Glomerular filtration

Question 1

What are the three main processes performed by the nephron?

Answer 1

1) Filtration
2) Reabsorption
3) Secretion

Question 2

How do you calculate urinary excretion rate?

Answer 2

Urinary excretion rate = filtration rate + secretion rate - reabsorption rate

Question 3

Why is a high rate of filtration needed?

Answer 3

High rate of filtration needed to help clear waste products efficiently

Question 4

What is the blood flow to the kidney?

Answer 4

Filtration requires good blood flow to kidney

Kidneys receive ~20% cardiac output

∴ Renal blood flow ~1 litre / min (haematocrit ~0.4  400ml/min)

∴ Renal plasma flow ~600ml / min

Question 5

Define Glomerular Filtration Rate

Answer 5

The volume of filtrate formed by all the nephrons in both kidneys per unit time

Question 6

What determines GFR?

Answer 6

1) Glomerular capillary filtration coefficient, Kf
2) Net filtration pressure (NFP)

GFR = Kf x NFP

Question 7

What does the glomerular capillary filtration coefficient Kf reflect?

Answer 7

1) Surface area available for filtration (a small sieve vs big sieve)
2) Hydraulic conductivity (‘permeability’) of the filtration barrier (how big are the holes in the sieve?)

Changes in Kf aren’t the major part of the physiological regulation but may be affected in disease processes

Question 8

How do you calculate Net Filtration Pressure (NFP)?

Answer 8

NFP = PG – PB – πG + πB

Question 9

How does most physiological regulation of GFR occur?

Answer 9

By changing glomerular hydrostatic pressure (PG)

Question 10

What does glomerular hydrostatic pressure (PG) depend on?

Answer 10

1) Arterial pressure
2) Afferent arteriole resistance
3) Efferent arteriole resistance

Question 11

How can you vary glomerular hydrostatic pressure (PG) independently of arterial pressure?

Answer 11

By varying the resistance of the afferent and efferent arterioles

Question 12

Would afferent arteriole dilation and efferent arteriole constriction increase or reduce GFR?

Answer 12

Increases GFR

Question 13

Would afferent arteriole constriction and efferent arteriole dilation increase or reduce GFR?

Answer 13

Reduces GFR

Question 14

How are pressures in the peritubular capillary different from the glomerular capillary?

Answer 14

Peritubular hydrostatic pressure is lower and colloid osmotic pressure is higher meaning that peritubular capillaries favour absorption rather than filtration

Question 15

Name two mechanisms of auto regulation of GFR

Answer 15

1) Myogenic response

2) Tubuloglomerular feedback

Question 16

Describe the stages of myogenic autoregulation of GFR

Answer 16

Myogenic response = inherent ability of smooth muscle in afferent arterioles to respond to changes in vessel circumference by contracting or relaxing

1) Increase in arterial blood pressure
2) Increased renal blood flow and increased GFR
3) Increased stretch of afferent arteriole (AA) smooth muscle cells
4) Opens Ca2+ channels
5) Reflex contraction of AA smooth muscle
6) Vasoconstriction of AA
7) Increased resistance to flow
8) Prevents changes in renal blood flow and GFR

Question 17

Describe the stages of tubuloglomerular feedback in autoregulation of GFR if blood pressure increased

Answer 17

1) Increase in arterial blood pressure
2) Increased renal blood flow and increased GFR
3) Increased [NaCl] delivered to macula densa cells
4) Release of paracrine factors (eg. adenosine)
5) Constriction of AA smooth muscle
6) Vasoconstriction of AA
7) Increased resistance to flow
8) Restores renal blood flow and GFR

Question 18

Describe the stages of tubuloglomerular feedback in autoregulation of GFR if blood pressure decreased

Answer 18

1) Decrease in arterial blood pressure
2) Decreased renal blood flow and decreased GFR
3) Decreased [NaCl] delivered to macula densa cells
4) Release of renin and angiotensin II
5) Increased EFFERENT arteriolar resistance

Question 19

What happens to GFR in kidney diseases?

Answer 19

GFR is directly related to the function of the nephrons and declines in all forms of progressive kidney diseases. GFR is usually accepted as the best overall index of kidney function in health and disease.