Lecture 31: Glomerular function

Question 1

What percentage of cardiac output does the kidneys receive? (Renal Blood Flow)

Answer 1

20% of CO per min (high flow for filtration, rather than metabolism)

Question 2

What are the two types of pressures that drive glomerular filtration?

Answer 2

Hydrostatic pressures
▪ Pressure due to the volume of fluid
▪ “Pushes” fluid away

Colloid osmotic pressures
▪ Osmotic pressure due to protein
▪ “Pulls” fluid towards

Question 3

What are the four pressures that effect glomerular filtration?

Answer 3

(Positive pressures favour filtration, Negative pressures oppose filtration)

Glomerular hydrostatic pressure (GHP)
▪ = blood pressure (+ 50 mmHg)

Blood colloid osmotic pressure (BCOP)
▪ = albumin (- 25 mmHg)

Capsular hydrostatic pressure (CsHP)
▪ = pressure of filtrate already present (- 15 mmHg)

Capsular colloid osmotic pressure (CsCOP) ▪ = no protein in capsular space (+ 0 mmHg)

Net filtration pressure
(+50) + (-25) + (-15) + (0) = 10 mmHg

Question 4

What does Glomerular Hydrostatic Pressure (GHP) do in regards to glomerular filtration?

Answer 4

Definition:
The pressure exerted by blood within the glomerular capillaries.

Value:
+50 mmHg (Favors Filtration)

Mechanism:
- This pressure is primarily generated by the heart’s pumping action and systemic blood pressure.
- It pushes water and solutes out of the blood and into the glomerular capsule.

Effect on Filtration:
The main driving force for filtration, promoting the movement of fluid and small molecules from the blood into the nephron to form filtrate.

Question 5

What does Blood Colloid Osmotic Pressure (BCOP) do in regards to glomerular filtration?

Answer 5

Definition:
The osmotic pressure created by plasma proteins (mainly albumin) in the blood.

Value:
-25 mmHg (Opposes Filtration)

Mechanism:
- Proteins in the blood (which are not filtered out) create an osmotic gradient that pulls water back into the capillaries.
- This force opposes the loss of water from the blood into the filtrate.

Effect on Filtration:
Counteracts the filtration force, reducing the amount of water and solutes that can be filtered out of the blood.

Question 6

What does Capsular Hydrostatic Pressure (CsHP) do in regards to glomerular filtration?

Answer 6

Definition:
The pressure exerted by the fluid (filtrate) already present within the glomerular capsule.

Value:
-15 mmHg (Opposes Filtration)

Mechanism:
- As filtrate accumulates in the glomerular capsule, it creates back pressure that resists the entry of more fluid.
- This pressure opposes the Glomerular Hydrostatic Pressure (GHP), slowing down filtration.

Effect on Filtration:
Reduces net filtration pressure by pushing back against the fluid coming from the glomerulus.

Question 7

What does Capsular Colloid Osmotic Pressure (CsCOP) do in regards to glomerular filtration?

Answer 7

Definition:
The osmotic pressure within the glomerular capsule.

Value:
+0 mmHg (No Significant Effect on Filtration)

Mechanism:
- There are no significant proteins in the capsular space (filtrate) because proteins are generally not filtered.
- As a result, there is no significant osmotic pressure pulling fluid back into the capsule.

Effect on Filtration:
Since CsCOP is zero, it does not significantly contribute to filtration, allowing the net filtration to depend on the balance between other pressures.

Question 8

What percentage of plasma is filtered in the kidneys (plasma makes up 55% of blood)?

Answer 8

20% of the plasma which enters the kidneys (renal plasma flow) is filtered through the glomeruli into the nephrons

Question 9

What is glomerular filtration rate (GFR)?

Answer 9

The amount of plasma filtered per unit time by the kidneys

Renal plasma flow (RPF) x Filtration fraction (FF) = Glomerular filtration rate (GFR)

eg.
625 mL/min (RPF) x 20% (FF) = 125 mL/min (GFR)

Question 10

What is renal filtered load?

Answer 10

= the amount of a particular substance (solute) in the plasma is filtered per unit of time (per minute/hour/day)

Question 11

What is the equation for filtered load?

