Glomerular Filtration lecture - Theo's and been through

Question 1

What does a fall in glomerular filtration lead to?

Answer 1

Renal Failure

Question 2

What is Glomerular Filtration?

Answer 2

Passive process: fluid is ‘driven’ through the semipermeable (fenestrated) walls of the glomerular capillaries into the Bowmans capsule space by the hydrostatic pressure of heart.

Question 3

What is meant by something is a ‘passive process’?

Answer 3

Doesn’t require ATP

Question 4

Describe the filtration barrier?

Answer 4

The filtration barrier (fenestrated endothelium of capillaries and semipermeable Bowman’s capsule)

Question 5

What is the barrier permeable to? (x2)

Answer 5

• fluids • small solutes (these are “freely filtered”: same concentration in filtrate and plasma)

Question 6

What is it impermeable to? (x3)

Answer 6

• cells • proteins • drugs etc carried bound to protein

Question 7

What drives Hydrostatic pressure?

Answer 7

Cardiac output

Question 8

What is primary urine?

Answer 8

A clear fluid that is completely free from blood and proteins, is produced containing electrolytes and small solutes.

Question 9

What is the difference of concentration of solutes in the filtrate vs the plasma in the Bowman’s capsule?

Answer 9

No difference

Question 10

Pressure forcing filtrate out of capillary? Notation?

Answer 10

Hydrostatic pressure in glomerulus capillaries due to blood pressure = Pgc.

Question 11

What are the opposing pressures to hydrostatic pressure (P gc)? (x2)

Answer 11

Hydrostatic pressure of Tubule (Pt) AND osmotic pressure of plasma proteins in the glomerular capillaries (Pi gc)

Question 12

What values do you use to calculate the net ultrafiltration pressure (P uf)?

Answer 12

P uf = (P gc)- (P t)- (Pi gc)

Question 13

What is the equation for Glomerular filtration Rate (GFR)?

Answer 13

GFR= (P uf) x (Kf)

(Remember, Puf = Pgc - Pt - Pigc).

Any changes in filtration forces or Kf will result in GFR imbalances. Kidney diseases may reduce number of functioning glomeruli = reduced surface area = dilation of glomerular arterioles by drugs/hormones.

Question 14

What is Kf and what does it describe? (x2) What does this mean practically?

Answer 14

Where Kf is an ultrafiltration coefficient (membrane permeability and surface area available for filtration).

These two variables are mechanisms by which we can therefore change GFR without changing pressure.

Question 15

How can surface area be changed in the kidneys (= change in GFR)? (x4)

Answer 15

Surface area is affected by number of functioning glomeruli (= reduced Kf), dilation of glomerular arterioles (= increased Kf) by drugs and hormones. Inflammation can also alter this.

Question 16

SUMMARY: what can we change to alter GFR?

Answer 16

• PRESSURE (Pgc, πgc, Pt).
• Membrane permeability.
• Surface area.

This all relates to the equation: GFR = (P uf) x (Kf)

(Remember, Puf = Pgc - Pt - Pigc).

Question 17

What is GFR?

Answer 17

• GFR = The amount of fluid filtered from the glomeruli into the Bowmans capsule per unit of time (ml/min). • Sum of filtration rate of all functioning nephrons

Question 18

What is normal GFR?

Answer 18

120ml/min

Question 19

How much of cardiac output enters kidneys? What term describes this?

Answer 19

20%, so 1L/min (because CO is 5L/min) = renal blood flow.

Question 20

What is approximate CO?

Answer 20

5L/min

Question 21

What proportion of blood is plasma?

Answer 21

60%

Question 22

What is the value for renal plasma flow approx.?

Answer 22

0.6L/min

Question 23

What is the filtration fraction approx.?

Answer 23

20% of plasma flow.

The proportion of the renal PLASMA flow that is filtered.

Question 24

What does GFR equal relative to Renal Plasma flow and Filtration Fraction?

Answer 24

GFR= Renal Plasma Flow x Filtration Fraction.

Renal blood flow = 1L/min
Renal plasma flow = 0.6L/min.
Filtration fraction = 20%.
THEREFORE, GFR = 0.6L/min x 0.2 = 120mL/min!

Question 25

What is the mechanism of autoregulation in the kidney called?

Answer 25

MYOGENIC AUTOREGULATION.

Question 26

What is the myogenic mechanism of autoregulation in the kidneys?

Answer 26

Vascular smooth muscle constricts when stretched. Keeps GFR constant when blood pressure rises. Arterial pressure rises → afferent arteriole stretches → myogenic mechanism contracts the same afferent arteriole → (vessel resistance increases)→ blood flow reduces and GFR remains constant.

Question 27

What is the purpose of the myogenic mechanism of autoregulation in the kidneys?

Answer 27

During exercise, blood pressure increases. According to what we know already, this increases GFR. This doesn't make sense functionally, because you dont want to urinate or lose lots of water in exercise. Therefore, arteriole contraction in the kidneys reduces the surface area, which reduces Kf - compensating for the increase in Puf and therefore keeping GFR constant overall.

Question 28

How does Haemorrhage affect GFR?

Answer 28

Decrease in cardiac output, therefore decreasing GFR.

Question 29

How does obstruction in nephron affect GFR?

Answer 29

It increases tubular pressure, therefore decreases GFR. Remember, tubular pressure, like osmotic pressure, is an opposing pressure to the hydrostatic pressure pushing filtrate into te Bowman's capsule.

Question 30

How does reduced plasma protein affect GFR?

Answer 30

Decrease of opposing osmotic pressure, thereby increasing GFR.

Question 31

How does a small increase in blood pressure affect GFR?

Answer 31

It doesn’t affect GFR because of myogenic autoregulation.

Question 32

What is renal clearance?

Answer 32

Clearance is the number of litres of plasma that are completely cleared of the substance per unit time.

Question 33

What is the equation for renal clearance?

Answer 33

Clearance of urine = (Concentration of substance in urine (U))x(Rate of urine production (V))/(Concentration of substance in plasma (P)). Units are in mL/min. Note: you don't need to change units for concentration of P and U if they are BOTH the same, because are measuring the relative proportions to eachother: U/P. e.g. for Na+: suppose P = 145mM, U = 290mM, V = 1ml/min. Therefore, renal clearance of Na+ = 290/145 x 1 = 2mL/min.

Question 34

What is Creatinine?

Answer 34

* a waste product from creatine in muscle metabolism * amount of creatinine released is fairly constant * if renal function stable, amount creatinine in urine is stable * low values of creatinine clearance may indicate renal failure * high plasma creatinine may indicate renal failure Rate of creatinine clearance = GFR

Question 35

What causes high plasma creatinine? (x3)

Answer 35

Protein shakes (temporarily), Muscular person, Renal Failure.

Question 36

What can renal clearance of PAH (Para aminohippurate)

Answer 36

Plasma Renal Flow

Question 37

What does amount excreted equal?

Answer 37

Amount Excreted = Amount filtered – amount reabsorbed +namount secreted

Question 38

SUMMARY: what can be determined about a substance if it's excretion rate is below, the same as, and above 120mL/min? (And clinical application?) [CLUE: normal GFR = 120mL/min].

Answer 38

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Question 39

What happens in the plasma if GFR falls?

Answer 39

Excretory products build up in plasma.