Renal Physiology - Renal clearance

Question 1

what is renal clearance?

Answer 1

it is the removal of a given substance from the plasma by the kidney

Question 2

what is renal clearance a function of?

Answer 2

of: filtration (GFR)
Tubular reabsorption
tubular secretion

Question 3

what does the collective work of filtration (GFR)
Tubular reabsorption and tubular secretion lead to?

Answer 3

urine formation and determines the amount of a specific substance in the urine

Question 4

what are clearance values?

Answer 4

these provide numerical expressions of how efficient the kidneys excrete various substances

Question 5

what is the renal clearance rate?

Answer 5

it is defined as the volume of plasma containing being completely cleared of and excreted in the urine per minute

Question 6

what needs to be done in order to calculate renal clearance rate?

Answer 6

collect urine for known period of time
measure conc of substance(s) in urine + calculate its excretion in the urine per minute

Question 7

what is the ‘clearance of the substance’?

Answer 7

i.e. the volume of plasma that would have to be cleared completely of the substance to result in the measured urinary excretion can then be calculated

Question 8

calculating the renal clearance of a filtered only substance gives us?

Answer 8

gives us GFR and an indication of renal health

Question 9

what criteria do the substances have to meet - the clearance of a substance which meet the criteria will give a measure of GFR?

Answer 9

it is freely filtered at glomerulus
it is not reabsorbed from the filtrate
it is not secreted into the filtrate
it is not metabolised by the tubular cells
it does not interfere w/ kidney function - not toxic
easily measured

Question 10

IN OTHER WORDS - what criteria do the substances have to meet - the clearance of a substance which meet the criteria will give a measure of GFR?

Answer 10

such a substance enters the tubular filtrate via filtration only and then flows through the tubules and exits in the urine - there should not be any reabsorption or secretion

Question 11

what is inulin?

Answer 11

a polymer of fructose w/ molecular weight 5.2kDa
NOT produced in body

Question 12

how can you use inulin?

Answer 12

give to animals - measure what is in urine which is an indicator of what has been filtered so can get measure of GFR

Question 13

does inulin meet the criteria?

Answer 13

YES

Question 14

rate filtered (of inulin) =

Answer 14

rate excreted

Question 15

how must inulin be administered?

Answer 15

intravenously (as continuous rate infusion)

Question 16

inulin isn’t very clinically useful - what substance is?

Answer 16

creatinine

Question 17

what is creatinine?

Answer 17

it is the degradation product of creatine phosphate in muscles

Question 18

what part of the criteria does creatinine not meet?

Answer 18

it doesn’t meet the ‘it is not secreted into the filtrate’ part of the criteria

Question 19

slight secretion of creatinine means what?

Answer 19

that creatinine enters the filtrate - not only by filtration but also by secretion
meaning that the ‘U’ in the equation is erroneously high

Question 20

Why is the ‘P’ in the equation also erroneously high?

Answer 20

because the method used for estimation of plasma creatinine conc also detects another compound

Question 21

two errors in calculation for creatinine?

Answer 21

the two errors (w/ P and U value) cancel each other out so creatinine clearance is commonly used to measure GFR

Question 22

how else can you measure substances that are normally excreted by the kidneys?

Answer 22

in plasma

Question 23

what substances can you measure that are normally excreted by the kidneys in plasma (or serum)?

Answer 23

urea (blood urea nitrogen, BUN)
Creatinine
SDMA

Question 24

what does SDMA stand for?

Answer 24

symmetric dimethylarginine

Question 25

where is urea reabsorbed?

Answer 25

in PCT and CD

Question 26

where is urea secreted?

Answer 26

in descending loop of Henle for hyperosmotic gradient

Question 27

what does the variable concentration of urea depend on?

Answer 27

depends on diet and hydration

Question 28

how bad does the damage have to be in order to recognise kidney damage with the use of plasma concentrations of urea?

Answer 28

requires significant kidney damage before noticeable effect in plasma concentration

Question 29

where is creatinine get reabsorbed?

Answer 29

it doesn't

Question 30

where is creatinine get secreted?

Answer 30

only small amount secreted into renal tubules

Question 31

what is the production/degradation ratio for creatinine?

Answer 31

it is stable

Question 32

when is kidney damage raised relying on the plasma conc of creatinine?

Answer 32

when up to 75% reduction in GFR

Question 33

SDMA is a ... and is produced alongside?

Answer 33

it is a methylated amino acid produced alongside asymmetric dimethylarginine (ADMA)

Question 34

what is SDMA (alongside ADMA) produced as a biproduct of?

Answer 34

of intranuclear methylation of regulatory proteins and subsequent proteolysis

Question 35

SDMA is biologically... and excreted by?

Answer 35

Biologically inactive and excreted by the kidneys

Question 36

what happens to ADMA?

Answer 36

it is metabolised

Question 37

when is the rise in SDMA detected?

Answer 37

when 25-40% reduction in GFR

Question 38

how are plasma concentrations of substances used to indicate GFR?

Answer 38

combination of serum creatinine and SDMA used to help stage chronic renal failure used in conjunction with clinical signs

Question 39

what other tests are used to detect kidney damage?

Answer 39

urine protein: creatinine (UPC)

Question 40

what is UPC used to detect?

Answer 40

used to detect protein loss in urine e.g. if glomerular disease is causing destruction of the filtration barrier

Question 41

what is UPC needed to first rule out?

Answer 41

urinary tract infection inflammation haematuria (blood in urine) not always high in renal disease