Glomerular Filtration NOT FINISHED

Question 1

What is the blood flow to the kidneys?

What is the renal plasma flow rate?

Answer 1

Blood flow = 1.1 L/min

Renal plasma flow 600ml/min (allowing for volume of cells in blood)

Question 2

Filtration in the kidney occurs in the ……………, where a …………… membrane separates the cells and plasma in the capillaries from the ………… which forms in the ……………. space . The filtration ……. flow, rather than strictly due to diffusion.

Answer 2

glomerulus
semipermeable
filtrate
Bowman's
bulk

Question 3

The two forces which drive the flow are identical to hose determining movement in all capillary beds:
1.
2.

Answer 3

1. Hydrostatic: higher hydrostatic pressure (50mmHg) in the capillaries drives fluid out; much higher than most capillaries
2. Osmotic/oncontic pressure: higher osmotic pressure in the capillaries (partially due to plasma protein) impedes the flow.
   The osmotic force increase along the length of the capillaries but equilibrium is not usually achieved

Question 4

What is the pressure of blood entering the glomerulus?

Answer 4

50mmHg

Question 5

What are the 2 main ways to locally increase the glomerular capillary pressure?

Answer 5

1. dilate the afferent arteriole

2. contact the efferent arteriole

Question 6

Describe osmotic and oncontic pressures across the golmerular capillaries

Answer 6

The total osmotic pressure of plasma is very high (due to lots of water). The ions however are free to move into Bowman’s space. They equalise on either side of the membrane, and hence so does the water. So the pressure is only due to the plasma protein.

Question 7

What are the 3 layers separating the blood from the lumen of the Bowman’s capsule?

Answer 7

1. endothelial cells of the glomerular capillaries
2. glomerular basement membrane
3. epithelial cells of Bowman’s capsule (podocytes)

Question 8

Describe how the glomerular barrier is adapted to its function

Answer 8

The endothelial cells have small (60nm) holes (fenestrations) between them. Tight junctions between cells.
The endothelial cells have a negatively charged glycocalyx, which creates a charge barrier that is particularly effective for proteins
The basement membrane also consists of fixed, negatively charged proteins e.g. collagen

Question 9

What are podocytes? What is their function?

Answer 9

Epithelial cells of Bowman’s capsule
They have small process that project and interdigitate with their neighbours to form another barrier to the movement of fluid. They form narrow slits for filtration.

Question 10

Describe and explain the fraction of molecules filtered due to size and charge:

• anionic dextran
• natural dextran
• cationic dextran

Answer 10

See graph in lecture
For all: as size increases, fraction filtered decreases
Anionic dextran filtered the least as the barrier has a fixed negative charge (glycocalyx)
Cationic dextran are filtered.
This is good as plasma proteins are negative so will remain in the blood. and small unusual (+ve) things will be excreted

Question 11

-

Answer 11

• most molecules less than 10kDa in size (Na, K, Mg, Ca, Cl, HCO, glucose and urea)
• larger molecules may be found, particularly if positively charged and if the glomerulae are damaged (leaky)

Question 12

What is GFR in ml/min and in L/day?

Answer 12

120ml/min

180L/day

Question 13

How much does each nephron filter in nL/min?

Answer 13

30-50nL/min

Question 14

The hydrostatic pressure in Bowman’s space helps to drive the filtrate along the rest of the nephron. What is this pressure?

Answer 14

10mmHg

Question 15

What is the filtration fraction?

What is this normally?

Answer 15

This is the proportion of the plasma flow that is filtered by the glomerulus
GFR/RPF
GFR = glomerular filtration rate
RPF = renal plasma flow
normally approx 20% (120/600)

Question 16

How can GFR be measured?

Answer 16

Using substances that are freely filtered, but neither secreted nor reabsorbed over the length of the tubules.
Such substances can either by injected intravenously (inulin) or be produced by the body at a steady rate (creatinine)

Question 17

How can the concentration of creatinine in the urine be used to calculate GFR?

Answer 17

amount of creatine = conc x volume
rate of creatine production in urine = amount/collection time
The amount excreted is the same as the amount filtered

Question 18

GFR calculation

Answer 18

u

Question 19

Empirical estimation

Answer 19

s

Question 20

How does GFR change with age?

How does this affect urine creatinine?

Answer 20

As we age, we lose nephrons and GFR falls.
Increases creatinine in urine, however creatinine levels stay fairly constant as production is also decreased.
The normal loss of renal function with age with place a limit of longevity, should other causes of death be significantly reduced.

Question 21

What does proteinuria suggest?

Answer 21

Glomerular dysfunction as protein is not usually present in urine.
This is a key feature of a key set of renal failures, which when sufficiently severe constitute nephrotic syndrome.
The set of conditions where there is a predominant dysfunction of the glomerulus are called glomerulonephritis

Question 22

What is congenital nephrotic syndrome?

Answer 22

The is a rare genetic condition involving a component of the glomerular barrier between podocytes
In this condition the glomerulus is more permeable to plasma proteins and so more protein is found in the urine