Glomerular filtration Rate

Question 1

What is azotaemia?

Answer 1

High urea and creatinine in the blood

Question 2

What are the three types of azotaemia?

Answer 2

Pre-renal (e.g. hypertension)
Renal (intrinsic; e.g. interstitial fibrosis)
Post-renal (e.g. urethral blockage, stenosis)

Question 3

What is GFR?

Answer 3

Glomerular filtration rate (GFR) is the volume of fluid filtered from the glomerular capillaries into the Bowman’s capsule per unit time and relatates to functional renal mass.

Question 4

How do we measure GFR in practice?

Answer 4

Via SCr /BUN, a high one indicates 75% nephron loss, but SCr is influenced by lean mass, breed, diet.

Question 5

What factors affect GFR?

Answer 5

Pressure in afferent/efferent arteriole
Colloid osmotic pressure
Hydrostatic pressure
Renal plasma flow (RPF)
Electrical charges across GBM

Question 6

What passes through the glomerulus easily?

Answer 6

Neutral or +ve charged molecules
Small molecules <8nm diameter
Round rather than ellipsoid molecules
Albumin is -ve charged thus does not freely filter
If drugs bind to proteins they are not filtered, thus in body for longer

Question 7

What is hydrostatic pressure?

Answer 7

Pressure exerted by water in blood

Question 8

What is Colloid osmotic pressure?

Answer 8

Pressure exerted by proteins in blood

Question 9

Why does protein in the urine cause oedma?

Answer 9

In Proteinuria you lose protein to urine, therefore less proteins in plasma, more in tubular fluid which leads to less net filtration force, less fluid moves across so fluid retention and ultimately oedema

Question 10

What determines hydrostatic pressure?

Answer 10

Cardiac output generates pressure
Afferent and efferent arterioles determine relative pressure difference across glomerulus
Concentration of plasma proteins

Question 11

What is clearance?

Answer 11

Clearance is the volume of plasma from which a substance is completely removed by the kidney in minute (could be any length of time though)

Question 12

What do we use to measure clearance?

Answer 12

Creatinine

Question 13

What affects clearance?

Answer 13

Renal clearance depends on glomerular filtration rate (GFR), tubular reabsorption, tubular secretion and excretion from other sources eg. saliva

Question 14

What is creatinine?

Answer 14

Creatinine is a non-protein nitrogenous substance from muscle
Daily production is constant and related to muscle mass.
Excretion is entirely proportional to glomerular filtration i.e. no net (<5%) tubular re-absorption or excretion occurs.
The concentration of creatinine in the urine reflects the amount of substance filtered as well as urine volume

Question 15

How do we measure creatinine in practice?

Answer 15

Catheterize bladder, remove urine.
Collect all urine over given time (e.g. 24h).
Collect blood sample in middle
At end, measure Cr in urine
Perform measurement of [creatinine] on blood sample.

Question 16

What can we use to measure GFR instead of creatinine?

Answer 16

Iohexol (most often used)

Question 17

What is the principle organ for electrolyte homeostasis?

Answer 17

Kidney

Question 18

How can we differentiate between pre-renal and renal azotemia?

Answer 18

Using Fractional excretion of electrolytes (the percentage excretion of the electrolytes in the serum)

Question 19

What do we look for in sedimentary analysis of urine?

Answer 19

Rbc & wbc				
Epithelial cells		
Micro-organisms		
Crystals: type, concentration, pH
Some in normal urine but there should be no casts