GFR

Question 1

What % of blood that passes through an artery at one time can be filtered into the glomerulus? What is the normal glomerular filtration rate?

Answer 1

20% of plasma, which makes up 60% of blood

90-120 ml/min/1.73m2

Question 2

How is the filtration barrier created?

Answer 2

• Epithelium of capillary allows water, salt, glucose ect to move between cells through fenestrations
• The basement membrane is an acellular gelatinous layer of collagen and glycoproteins.
• Podocytes deep to the basement membrane have pedicles which are projections which lay across the basement membrane, these create fenestration slits to prevent movement of large molecules

Question 3

What causes repulsion of proteins, meaning they are not filtered into the bowmans capsule

Answer 3

The glycoproteins are negatively charged, and so repel postivitely charged proteins. Cations need to be very small to be able to filter through.

Question 4

What is the difference between the bowmans capsule and the glomerulus.

Answer 4

The glomerulus is the blood vessels/ capillaries between the afferent and efferent arteriole. The bowmans capsule is the end of the nephron which wraps around the glomerulus. Together they’re called the renal corpuscle.

Question 5

What 3 forces act on plasma at the glomerulus and what result to they have on plasma movement?

Answer 5

• Outward (into bowmans capsule) hydrostatic pressure form arteriole
• Inward hydrostatic pressure from the fluid already within the bowmans capsule
• Inwards oncotic pressure due to lots of proteins in glomerulus but not many in the capsule- gradient draws water in
  Overall outward hydrostatic pressure is greater than the other two so net movement out of the glomerulus, into the capsule.

Question 6

What is the name for the net pressure as a result of the hydrostatic pressures and the oncotic pressure?

Answer 6

Net filtration pressure

Question 7

Why is renal autoregulation of filtration pressure important?

Answer 7

Because we need to keep GFR normal despite changes in hydrostatic pressure on coughing, sitting, standing ect

Question 8

What mechanisms are there to maintain net filtration pressure?

Answer 8

• Myogenic mechanisms
• Tubular- glomerular feedback
• Glomerular tubular balance (technically doesn’t regulate NFP but does dampen effects of GFR changing, so like a 2nd line of defence)

Question 9

What is the myogenic response to a drop in GFR?

Answer 9

Stretch receptors in the afferent and efferent arterioles detect drop in stretch (so drop in BP), this triggers the afferent arteriole to dilate (smooth muscle to relax) and efferent arteriole to constrict (smooth muscle to constrict).
If GFR/ blood pressure were to increase the opposite would occur.

Question 10

Describe the tubular- glomerular feedback detection and response of a decreased GFR

Answer 10

• If GFR drop, more NaCl reabsorbed before DCT as slower flow rate–> decreased conc of NaCl in DCT
• The drop in Cl- conc is detected by the macula densa cells of the DCT
• This stimulates prostaglandins release from the afferent arteriole. They cause vasodilation but only act very locally so dont affect the efferent.

Question 11

Describe the tubular- glomerular feedback detection and response of a increased GFR

Answer 11

• If GFR increase, less NaCl reabsorbed before DCT as faster flow rate–> increased conc of NaCl in DCT
• The rise in Cl- conc is detected by the macula densa cells of the DCT
• It causes adenosine release from the juxtaglomerular aparatus, which interacts with A1 on the afferent arteriole (causes vasoconstriction) and A2 on the efferent arteriole (vasodilation)

Question 12

What is the affect of sympathetic stimulation on GFR?

Answer 12

If sympathetic stimulated- divert blood flow and conserve blood (may be in shock).
Therefor causes vasoconstriction of both the afferent and efferent and hence decreases GFR

Question 13

What is released by the parasympathetics to cause vasodilation of the AA and EA?

Answer 13

Nitric oxide

Question 14

What is the glomerular tubular balance and what is its significance?

Answer 14

Absolute reabsorbtion is not constant but % of a certain ion/ molecule is, so that if GFR increases momentarily, reabsorption of, for example, Na+ also increases.
This is important to blunt changes to ion concentrations as a result of GFR changes when myogenic and TG feedback fails.

Question 15

What will happen to GFR in pregnancy?

Answer 15

increase to 130- 180ml/min as the kidneys hypertrophy, youll go back to normal after 6 months

Question 16

Other than pregnancy, when may GFR change?

Answer 16

When you get older it declines as past 30, the number of functional nephrons decline.
When you get some kind of kidney disease it is almost alway affected.

Question 17

What is GFR at birth? How long till it should be normal?

Answer 17

20ml/min, normal after 18 months

Question 18

Why do older people get compensatory hypertrophy?

Answer 18

because their nephron numbers are declining, so theyr try to increase size to keep up with demand

Question 19

What is the calculation for renal clearance?

Answer 19

excretion rate/ plasma concentration

Question 20

How is excretion rate calculated?

Answer 20

conc in urine x urine flow rate

Question 21

What is renal clearance?

Answer 21

The volume of plasma from which all the substance has been removed

Question 22

How can renal clearance be used to calculate GFR?

Answer 22

Because if a substance is produced in the body at a constant rate, freely filtered across the glomerulus, only excreted by the kidneys (not sweat ect) and not secreted or reabsorbed by the nephron, renal clearance will be the same as GFR

Question 23

Why is inulin good to determine GFR? What are its disadvantages?

Answer 23

Good: Freely filtered at glomerulus, not reabsorbed or secreted by the nephron and only excreted by the kidneys
BUT: not produced at constant rate in body so needs IV drip administration and a catheter is needed to time urine collections

Question 24

What else can be injected and used to measure renal clearance and so GFR?

Answer 24

51 Cr- EDTA- a radioactive marker, can measure plasma concs every 2 hrs to measure rate of clearance and so GFR

Question 25

Why is 51 Cr- EDTA not ideal to measure renal clearance?

Answer 25

Clearance is slightly lower than inulin, suggesting some reabsorbtion by the nephron. Also cant be used in preggers because radioactive

Question 26

When is 51 Cr EDTA used?

Answer 26

In children before renal transplants to get most accurate GFR

Question 27

Why can creatinine be used to estimate GFR?

Answer 27

• An endogenous substance created at a constant rate
• Freely filtered in glomerulus
• Not reabsorbed
• almost only excreted by kidney
  So if serum creatinine high-> low GFR as not getting rid of it well

Question 28

What are the limitations of using creatinine to estimate GFR?

Answer 28

• There is some secretion so creatinine is overestimation of GFR
• Some groups of ppl naturally have higher production and intake of creatinine so may make their GFR look low
• It is not accurate in ppl with GFR >120ml/min but this is ok because beyond that theyre healthy anyway
• As GFR decreases in disease state, proportion of secretion and extra renal excretion of creatinine increases, making GFR look slightly better than it is

Question 29

How can creatinine clearance be calculated accurately?

Answer 29

• Take blood and urine samples over 24 hr period

- multiply urine conc by urine production and then divide by the avg plasma conc

Question 30

In who will serum creatinine be naturally highest?

Answer 30

• Muscley
• men
• african
• eat lots of meat
• in their 20s

Question 31

What is eGFR?

Answer 31

an algorithm which estimates true GFR based on creatinine and takes into account 4 variables which affect creatinine (race, age ect)