Glomerular Filtration and Renal Blood Flow

Question 1

What are some barriers to filtration in the kidneys?

Answer 1

Glomerular capillary endothelium = red blood cell barrier
Basement membrane = plasma protein barrier
Slit processes of podocytes = plasma protein barrier

Question 2

What must fluid filtered from the glomerulus into the Bowman’s capsule pass through?

Answer 2

The three layers that make up the glomerular membrane

Question 3

What are the forces that make up the net filtration pressure?

Answer 3

Glomerular capillary blood pressure (BPgc)
Bowman’s capsule oncotic pressure (COPbc)
Bowman’s capsule hydrostatic pressure (HPbc)
Capillary oncotic pressure (COPgc)

Question 4

How is the net filtration pressure calculated?

Answer 4

(BPgc + COPbc) - (HPbc + COPgc)

Question 5

What is the oncotic pressure equal to?

Answer 5

The colloid osmotic pressure

Question 6

What is the balance of hydrostatic forces and osmotic forces?

Answer 6

Staring forces

Question 7

What is the glomerular filtration rate (GFR)?

Answer 7

Rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time

Question 8

How can the glomerular filtration rate be calculated?

Answer 8

GFR = Kf x net filtration pressure

Question 9

What is Kf?

Answer 9

The filtration coefficient = how holey the glomerular membrane is

Question 10

What is the normal glomerular filtration rate?

Answer 10

125 ml/min

Question 11

What is the major determinant of glomerular filtration rate?

Answer 11

Glomerular capillary fluid (blood) pressure (BPgc)

Question 12

How is GFR regulated extrinsically?

Answer 12

Sympathetic control via baroreceptor reflex

Question 13

How is GFR regulated intrinsically?

Answer 13

Autoregulation = myogenic mechanism and tubuloglomerular feedback mechanism

Question 14

How does increased arterial blood pressure affect the GFR?

Answer 14

Increases blood flow into glomerulus = glomerular blood pressure and net filtration both increase, which increases GFR

Question 15

What happens to the glomerular filtration rate if the glomerular blood pressure (BPgc) falls?

Answer 15

GFR decreases = occurs after constriction of afferent arteriole

Question 16

Why don’t changes in systemic blood pressure always result in changes in GFR?

Answer 16

Autoregulation prevents short term changes in systemic arterial pressure affecting GFR

Question 17

If GFR protected form changes in mean arterial blood pressure over a wide range of values?

Answer 17

Yes = as is renal blood flow

Question 18

What occurs in the myogenic mechanism in the kidneys?

Answer 18

If vascular smooth muscle is stretched (increases arterial pressure), it contracts to constrict the arteriole

Question 19

What occurs in the tubuloglomerular feedback mechanism?

Answer 19

If GFR rises then more NaCl flows through the tubule leading to constriction of the afferent arteriole

Question 20

What part of the kidney is involved in the tubuloglomerular feedback mechanism?

Answer 20

The juxtaglomerular apparatus

Question 21

What senses the NaCl of tubular fluid?

Answer 21

The macula densa

Question 22

What are some things that may cause a decrease in GFR?

Answer 22

Increased HPbc (i.e due to kidney stone)
Increased COPbc (i.e diarrhoea)

Question 23

What are some things that may cause an increase in GFR?

Answer 23

Decreased COPgc (e.g severely burned patient)
Decreased Kf = due to change in surface area available for filtration

Question 24

What is plasma clearance a measure of?

Answer 24

How effectively the kidneys can clean the blood of a substance

Question 25

What is the plasma clearance equal to?

Answer 25

The volume of plasma completely cleared of a substance per minute

Question 26

Is plasma clearance unique to each substance?

Answer 26

Yes

Question 27

How can plasma clearance be calculated?

Answer 27

Rate of excretion/plasma concentration | Urine concentration x urine volume/plasma concentration

Question 28

What is the clearance of inulin equal to?

Answer 28

The GFR = measurement of clearance can be used to clinically determine GFR

Question 29

What are some features of inulin?

Answer 29

Freely filtered at the glomerulus, neither absorbed nor secreted, not metabolised by kidney

Question 30

How does inulin enter the urine?

Answer 30

By filtration alone

Question 31

Where can inulin be measured?

Answer 31

Easily measured in urine and blood

Question 32

What can be used instead of inulin to measure GFR?

Answer 32

Creatine clearance

Question 33

How are the plasma and urine concentrations of inulin related?

Answer 33

Plasma inulin x GFR = Urine inulin x urine volume

Question 34

What is the clearance of substances which are filtered, completely reabsorbed and not secreted?

Answer 34

Clearance = 0 (e.g glucose, also applies to substances not filtered and not secreted)

Question 35

What is the clearance of substances which are filtered, partly reabsorbed and not secreted?

Answer 35

Clearance < GFR (e.g urea, only portion of plasma is cleared)

Question 36

What is the clearance of substances which are filtered, secreted but nor reabsorbed?

Answer 36

Clearance > GFR (e.g H+, all of filtered plasma is cleared of a substance, as is the peritubular plasma from which it was secreted?

Question 37

What happens of clearance is less than GFR?

Answer 37

Substance is reabsorbed

Question 38

What happens if clearance equals GFR?

Answer 38

Substance is neither reabsorbed nor secreted

Question 39

What happens if clearance is greater than GFR?

Answer 39

Substance is secreted

Question 40

What is used to calculate renal plasma flow?

Answer 40

Para-amino hippuric acid (PAH) = exogenous organic anion, used clinically to measure RPF (= 650ml/min)

Question 41

What are some features of PAH?

Answer 41

Freely filtered at the glomerulus, secreted into tubule (not reabsorbed), and completely cleared from plasma

Question 42

What happens to the PAH that escapes filtration?

Answer 42

Secreted from the peritubular capillaries

Question 43

What are some example clearance values?

Answer 43

``` Glucose = 0 PAH = 650 ml/min Inulin = 125 ml/min Creatine = 125 ml/min ```

Question 44

What are some benefits of measuring creatine clearance?

Answer 44

Produced at near constant rate, freely filtered and not reabsorbed but is slightly secreted, gives close approximation of GFR

Question 45

What is the drawback of using inulin clearance as a marker for GFR?

Answer 45

Requires constant infusion to insure constant plasma concentration

Question 46

What are the features of a good clearance marker?

Answer 46

Non-toxic, inert (doesn't metabolise) and easy to measure

Question 47

What are some features of a good GFR marker?

Answer 47

Filtered freely, not secreted or reabsorbed

Question 48

What are some features of a good RPF marker?

Answer 48

Should be filtered and completely secreted

Question 49

What is the filtration fraction?

Answer 49

Fraction of plasma flowing through the glomeruli that is filtered into the tubules

Question 50

How is the filtration fraction calculated?

Answer 50

GFR/Renal plasma flow

Question 51

How does GFR compare to the plasma volume?

Answer 51

GFR is much greater

Question 52

How is renal blood flow calculated?

Answer 52

RPF x (1/1-Hct)

Question 53

What is Hct?

Answer 53

Haematocrit

Question 54

Does the afferent or the efferent arteriole have a wider diameter?

Answer 54

The afferent arteriole

Question 55

Where are the pores in the glomerular capillaries located?

Answer 55

In between endothelial cells = exclude red blood cells

Question 56

What is the basal lamina (basement membrane) made of?

Answer 56

Collagen and glycoproteins = confer net negative charge which acts as barrier to large plasma proteins