Renal Blood Flow & Glomerular Filtration

Question 1

What is a cardinal sign of renal failure?

Answer 1

Sharp drop in GFR

Question 2

What is glomerular filtration?

Answer 2

The formation of an ultrafiltrate of plasma in the glomerulus of a kidney nephron

Question 3

What does ‘freely filtered’ mean?

Answer 3

A substance that has the same concentration in the filtrate as in the plasma (e.g. small solutes)

Question 4

What is the driving force for ultrafiltration?

Answer 4

It is a passive process driven by the hydrostatic pressure of the heart

Question 5

What are the two forces opposing the hydrostatic pressure?

Answer 5

Hydrostatic pressure of the tubule

Osmotic pressure of the plasma proteins in glomerular capillaries

Question 6

What is the net ultrafiltration pressure?

Answer 6

Puf = 10 - 20 mm Hg

Question 7

Define Glomerular Filtration Rate.

Answer 7

The amount of fluid filtered from the glomerulus to the Bowman’s capsule per minute (ml/min)

Question 8

What is the equation for glomerular filtration rate?

Answer 8

GFR = Puf x Kf 
Kf = ultrafiltration coefficient (based on membrane permeability)

Question 9

Explain, using the equation, how renal disease can cause a decrease in GFR.

Answer 9

You can lose nephrons hence you lose surface area –> decrease in Kf –> decrease in GFR

Question 10

What proportion of cardiac output goes to the kidneys?

Answer 10

20%

Question 11

What is filtration fraction? Give an approximate value.

Answer 11

The ratio between the renal plasma flow and the amount of filtrate filtered by the glomerulus (usually around 20%)

Question 12

State another equation for glomerular filtration rate using filtration fraction.

Answer 12

GFR = RPF x FF

Question 13

State four factors that affect GFR?

Answer 13

Glomerular capillary pressure
Plasma oncotic pressure
Tissue pressure
Glomerular capillary surface area or permeability (Kf)

Question 14

State the two mechanisms involved in autoregulation of GFR.

Answer 14

Myogenic Mechanism

Tubuloglomerular Feedback

Question 15

Describe the myogenic mechanism.

Answer 15

Vascular smooth muscle constricts when it is stretched (e.g. due to an increase in blood pressure)
This keeps GFR constant when blood pressure rises

Question 16

Describe the tubuloglomerular feedback mechanism.

Answer 16

NaCl concentration of the urine is detected by the macula densa. The macula densa sends signals to the afferent arterioles to change the resistance and hence change GFR.

Question 17

Describe how changes in the afferent and efferent arterioles can affect ultrafiltration.

Answer 17

They can be constricted or dilated to affect the net ultrafiltration. (e.g. vasodilating the efferent means that more blood moves away from the glomerular capillaries so the GFR decreases)

Question 18

What is the normal GFR?

Answer 18

120 ml/min

Question 19

Define clearance.

Answer 19

The number of litres of plasma that are cleared of a substance per minute.

Question 20

State the equation for clearance.

Answer 20

C = V x U/P
V = rate of urine production
U = concentration of substance in urine
P = concentration of substance in plasma

Question 21

What are the properties of a substance used to measure GFR? Give an example.

Answer 21

Inulin - is freely filtered and is neither reabsorbed nor secreted

Question 22

What is used practically to measure GFR?

Answer 22

Creatinine - this is a naturally produced waste product of creatine in muscle metabolism
It is released at a fairly constant rate in the urine
If renal function is stable then the amount of creatinine in the urine is stable.

Question 23

What is used to measure Renal Plasma Flow Rate?

Answer 23

PAH - para aminohippurate

Question 24

What properties does this substance have that makes it appropriate for measuring RPF?

Answer 24

It is totally cleared from the plasma when it passes through the kidneys. The normal glomerular filtration removes around 20% of the total PAH and then secretion from the blood into the tubules removes the rest of the PAH. There is no PAH in the renal vein.
RPF = V x Upah/Ppah

Question 25

What change in creatinine concentration indicates renal disease?

Answer 25

Increase = the cretinine is building up because the kidneys are unable to remove it