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Medicine Nephrology > Renal Physiology > Flashcards

Renal Physiology Flashcards

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1
Q

What is the definition of Glomerular Filtration Rate (GFR)?

A

Amount of urine filtered by all the nephrons in both kidneys in one minute.

GFR is an important indicator of kidney function.

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2
Q

How is GFR calculated?

A

GFR = Net ultra filtration pressure X ultrafiltration (UF) coefficient.

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3
Q

What is the formula for calculating net filtration pressure?

A

Net Hydrostatic Pressure (H.P) - Net Oncotic Pressure (O.P)

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4
Q

What is the normal range for GFR?

A

90 - 125 ml/min.

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5
Q

What is the glomerular hydrostatic pressure in the GFR calculation?

A

60 mm Hg

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6
Q

What is the value of Glomerular osmotic pressure?

A

32 mm Hg

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7
Q

What is the value of Bowman’s capsule pressure?

A

18 mm Hg

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8
Q

What is Chronic Kidney Disease (CKD) classified based on ?

A

It is classified based on GFR.

GFR less than 15ml/min is considered end stage renal disease

In these cases dialysis required

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9
Q

What is the typical net ultra filtration pressure used in GFR calculations?

A

10 mm Hg

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10
Q

Fill in the blank: GFR is calculated using the formula GFR = Net filtration pressure X _______.

A

ultrafiltration (UF) coefficient whose value is 12.5ml/min

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11
Q

True or False: An increase in glomerular colloid osmotic pressure would increase GFR.

A

False

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12
Q

What is the best marker to estimate GFR?

A

Creatinine

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13
Q

In what condition is a high creatinine value significant independently of muscle mass?

A

Pregnancy

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14
Q

What is the molecular weight of creatinine?

A

113 kDa

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15
Q

What equation was historically used to calculate GFR in bedside but is now obsolete?

A

Cockcroft & Gault equation

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16
Q

What is the formula for Creatinine Clearance (CrCl) in males according to Cockroft and gault equation?

A

CrCl = (140 - age) × body weight.
———————————-
(72 × serum creatinine)

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17
Q

How is the CrCl formula adjusted for females?

A

Multiply the result obtained for males by 0.8

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18
Q

Name two modern equations used to estimate GFR.

A

1) MDRD (Modification of diet and renal disease)

2) CKD-EPI (CKD - Epidemiology Problem Initiative Group)

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19
Q

Which equation is preferred when GFR > 60?

A

CKD-EPI equation

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20
Q

What is a major disadvantage of Cockcroft & Gault equation?

A

1) Here our assumption is that
CrCl = GFR

But in reality Creatinine is secreted in kidneys so CrCl = GFR + Tubular secretion

Due to this It overestimates GFR

2) This Equation was developed when Creatinine was estimated using Jaffee’s method which is not standardised method so value is not accurate

3) This Equation overemphasises on body wt but Creatinine levels are influenced by mussle mass not body weight

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21
Q

Why is urea clearance not equal to GFR ?

A

Urea is absorbed by tubules

So Urea Clearance = GFR - Tubular Absorption

Due to this we may underestimate GFR

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22
Q

What is IDMS and why is it preferred?

A

Isotope Dilution Mass Spectroscopy; it provides more accurate creatinine estimation.

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23
Q

Why is body weight not ideal in GFR estimation?

Study These Flashcards
A

Creatinine depends on muscle mass, not body weight.

24
Q

Which equation is used to estimate GFR in children?

