Topic 2 - The Glomerulus as a Filter

Question 1

What makes up the glomerular filter?

Answer 1

1. fenestrated capillary endothelium
2. basement membrane
3. filtration slits of podocytes (slit diaphragm, nephrin)

Question 2

What are podocytes?

Answer 2

glomerular visceral epithelial cells

express specific proteins (nephrin and others)

slit diaphragm proteins form an important part of the glomerular filtration barrier (GFB) by forming a zipper-like structure between adjacent foot processes

Question 3

What factors determine the permeability of glomerulus to macromolecules?

Answer 3

size selectivity: larger molecules are restricted more than smaller molecules

charge selective: anionic molecules are restricted more than neutral or cationic molecules

• renal glomeruli usually filter small molecules such as glucose
• restrict the filtration of large molecules such as plasma proteins
  
  • albumin is usually the most abundand circulating protein and healthy persons excrete less than 30mg of urinary albumin each day
  • as the kidneys, however, bevome damaged by, for example, the complications of diabetes, ‘leakage’ of larger molecules such as albumin occurs

Question 4

The filtration of serum albumin is normally severely restricted at the glomerulus (<1%), yet a neutral molecule of the same effective molecular size is filtered to a much higher degree (15-25%). this is best explained by:

a. flexibility of the dextran molecule
b. secretion of dextran by the tubule
c. fixed negative charge of the glomerular membrane
d. sodium-dextran co-transport at the glomerulus
e. greater solvent-drag of dextran compared to albumin

Answer 4

c. fixed negative charge of the glomerular membrane

Question 5

The main barrier precluding the free passage of albumin across the glomerular capillary walls is formed by:

a. the fenestrated glomerular endothelium
b. anionic proteoglycan clusters within the glomerular basement membrane
c. the filtration slits in between visceral epithelial cells (podocytes)
d. none are correct
e. all are correct

Answer 5

b. anionic proteoglycan clusters within the glomerular basement membrane

Question 6

which of the following statements about glomerular filtration is NOT true?

a. the endothelium of the glomerular capillary is an important barrier to the passage of plasma proteins
b. the glomerular basement membrane prevents the filtration of large plasma proteins
c. the filtration slits between the podocytes help to retard the filtration of plasma proteins
d. about 7g of albumin are filtered daily and reabsorbed by the proximal tubule
e. reducing the net negative charge on the components of the glomerular barrier will result in increased filtration of plasma proteins

Answer 6

a. the endothelium of the glomerula capillary is NOT an important barrier to the passage of plasma proteins

Question 7

Which of the following statements about glomerular filtration is NOT true?

a. the basal lamina (basement membrane) of the glomerular capillaries is the most important barrier to passage of plasma proteins
b. if myoglobin is present in plasma, much of it will be filtered at the glomerulus
c. essentially no serum albumin is filtered, thus its daily filtered load is close to zero
d. the filtration fraction is defined as GFR/RPF and it is normally between 0.15 and 0.20
e. a disease that diminished the density of negative sialic acid moieties in the glomerular filtration barrier would result in greater filtration of plasma proteins

Answer 7

c. albumin IS filtered

Question 8

All of the following statements concerning the renal corpuscls are true EXCEPT:

a. a basal lamina represents the middle element of the filtration barrier
b. the endothelium of the capillary loops is fenestrated
c. the slit membranes prevent the passage of anything equal to or larger than serum albumin
d. everything except the largest protein molecules can pass through the basal lamina
e. the foot processes of the podocytes close the fenestrations of the capillaries

Answer 8

e. the glomerulus is composed of an afferent and efferent arteriole

intervining lined by endothelial cells (tuft)

• outer surface of capillaries is covered by epithelial cells (podocytes) which are continuous with the epithelium of Bowman’s space and proximal tubule
• the mesangium and basement membrane