Glomerular Filtration

Question 1

What is the combined blood flow to the two kidneys?

Answer 1

~1.1L/min.

Question 2

What is the main driving force behind glomerular ultrafiltration?

Answer 2

Bulk flow (not diffusion).

Question 3

Define capillary tuft.

Answer 3

The intricate vascular bed that supplies the bowman’s capsule with blood to be filtered.

Question 4

Which two forces contribute to the rate of flow through afferent and efferent arterioles flowing to and from the capillary tuft?

Answer 4

1 - Hydrostatic pressure (drives the flow).

2 - Oncotic pressure (resists the flow).

Question 5

How does oncotic pressure change along the length of the capillaries supplying the capillary tuft?

Answer 5

It increases.

Question 6

List 2 ways by which glomerular capillary pressure can be increased.

Answer 6

1 - Dilate the afferent tubule.

2 - Contract the efferent tubule.

Question 7

How can osmotic pressure be calculated?

Answer 7

• Using the Morse equation:
• Osmotic pressure = nCRT
• nC = osmolality, R = ideal gas constant, T = temperature in kelvin.

Question 8

Where does the efferent glomerular arteriole go after leaving the glomerulus?

How do the forces driving and resisting osmosis compare here to that of the glomerular arterioles?

Answer 8

• It enters a portal vein and travels to a second capillary bed surrounding the loop of henle.
• Here, it is known as the vasa recta.
• Here, the hydrostatic pressure is lower and the oncotic pressure is higher.

Question 9

Why is the oncotic pressure higher towards the efferent end of the glomerular arteriole?

Answer 9

Because water is filtered out of the arteriole into the bowman’s capsule, concentrating the plasma proteins.

Question 10

List the barriers to diffusion across at the glomerulus.

Answer 10

1 - Endothelial cells.

2 - A basement membrane.

3 - Podocytes.

Question 11

Define podocyte.

Answer 11

A cell in the Bowman’s capsule with projections known as pedicels that interdigitate with each other and wrap around the capillaries of the glomerulus.

Question 12

List 3 specialised features of glomerular endothelial cells that improves filtration rate.

Answer 12

1 - 60nm wide fenestrations.

2 - The endothelial cells have a negatively-charged glycocalyx (pericellular matrix), which creates a negative charge barrier that is particularly effective for proteins.

3 - The basement membrane and podocytes also consist of fixed, negatively charged proteins (eg. collagen in the basement membrane and nephrin in podocytes).

Question 13

What structure lies in between the pedicels of podocytes?

Answer 13

Filaments that bridge the pedicels.

Question 14

List 2 factors that affect how easily a molecule is filtered through the glomerulus.

Answer 14

1 - Molecular radius.

2 - Charge (positively charged ions are filtered more easily).

Question 15

Define bulk flow / advection (in the context of glomerular filtration).

Answer 15

The movement of solutions from an area of high pressure to an area of low pressure.

Question 16

Define solvent drag.

Answer 16

The movement of solutes due to the movement of the solvent in which they are dissolved.

Question 17

List 9 molecules that are filtered through the glomerulus.

Answer 17

1 - Na+.

2 - K+.

3 - Mg2+.

4 - Ca2+.

5 - Cl-.

6 - HCO3-.

7 - Glucose.

8 - Urea.

9 - Water.

Question 18

What is the average glomerular filtration rate?

Answer 18

~120ml min^-1.

Question 19

What is the average filtration rate of an individual nephron?

Answer 19

30-50nL min^-1.

Question 20

What drives the movement of filtrate along a nephron?

Answer 20

The hydrostatic pressure in the Bowman’s space.

Question 21

List the values of:

1 - Glomerular blood hydrostatic pressure.

2 - Glomerular blood oncotic pressure.

3 - Capsular hydrostatic pressure.

From this, calculate the net value of the hydrostatic pressure in the Bowman’s space.

Answer 21

1 - Glomerular blood hydrostatic pressure is 55mmHg.

2 - Glomerular blood oncotic pressure is 30mmHg.

3 - Capsular hydrostatic pressure is 15mmHg

• The hydrostatic pressure in the Bowman’s space is therefore 10mmHg.

Question 22

Define filtration fraction.

Answer 22

The proportion of the plasma flow that is filtered by the glomerulus.

Question 23

What is the average value for filtration fraction?

Answer 23

~0.2.

Question 24

How can glomerular filtration rate be measured using urine concentrations?

Answer 24

• By measuring the amount of a substance in the urine (such as creatinine) that is freely filtered, but neither secreted nor reabsorbed over the length of the renal tubules (measure concentration and volume of urine), and dividing this value over the time to produce that volume of urine.
• This will give the filtration rate for that molecule, so to find the total GFR, divide by the proportion of the blood that is composed of that molecule.

Question 25

How can glomerular filtration rate be calculated using a single blood sample?

Answer 25

• GFR is inversely proportional to creatinine concentration in the plasma:

GFR = k / Ccr

• k = (140-age) * mass * 1.23 (or 1.04 for women)
• This formula relies on the fact that creatinine production = creatinine loss at the kidney.

Question 26

What does proteinuria indicate?

Answer 26

Glomerular dysfunction.

Question 27

Define glomerulonephritis.

Answer 27

A set of conditions where there is a predominant dysfunction of the glomerulus.

Question 28

Define nephrotic syndrome.

Answer 28

A set of renal failures that result in abnormal protein loss through the urine.

Question 29

Define congenital nephrotic syndrome.

Answer 29

Congenital nephrotic syndrome is a genetic disorder involving a component of the glomerular barrier between podocytes (either nephrin or podocin), resulting in a glomerulus that is more permeable to plasma proteins.