Study These Flashcards
A

Schwartz equation

25
Advantages of MDRD and CKD EPI equations over Cockroft And Gault equation.
1) They directly measure GFR but Cockroft And Gault equation measures Creatinine Clearance 2) Equation uses estimation of Creatinine using standard assays (IDMS) 3) Race used instead of weight
26
What is Schwarts equation ?
GFR = K * length of Child -------------------------- S.Cr K = Constant <1yr Preterm 0.33 <1yr Term o.45 1-12yrs and adolescent girls 0.55 Adolescent boys 0.7
27
How long does it take for serum creatinine to start rising in Acute Kidney Injury (AKI)?
24–48 hours.
28
What factors affect serum creatinine (S.Cr) levels?
1. Muscle mass 2. Race (higher in African Americans) 3. Age (declines after 40 years due to muscle mass loss) 4. Sex (higher in males) 5. Food 6. Drugs
29
How does diet influence serum creatinine?
Protein supplementation increases muscle mass → ↑ S.Cr Malnutrition decreases muscle mass → ↓ S.Cr
30
Does obesity affect serum creatinine? Why or why not?
No, because serum creatinine depends on muscle mass and not on fat so obesity doesn’t affect S.Cr.
31
Which drugs affect serum creatinine by competing with creatinine for tubular secretion?
Cimetidine and Trimethoprim
32
What is the relationship between GFR and S.Cr?
Inverse relationship – as S.Cr rises, GFR significantly decreases.
33
What is the clearance formula of a drug/substance?
Clearance = Urine conc. × (Volume of urine in ml/min) --------------------------------- Plasma conc.
34
What happens to GFR when S.Cr is 2 mg/dL?
GFR falls to approximately 60 mL/min.
35
What is the molecular weight of Cystatin C?
13 kDa
36
What type of inhibitor is Cystatin C?
Cysteine protease inhibitor.
37
Where is Cystatin C produced?
At a constant rate by all nucleated cells
38
How is Cystatin C handled in the kidney?
1) Filtered but not secreted 2) Reabsorbed by proximal convoluted tubule (PCT).
39
What factors do not affect Cystatin C levels?
Race Diet Muscle mass Drugs
40
What are some disadvantages of using Cystatin C?
Non-specifically elevated in: Inflammation Patients with thyroid disorders People taking steroids Smokers Therefore, cannot replace creatinine (Cr).
41
What is renal auto regulation ?
When Mean Arterial Pressure is within autoregulation range (80 - 180 mmHg) Renal blood flow and GFR will be maintained constant
42
What are the two mechanisms involved in renal autoregulation?
1) Myogenic reflex (stretch reflex) 2) Tubuloglomerular feedback (adenosine mediated).
43
What happens in response to decreased GFR in tubuloglomerular feedback?
Afferent arteriolar vasodilation.
44
What is the filtration fraction (FF) formula?
FF = GFR / Renal Plasma Flow (RPF).
45
What is the estimated RPF using Para Amino Hippuric Acid (PAH)?
700 ml/min.
46
Using what material we can calculate renal plasma flow ?
Para amino Hippuric Acid (PAH)
47
What is the normal range for filtration fraction (FF)?
0.1 to 0.2
48
What are the consequences of increased filtration fraction?
Increased peritubular capillary oncotic pressure More fluid taken from interstitial space Decreased renal interstitial hydrostatic pressure (RIHP) Increased reabsorption from proximal convoluted tubule (PCT)
49
What is the concept of pressure natriuresis?
Increased cardiac output leads to increased Na+ and water excretion; Therefore sustained increase in BP requires renal impairment. In otherwords sustained increase in BP cannot be produced only by increasing CO unless there is renal impairment
50
Mechanism of pressure natriuresis
Increase CO Increses RBF Which increases Medullary blood flow Which in turn increases RIHP Which reduces reabsorption from PCT Which increases sodium and water excretion
51
What mediates afferent arteriolar dilatation?
Prostaglandins
52
What mediates efferent arteriolar constriction?
Angiotensin II
53
What is the effect of efferent arteriolar constriction on medullary blood flow?
Decreases blood flow to medulla → medullary hypoxia
54
What is the effect of ACE inhibitors on efferent arterioles?
Block angiotensin II → efferent arteriolar dilatation → ↓ GFR (↑ S. creatinine), but ↑ blood flow
55
Why are ACE inhibitors contraindicated in bilateral renal artery stenosis?
Because they block efferent arteriolar constriction → risk of frank renal failure
56
What is the effect of NSAIDs on renal arterioles?
Block prostaglandins → block afferent arteriolar dilation → afferent constriction → ↓ GFR & ↓ RBF

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