Filtration and Clearance

Question 1

where does plasma filter in the kidney? what type of ultrafiltrate is produced?

Answer 1

filters in the glomerular capillaries into Bowman’s space

forms protein free ultrafiltrate with plasma ion levels

Question 2

what causes the net efflux of ultrafiltrate?

Answer 2

differences in the oncotic and hydrostatic pressure across the glomerular capillaries

Question 3

what is the normal GFR? how long does it take to filter out the ECF volume?

Answer 3

125 ml/min or 180L per day

2 hours

Question 4

what is the filtration coefficient of the glomerular capillary?

Answer 4

Kf

the product of capillary hydraulic conductivity and the surface area of filtration

Question 5

how does capillary hydrostatic pressure change from beginning to end of the glomerular capillary? what is this due to?

Answer 5

only by 3 mmHg (small proportion of the total)

this is due to post capillary efferent arteriole constriction

Question 6

how does the oncotic pressure of the glomerular capillary change from beginning to end? how about the oncotic pressure of the bowman’s capsule?

Answer 6

glomerular capillary- from 25 mmHg to 35mmHg

bowman’s capsule- always 0 mmHg

Question 7

what is the hydrostatic pressure in the bowman’s capsule?

Answer 7

10 mmHg

Question 8

why does so much more plasma filter into the bowman’s capsule compared to other body tissues?

Answer 8

because the filtration coefficient is so large and the capillary hydrostatic pressure does not change much due to arteriole constriction

Question 9

where does glomerular filtration cease?

Answer 9

where starling forces cancel each other out towrad the efferent end of the capillary

Question 10

describe the basment membrane of the glomerular capillaries and how it effects filtration`

Answer 10

their basement membrane restrict filtration of solutes >1 KDa and its anionic charge favors cation and restricts anion filtration

Question 11

what cells have foot processes covering glomerular capillaries? what are their purpose?

Answer 11

podocytes

their anionic charge further restricts anionic protein filtration

Question 12

what do the fenestrations of the glomerular capillaries filter out?

Answer 12

prevent cellular components from being filtered towards the basement membrane

Question 13

which molecules are freely filtered through the bowman’s capsule?

Answer 13

water and solutes with a diameter of <4nm

the greater the size, the less likely it is to get filtered out

Question 14

out of the plasma proteins myoglobin, hemoglobin, albumin and insulin, which ones are most likely to be seen in the ultrafiltrate?

Answer 14

insulin (freely filtered) > myoglobin > hemoglobin > albumin

Question 15

what would occur if charge was removed from the glomerular barrier? what disease is this associated with?

Answer 15

it would increase the passage of anions and overall proteins into the ultrafiltrate
associated with nephritis

Question 16

what is the cardiac output at rest equivalent to? how much of this perfuses the kidney?

Answer 16

equivalent to blood volume

20% of CO perfuses kidney

Question 17

how do renal blood flow and renal plasma flow relate? how much of this plasma is filtered at the glomeruli?

Answer 17

RPF is 55% of RBF

GFR is 20% of RPF

Question 18

what is the filtration fraction?

Answer 18

FF=GFR/RPF (fraction of plasma that is filtered out of the blood and into the bowman’s capsule)
about .2 in a normal person

Question 19

how many liters of plasma does the kidney filter per day? how does this compare to the ECF volume?

Answer 19

180L which is 10 times the total ECF volume

Question 20

how does GFR and the rate of urine excretion change with fluid consumed?

Answer 20

GFR stays the same but the amount of urine excreted varies

Question 21

what occurs to GFR and FF with increasing RPF? how does this occur?

Answer 21

GFR increrases and FF decreases
a greater surface area of the glomerular capillaries filter plasma until maximal rate is achieved
this is still a smaller fraction due to the increased flow

Question 22

describe the relationship between FF and RPF. why does this occur?

Answer 22

greater fraction of the RPF is filtered at lower rates and a smaller fraction is filtered at high RPF
maintains GFR at levels necessary for renal function when RPF is compromised

Question 23

what does afferent arteriolar constriction doe to glomerular capillary hydrostatic pressure, RPF and GFR?

Answer 23

decreases capillary hydrostatic pressure
RPF decreases
GFR decreases

Question 24

what does efferent arteriolar constriction doe to glomerular capillary hydrostatic pressure, RPF and GFR?

Answer 24

increases capillary hydrostatic pressure
RPF decreases
GFR increases

Question 25

what does efferent and afferent arteriolar constriction doe to glomerular capillary hydrostatic pressure, RPF and GFR?

Answer 25

no change on capillary hydrostatic pressure RPF decreases GFR stays the same

Question 26

what effects does increasing or decreasing plasma protein levels have on GFR?

Answer 26

increasing- decreases GFR | decreasing- increases GFR

Question 27

what effect does ureter obstruction have on RPF and GFR?

