Glomerular filtration and renal blood flow

Question 1

What is glomerular filtration?

Answer 1

Process whereby a proportion of the plasma enters the bowman’s capsule and forms the initial tubular fluid

Question 2

Which structures compose the glomerular epithelium? What does each form a barrier to?

Answer 2

Capillary endothelium - RBC
Basal lamina/basement membrane - proteins
Slit processes of podocytes - proteins

Question 3

Across which structure does the process of filtration occur?

Answer 3

Glomerular epithelium

Question 4

What feature of the basement membrane allows it to act as a barrier to plasma proteins?

Answer 4

Negatively charged glycoproteins repel negatively charge plasma proteins

Question 5

Which (starling) forces comprise net filtration pressure?

Answer 5

Capillary hydrostatic pressure
Bowman’s oncotic pressure
Capillary oncotic pressure
Bowman’s hydrostatic pressure

Question 6

Which of the forces involved in net filtration favour it?

Answer 6

Capillary hydrostatic pressure

Bowman’s oncotic pressure

Question 7

Which of the forces involved in net filtration oppose it?

Answer 7

Bowman’s hydrostatic pressure

Capillary oncotic pressure

Question 8

Is glomerular filtration an active or passive process?

Answer 8

Passive

Question 9

What is oncotic pressure?

Answer 9

The pressure of plasma proteins (drags fluid towards it)

Question 10

Why is capillary blood pressure constant throughout the nephron?

Answer 10

Because of the back pressure created by the afferent arteriole being of larger diameter than the efferent arteriole

Question 11

Why is bowman’s oncotic pressure 0 in a healthy adult?

Answer 11

Plasma proteins should be unable to cross the glomerular membrane

Question 12

What is GFR?

Answer 12

Glomerular filtration rate - the rate at which protein free plasma is filtered from the glomeruli into the bowman’s capsule per unit time

Question 13

Write the equation to calculate GFR

Answer 13

GFR = filtration coefficient x net filtration pressure

Question 14

What is the filtration coefficient?

Answer 14

A measure of how holey (i.e permeable to substances) the glomerular membrane is

Question 15

What is the average GFR in a healthy adult?

Answer 15

125ml/min

Question 16

Which starling force is the major determinant of GFR?

Answer 16

Capillary hydrostatic (blood) pressure

Question 17

How is GRF regulated?

Answer 17

Extrinsic mechanisms (sympathetic activity)
Intrinsic mechanisms/autoregulation (myogenic, tubuloglomerular feedback)

Question 18

How does arterial blood pressure affect GFR?

Answer 18

Increase in BP –> increase in capillary hydrostatic pressure –> increase in net filtration pressure –> increase in GFR

(and vice versa)

Question 19

How does vasoconstriction and vasodilation affect GFR respectively?

Answer 19

Vasoconstriction - decreased blood flow –> decreased GFR

Vasodilation - increased blood flow –> increased GFR

Question 20

Do changes in systemic arteriolar pressure always cause changes in GFR?

Answer 20

No - autoregulation helps prevent short term changes in mean arterial BP from affecting renal blood flow and GFR

Question 21

How is mean arterial blood pressure calculated?

Answer 21

1/3 pulse pressure plus diastolic pressure

Question 22

How is autoregulation of GFR protective?

Answer 22

Helps prevent serious electrolyte imbalance

Question 23

Explain myogenic autoregulation

Answer 23

Vascular smooth muscle within the kidneys contracts when stretched (i.e by increased blood pressure) thereby limiting blood flow

Question 24

Explain tubuloglomerular autoregulation

Answer 24

Increased GFR causes increased salt flowing through the tubules which is detected by the macula densa and results in the constriction of afferent arterioles

Question 25

How does the macula densa cause constriction of afferent arteriole?

Answer 25

Release of vasoactive mediators which cause vasoconstriction

Question 26

When does extrinsic control of GFR override intrinsic control?

Answer 26

When extrinsic changes are severe enough

Question 27

How does increased hydrostatic pressure within the Bowman's capsule change GFR? In which pathology might this be seen?

Answer 27

It decreases it | Kidney stones

Question 28

How does increased capillary oncotic pressure change GFR? In which pathology might this be seen

Answer 28

It decreases it | Diarrhoea (dehydration causes proportional increase in plasma proteins)

Question 29

How does decreased capillary oncotic pressure change GFR? In which pathology might this be seen?

Answer 29

``` It increases it Severe burns (due to leakage of plasma proteins from the damaged skin) ```

Question 30

How does a decrease in the filtration coefficient change GFR?

Answer 30

It decreases it (due to decreased surface area for filtration)

Question 31

What is plasma clearance?

Answer 31

The efficiency of the kidneys at clearing the plasma of a substance (the volume of plasma completely cleared of a substance per minute)

Question 32

What is the value of kidney plasma clearance?

Answer 32

Plasma clearance is a value unique to each substance being cleared

Question 33

What are the units of plasma clearance?

Answer 33

ml/min

Question 34

Write the equation used to calculate plasma clearance

Answer 34

Plasma clearance = rate of excretion / plasma concentration | i.e plasma clearance = [urine concentration x urine output]/plasma concentration

Question 35

For which substances does plasma clearance equal GFR?

Answer 35

Inulin | Creatinine (almost)

Question 36

Why are substances which have a plasma clearance equal to GFR useful?

Answer 36

They provide a clinical measure of GFR

Question 37

Why is creatinine used as a measure of GFR over inulin?

Answer 37

Creatinine is endogenous and therefore does not need to be infused NB - creatinine is secreted a little bit so doesn't give 100% accuracy

Question 38

Which important substance is filtered and completely reabsorbed (i.e clearance = 0) but not secreted?

Answer 38

Glucose (in a healthy adult)

Question 39

Which importance substance is filtered and partially reabsorbed (i.e clearance

Answer 39

Urea

Question 40

Which important substance is filtered and completed secreted (i.e clearance > GFR)?

Answer 40

Hydrogen ions

Question 41

How can we tell if a substance is reabsorbed, secreted or neither using clearance and GFR values?

Answer 41

Clearance GFR - secreted NB - GFR is taken to be 125ml/min

Question 42

Which substance is used clinically to measure renal plasma flow?

Answer 42

Para-amino hippuric acid (PAH)

Question 43

How is PAH treated by the kidneys?

Answer 43

Filtered Completely secreted Not reabsorbed (i.e completely cleared from plasma)

Question 44

What is creatinine?

Answer 44

A muscle metabolite produced at a nearly constant rate

Question 45

Which three properties should substances used as clearance markers have?

Answer 45

``` Non-toxic Inert Easy to measure GFR - not absorbed or secreted RPF - not absorbed but completely secreted ```

Question 46

What is the filtration fraction?

Answer 46

Fraction of plasma flowing through the tubules that gets filtered

Question 47

Write the equation used to calculate filtration fraction

Answer 47

Filtration fraction = GFR / RPF

Question 48

Write the equation used to calculate renal blood flow

Answer 48

Renal blood flow = renal plasma flow x 1/(1-haematocrit)