Renal blood flow and glomerular filtration

Question 1

Relevance of glomerular filtration to medicine

Answer 1

Fall in glomerular filtration= kidney failure

Question 2

Glomerular filtration definition

Meaning?

Answer 2

Passive process by which fluid is ‘driven’ through the semipermeable (fenestrated) walls of the glomerular capillaries into the Bowmans capsule space by the hydrostatic pressure of heart.

Doesn’t involve ATP

Question 3

Filtration barrier= permeable to?

Answer 3

Fluids

Small solutes= SAME CONCENTRATION in filtrate and plasma

Question 4

Filtration barrier= impermeable to?

Answer 4

Cells
Proteins
Drugs bound to protein

Question 5

What is produced at the end of glomerular filtration?

Answer 5

‘Primary urine’
Clear fluid, no blood+ proteins, contains electrolytes+ small solutes
Not necessarily what is finally excreted

Question 6

What determines how much pressure is exerted?

Answer 6

Hydrostatic pressure of heart= determines cardiac output

Question 7

Forces needed to be considered in glomerulus
Driving forces?
Opossing forces?
Final forces equation?

Answer 7

Driving force= P(gc)= hydrostatic pressure in glomerular capillaries (due to blood pressure)

Opposing forces:
P(t) Hyrostatic pressure of tubule
π(gc) Osmotic pressure of plasma proteins in glomerular capillaries (lots more proteins in tubule than capillary)

Total equation= P(uf) Net ultrafiltration pressure
P(uf)= P(gc) - P(t)- π(gc)

Question 8

Units of ultrafiltration pressure

Answer 8

mmHg

Question 9

Glomerular filtration rate equation

What leads to increase of Kf/ decreased Kf?

GFR units?

Answer 9

GFR= P(uf) x Kf
GFR= amount of fluid filtered from glomeruli into Bowman’s capsule per unit of time, index of kidney function
Kf= ultrafiltration coefficient (affected by membrane permeability+ available for filtration)
Therefore any changes in Kf/ P(uf) affects GFR

GFR units= ml/min

Kidney disease= decreased number of functioning glomeruli= decreased SA= decreased Kf

Dilation of glomerular arterioles by drugs/ hormones= increased Kf

Question 10

If the pressure is the same, what affects flow?

Answer 10

Membrane permeability

SA (e.g. loss of nephrons)

Question 11

What indicates inflammation

Answer 11

Nephrons enlarged by inflammation= indicated by protein in urine because WBCs need to enter tubules

Question 12

Renal Blood Flow approx no.?
Renal Plasma Flow approx no?
Filtration Fraction meaning? Approx no?

Answer 12

1L/min (1/5 cardiac output)
0.6L/min
FF= ratio between RPF+ amount of filtrate filtered by glomerulus, normally 20%

Question 13

Second GFR equation?

Normal value?

Answer 13

GFR= RPF x FF

120ml/min

Question 14

Concentration of filtrate taking into account GFR equations?

Answer 14

SAME AS BLOOD (not impacted by volume)

Question 15

Summary of things that affect GFR?

Answer 15

P(gc)
π(gc)
P(t)
K(f)
Neural/ Hormonal input to afferent/efferent arteriole= change in P(uf)

Question 16

Mechanisms of Autoregulation

Importance

Answer 16

1. Myogenic mechanism
Vascular smooth muscle constricts when stretched. 
Keeps GFR constant when blood pressure rises.
Arterial pressure rises → afferent arteriole stretches →
arteriole contracts (reflex) → (vessel resistance increases)→ reduced pressure in Bowman's capsule+ blood flow reduces and GFR remains constant

Importance= in exercise where increased CO= myogenic mechanism prevents increased urine output

Question 17

Effects on GFR:
Severe haemorrhage 			 
Obstruction in nephron tubule			
Reduced plasma protein concentration	
Small increase in blood pressure

Answer 17

Decrease
Decrease (opposing pressure increases (hydrostatic pressure is the same))
Increase
No effect (myogenic mechanism)

Question 18

Renal clearance definition

Importance?

Answer 18

Number of litres of plasma that are completely cleared of the substance per unit time
(rate at which substance is removed from plasma (rate at which excreted by kidney))
This is the way to indirectly measure GFR

Question 19

Renal clearance equation
Units?
Importance?

Answer 19

C= (U x V)/P
U = concentration of substance in urine
P = concentration of substance in plasma
V = rate of urine production
ml/min
Can measure these parameters in patients

Question 20

Low clearance meaning?

Answer 20

Something actively reabsorped from plasma because lots has gone from primary urine

Question 21

High clearance meaning?

Answer 21

Secreted actively into urine

Question 22

Which molecule is used to measure clearance to estimate GFR?
Properties of molecule?
Normal levels indication?
Kidney failure?
High creatine causes?

Answer 22

Inulin (can be infused into body but not made naturally)
Creatine

Waste product from creatine in muscle metabolism
Amount of creatinine released = constant
Only cleared through glomerular filtration
Not reabsorped+ not secreted

Stable renal function= creatinine amount in urine is stable
Kidney failure= low values of creatinine clearance+
High plasma creatinine

High creatine= kidney failure/ lots of exercise/ high creatine in diet

Question 23

What molecule helps to measure RPF
No.?
Properties of molecule?

Answer 23

PAH (Para aminohippurate) clearance
625ml/min
All of PAH removed from plasma passing kidney= clearance= RPF through active secretion of anything that hasn’t been filtered

Question 24

Amount excreted equation?

Answer 24

Amount excreted= Amount filtered- amount reabsorbed+ amount secreted

Question 25

Pharmacokinetics importance

Answer 25

Anyone with renal failure that gets given a potent drug with a narrow therapeutic window (too much= bad for patient)= at risk of overdose