Answer 11

Filtered Load = GFR x solute plasma conc.

Question 12

What percentage of the filtered plasma gets reabsorbed?

Answer 12

> 99 % is reabsorbed

Question 13

How much plasma is normally filtered per day?

Answer 13

Normally:
▪ 180 L/day
▪ 125 mL/min

▪ Tightly regulated
▪ Variation from person to person
▪ Declines slowly from age 30

Question 14

How can we calculate glomerular filtration rate and Renal plasma flow?

Answer 14

▪ Renal plasma flow and glomerular filtration rate cannot be measured directly…….. so the renal clearance of substances with particular characteristics is used to estimate them.

Question 15

The filtration barrier in the glomerulus of the kidney:
1. is not selective.
2. is not permeable to albumin.
3. is only permeable to large molecules.
4. is not permeable to glucose.

Answer 15

2

Question 16

What is renal clearance?

Answer 16

Clearance is the volume (mL) of plasma that is cleared of a substance by the kidneys per unit time (per minute)

Question 17

What is the renal clearance for inulin and creatinine?

Answer 17

Creatinine and inulin:
▪ Freely filtered
▪ Not secreted
▪ Not reabsorbed

So…

> 99% of the plasma which was filtered is reabsorbed

Meaning…

“ALL” of the plasma which was filtered has been reabsorbed and “cleared” of creatinine

Clearance of creatinine = Volume of plasma filtered per minute = GFR

Question 18

What is the renal clearance of medications and toxins eg. PAH?

Answer 18

▪ Freely filtered
▪ Entirely secreted
▪ Not reabsorbed

So…

PAH is entirely secreted so ALL PAH in the blood of the peritubular capillaries is secreted into the nephrons

Meaning…

“ALL” of the plasma which has passed through the kidneys has been cleared of PAH

Clearance of PAH = Volume of plasma flowing through the kidneys per minute = RPF

Question 19

What is the renal clearance of glucose?

Answer 19

▪ Freely filtered
▪ Not secreted
▪ Fully reabsorbed in the proximal tubule

So…

> 99% of the plasma which was filtered is reabsorbed, Glucose is fully reabsorbed

Meaning…

NO plasma has been “cleared” of glucose

Question 20

What is the renal clearance for sodium?

Answer 20

▪ Freely filtered
▪ Not secreted
▪ Almost fully reabsorbed in most parts of the nephron

So…

> 99% of the plasma which was filtered is reabsorbed, Almost all of the sodium which is filtered is reabsorbed

However this means…

A small amount of the plasma which was filtered has been
reabsorbed without sodium and has been “cleared” of sodium

Question 21

What is the formula for renal clearance?

Answer 21

Clearance (CX) (in mL/min) = (Ux x V) / (Px)

Clearance of substance X:

Ux = concentration of X in urine (mmol/L)

V = volume of urine produced per unit time (mL/min or L/hour)

Px = concentration of X in plasma (mmol/L)

Question 22

What two substances can be used to measure GFR?

Answer 22

Substance must be:
▪ Be freely filtered
▪ NOT be reabsorbed from the tubule
▪ NOT be secreted into the tubule

Inulin
* Polysaccharide, not metabolised by body
* Not found in body, must be injected

Creatinine (more commonly used)
* Waste product produced by muscles
* Already in the body, so most commonly used clinically

Question 23

How can plasma creatinine levels be an indicator of kidney function?

Answer 23

▪ If both kidneys are working (GFR = 125 mL/min) plasma creatinine is low

▪ Even if only one kidney is still working (GFR = 60 mL/min), plasma creatinine remains fairly normal

▪ When GFR < 25 mL/min plasma creatinine conc increases as the kidneys ability to clear waste products from the blood is reduced

▪ Kidney failure: GFR < 15 mL/min the kidneys ability to clear waste products from the blood is greatly reduced

Question 24

The glomerular filtration rate (GFR) is defined as the:
1. amount of substance filtered per unit of time (e.g. per minute).
2. volume of urine produced per day.
3. amount of water both kidneys reabsorb per day.
4. volume of plasma filtered per unit of time

Answer 24

4