45 Glomerular filtration and its control

Question 1

What are the 6 functions of the renal system?

Answer 1

1. Regulation of ECF volume and bp
2. Regulation of osmolarity
3. Maintenance of ion balance
4. Regulation of pH
5. Excretion of waste
6. Production of hormones

Question 2

Kidney structure?

Answer 2

• Outer cortex (contains superficial glomeruli 90% and the remaining glomeruli are Juxtamedullary)
• Inner medulla (divided into renal medulla pyramids with interlobular arteries/veins inbetween)
• Pyramid contain duct of Bellini that drain into the renal pelvis –> ureter

Question 3

• Outer cortex (contains superficial glomeruli 90% and the remaining glomeruli are Juxtamedullary)
• Inner medulla (divided into renal medulla pyramids with interlobular arteries/veins inbetween)
• Pyramid contain duct of Bellini that drain into the renal pelvis –> ureter

Answer 3

1. Filtration by glomerulus
2. Obligatory absorption (most of water and ions) and secretion (toxins etc) by proximal tubule
3. Generation of osmotic gradient by loop of Henle
4. Regulated absorption and secretion by distal tubule
5. Regulation of water uptake by collecting ducts

Question 4

Name the layers in the triple barrier which forms glomerular filtrate

Answer 4

1. Endothelial lining of capillaries
2. Basement membrane of capillaries (-ve charge)
3. Foot processes of epithelial cells (podocytes)

Question 5

What are the properties of the triple layer?

Answer 5

• Allows free passage of solutes up to ~60kDa
• Opposes movement of cells and large protein
• Negatively charged molecules are filtered less easily

Question 6

What are podocytes?

Answer 6

Podocytes are cells in the Bowman’s capsule in the kidneys that wrap around capillaries of the glomerulus

Question 7

As values of glomerular filtration, what does RBF and RPF stand for?
RPF equation?

Answer 7

• R.B.F - Renal blood flow
-Total amount of blood that traverses renal artery or vein per unit time = 1100ml/min

• R.P.F. - Renal plasma flow
-Total amount of plasma that traverses renal artery or vein per unit time.

RPF = (100-haematocrit)% x 1100

Question 8

What are the factors favouring movement into the tubule?

Answer 8

1. Hydrostatic pressure of the blood (+55 mmHg)

2. Oncotic pressure of the tubule (0 mmHg)

Question 9

What are the factors opposing movement into the tubule?

Answer 9

1. Hydrostatic pressure of tubule (-15 mmHg)

2. Oncotic pressure of blood (-30 mmHg)

Question 10

What is the net filtration rate?

Answer 10

Fnet = F (out of capillary) - F (into capillary)

= HPg - (OPg + HPc)

= 55mmHg - (30mmHg + 15mmHg)

= + 10mmHg

Question 11

What is GFR?

Answer 11

• Glomerular filtration rate (i.e. rate of filtrate production)
• Normally 125-130ml/min

Question 12

What are the auto regulation mechanisms of glomerular filtration?

Answer 12

Intrinsic or local control:

1. Myogenic mechanism: Mediated by stretch receptors in the arterioles
2. Tubuloglomerular feedback (nephrogenic)

Question 13

What are the 4 outcomes of myogenic auto regulation of glomerular filtration?

Answer 13

Afferent arteriole:
1. Constriction:
• ↓ filtration pressure 
• ↓ GFR 
2. Dilation:
• ↑ pressure driving ultrafiltration 
• ↑ GFR

Efferent arteriole:
3. Constriction:
• Causes pressure to back up within the capillary
• ↑ GFR
4. Dilation:
• Allows blood to easily escape the capillary and ↓ pressure
• ↓ GFR

• Afferent respond to ↑ BP by constricting
=> relatively consistent GFR and RBF

Question 14

What controls GFR?

Answer 14

• Major: changes to BP/ pressure
• Afferent arteriole dilation: ↑ GFR
• Efferent arteriole dilation: ↓ GFR

Question 15

What is renal blood flow?

Answer 15

• 1 – 1.2 L/min (20 – 25% of cardiac output)
• RBF essentially constant over wide range of BP
• Hence GFR essentially constant over wide range of BP

Question 16

What is the mechanism of Tubuloglomerular feedback?

Answer 16

• A function of the juxtamedullary nephron

* Flow in the DCT is indirectly monitored and high flow information fed back to the arterioles

Question 17

Which ion concentrations increase when the filtrate flow is high in glomerulus?

Answer 17

Na+ and Ca+

Question 18

What cells monitor Na+ and Ca+ levels?

Answer 18

• Macula densa cells adjacent to the DCT
• Signal sent to the afferent arteriole which causes it to constrict via adenosine release if ↑ Na+ and Ca+ levels
=> ↓ flow rate to normal limits

Question 19

What are the extrinsic hormonal factors that affect RBF and GFR, and decrease afferent blood flow?

Answer 19

Vasoconstriction:

1. Sympathetic nerves release norepinephrine
2. Circulating epinephrine
3. Angiotensin II