Answer 27

no change on RPF | decreases GFR by increasing glomerular hydrostatic pressure

Question 28

what causes fluid reabsorption in the post glomerular peritubular capillaries?

Answer 28

starling forces | the peritubular capillary oncotic pressure difference exceeds hydrostatic pressure difference that opposes absorption

Question 29

describe the relationship of the glomerular and peritubular capillaries

Answer 29

the peritubular capillary is a continuation of the glomerular efferent capillary

Question 30

describe the net absorptive pressure along the peritubular capillaries?

Answer 30

it decreases from 17mmHg to 12 mmHg | absorption is greater at the beginning of the peritubular capillary

Question 31

what measurement provides an index of the number of functioning nephrons in kidney disease? when does renal failure begin?

Answer 31

GFR measurement | renal failuer when GFR decreases to below 20 ml/min or a loss of function of 85% of nephrons

Question 32

what is renal clearance?

Answer 32

the virtual volume of plasma from which a solute is completely removed from by the kidney per unit of time (rate of ml/min)

Question 33

under what conditions can renal clearance of a solute be used to measure GFR?

Answer 33

if the solute is freely filtered at the glomerulus, if it is not reabsorbed or secreted in the nephron, and if it is not synthesized or metabolized in the kidney

Question 34

what is inferred about GFR by the renal clearance of an appropriate solute? (equation included)

Answer 34

plasma solute concentration x GFR = urine solute concentration x rate of urine flow can use this equation to measure GFR

Question 35

how does the amount of solute filtered in a given time compare to the amount of solute excreted at a given time? what is this fact used for?

Answer 35

they are equal | provides the derivation for finding the GFR

Question 36

what exogenous solute is used to measure GFR? how is this done?

Answer 36

inulin- a fructose polymer (measured very accurately) | given by IV infusion to maintain constant plasma levels and measured in plasma and urine

Question 37

what endogenous solute is used to measure GFR? how is this done?

Answer 37

creatinine in the absence of strenuous exercise and disease a constant rate of creatinine diffuses from skeletal muscle to plasma

Question 38

what does increased plasma creatinine indicate?

Answer 38

decreased clearance from the plasma (major way to detect GFR) decreased GFR resulting from decreased number of functioning nephrons

Question 39

what is the normal amount of plasma creatinine? how does the kidney metabolism of creatinine impact GFR estimation?

Answer 39

1mg/100ml | minor secretion by the kidney results in 10% over estimation of GFR

Question 40

what is the equation for clearance of any solute? what does clearance depend on?

Answer 40

clearance= (urine concentration of the solute x rate of urine flow)/plasma concentration of solute depends on amount that is reabsorbed in the nephron

Question 41

what are the possibilities of renal handling of a solute?

Answer 41

just resorption, just secretion or a combination of resorption and secretion in the same or different segments of a nephron

Question 42

what do clearance ratios use for comparison? what do they indicate?

Answer 42

compare clearance of a solute to inulin or creatinine. | 1 indicates net solute secretion into tubular fluid

Question 43

what is the fractional excretion of water?

Answer 43

the fraction of the glomerular filtrate not reabsorbed from the nepron (excreted)

Question 44

how does fractional excretion of water compare to the GFR?

Answer 44

FE H2O= rate of urine excretion/GFR

Question 45

how does fractional excretion of water relate to inulin concentrations? can this property be transferred to creatinine?

Answer 45

FE H2O= plasma concentration of inulin/ urine concentration of inulin this also can be used for creatinine clearance

Question 46

what is the fractional excretion of solute? how is it estimated?

Answer 46

the fraction of filtered solute which appears in urine | estimated as the ratio of solute clearance to GFR

Question 47

what are the equations for fractional excretion of solute?

Answer 47

=clearance of solute/clearance of creatine =(urine concentration of solute x plasma concentration of creatine)/ (urine concentration of creatine x plasma concentration of solute)

Question 48

when water and Na balance, what is the fractional excretion of water and Na? what happens in dehydration and positive water balance?

Answer 48

both are 1% dehydration: FE H2O < 1% and FE Na=1% positive: FE H2O > 5% and FE Na= 1%

Question 49

what is fractional reabsorption?

Answer 49

fraction of filtered water or solute which is reabsorbed | 1-FE

Question 50

how does the kidney modulate renal blood flow?

Answer 50

renal vascular resistance increases and decreases with increased and decreased MAP keeping blood flow fairly constant

Question 51

what is the equation for renal blood flow?

Answer 51

RBF= (renal A-V blood pressure) / renal vascular resistance

Question 52

what causes autoregulation of RBF?

Answer 52

myogenic response of renal vasculature to pressure changes

Question 53

what senses the increase or decrease in GFR? what is the result?

Answer 53

tubuloglomerular feedback at macula densa cells sense increase or decrease in GFR cause increased or decreased resistance of afferent arteriole by contraction or relaxation