more blood can get through which increases pressure

Question 27

What happens to GFR if resistance is increased in efferent vessel?

Answer 27

Increases even though less blood gets through as pressure is increased

Question 28

Renal vasculature has a BLANK ability to adjust its resistance in response to changes in BLANK What does this help do?

Answer 28

Intrinsic Arterial BP Helps keep blood flow and GFR constant = autoregulation

Question 29

What range of mean BP is the autoregulation effective over?

Answer 29

60-130 mmHg

Question 30

What happens below 60 mmHg mean BP?

Answer 30

Filtration starts to fall

Question 31

What happens below 50 mmHg mean BP?

Answer 31

Filtration stops

Question 32

What happens if mean arterial pressure increases? Why does this happen?

Answer 32

Autonomic increase in afferent arteriolar constriction - if this did not happen glomerular capillary pressure would increase and so would filtration

Question 33

What happens if mean arterial pressure decreases?

Answer 33

Afferent vessel dilates to restore Pgc

Question 34

What is autoregulation independent of?

Answer 34

Nerves or hormones as it happens in denervated and isolated perfused kidneys

Question 35

Is there any interaction between intrinsic and extrinsic controls?

Answer 35

Yes - in situations when blood volume drops very low (haemorrhage) The activation of sympathetic VC nerves and Angiotensin 2 overrides autoregulation (which would have sought to dilate afferent vessel to raise GFR) and instead constricts the vessels This stops filtration allowing blood to be redirected to important organs

Question 36

What does prolonged reduction in renal blood filtration cause?

Answer 36

Irreparable damage which may then lead to death due to disruption of the kidney’s role in homeostasis.

Question 37

What % of blood plasma volume filters in the glomerulus/bowmans capsule?

Answer 37

20% the rest returns to systemic circulation

Question 38

What % of fluid is re-absorbed?

Answer 38

19%

Question 39

Peritubular capillaries are responsible for ?

Answer 39

Reabsorption

Question 40

How do we know reabsorption of fluid occurs?

Answer 40

Since 180 l/day are filtered through the glomerulus into the renal tubule, but only 1-2 l/day are excreted as urine

Question 41

Why is the efferent arteriole good for reabsorption

Answer 41

Because it offers resistance along its entire length, there is a large P drop so that hydrostatic pressure is very low - 15mmHg So starlings forces are not opposed and actually increase and can cause reabsorption

Question 42

What causes Starling's forces to increase in peritubular capillaries?

Answer 42

Since 20% of the plasma has been filtered. The blood remaining in the efferent arteriole and then the peritubular capillaries has a higher concentration of plasma proteins and therefore a higher con. of osmotic pressure