Fluids, Filtration

Question 1

What % of body mass is total body water?

Answer 1

60%

Question 2

What fraction of total body water is extracellular? Ie, what % of total body mass is extracellular fluid?

Answer 2

1/3

1/3 * 60% = 20% of total body mass.

Question 3

What fraction of total body water is intracellular? Ie, what % of total body mass is intracellular fluid?

Answer 3

2/3

2/3 * 60% = 40% of total body mass.

Question 4

What can radiolabeled albumin measure?

Answer 4

plasma volume

Question 5

What can inulin measure?

Answer 5

extracellular volume

Question 6

What is normal osmolality?

Answer 6

285-295 mOsm.

Question 7

Of extracellular fluid, what % is interstitial fluid, and what % is plasma volume?

Answer 7

75% interstitial, 25% plasma.

Question 8

What does the glomerular filtration barrier filter plasma according to?

Answer 8

Size and net charge.

Question 9

What are the three components of the glomerular filtration barrier?

Answer 9

Fenestrated capillary endothelium
Fused basement membrane with heparan sulfate
Epithelial layer consisting of podocyte foot processes.

Question 10

Which component of the glomerular filtration barrier is the size barrier?

Answer 10

Fenestrated capillary endothelium

Question 11

Which component of the glomerular filtration barrier is the charge barrier?

Answer 11

Fused basement membrane w/ heparan sulfate

Question 12

Which barrier is lost in nephrotic syndrome?

Answer 12

Charge barrier. Leads to albuminuria, hypoproteinemia, hyperlipidemia, edema etc.

Question 13

What do podocytes wrap around?

Answer 13

Glomerular capillaries.

Question 14

Why can inulin be used to calculate GFR?

Answer 14

It is freely filtered and neither reabsorbed nor secreted.

Question 15

What is used clinically to calculate GFR?

Answer 15

Creatinine

Question 16

Why is creatinine an imperfect measure of GFR?

Answer 16

Creatinine is mildly secreted by renal tubules

Question 17

Does creatinine overestimate or underestimate GFR?

Answer 17

Overestimates, because the renal tubules are secreted - adding to the “cleared” number.

Question 18

What can be given to make creatinine a more accurate measure?

Answer 18

Cimetidine. It decreases tubular secretion of Cr.

Question 19

What does (Ux * V)/Px describe?

Answer 19

Clearance; volume of plasma from which the substance is completely clear per unit time.

Question 20

If Cx > GFR, what does that mean?

Answer 20

Net tubular secretion of X.

Question 21

If Cx < GFR, what does that mean?

Answer 21

Net tubular reabsorption of X.

Question 22

What is used to measure effective renal plasma flow?

Answer 22

para-aminohippuric acid (PAH). PAH is both filtered AND secreted in the PCT, resulting in near 100% excretion.

Question 23

What is the difference between GFR and effective renal plasma flow, conceptually?

Answer 23

Inulin measures what gets through the filtration barrier and assumes no action beyond that; this measures GFR.
PAH measures what gets through the filtration barrier and what is excreted from the blood - it is a more accurate measure of excretion.

Question 24

What is the equation for effective renal plasma flow?

Answer 24

Same as clearance, using PAH:

(Upah * V)/Ppah

Question 25

How does effective renal plasma flow relate to renal blood flow?

Answer 25

RBF= RPF /(1-Hct)

Question 26

How the heck does the eRPF to RBF formula make sense?

Answer 26

Rearrange! RBF * (1-Hct) = RPF RBF - RBF(Hct) = RPF Renal blood flow - (% that is RBCs) = Renal plasma flow

Question 27

Is calculated eRPF accurate?

Answer 27

It underestimates by 10% - eRPF is actually 90% of RPF.

Question 28

What is filtration fraction?

Answer 28

GFR/RPF

Question 29

What is filtration fraction conceptually?

Answer 29

Of the amount that is flows through kidney, what percent came through the glomerulus.

Question 30

What actions do NSAIDs have on renal arterioles?

Answer 30

Blocks dilation of the afferent arteriole.

Question 31

What effect does afferent arteriole constriction have on GFR, RPF, and FF? (NSAIDs)

Answer 31

GFR and RPF decrease, FF is constant. | NSAIDs

Question 32

What actions do ACEis have on renal arterioles?

Answer 32

Blocks constriction of the efferent arteriole.

Question 33

What effect does efferent arteriole constriction have on GFR, RPF, and FF?

Answer 33

Increases the GFR, and decreases the RPF. FF increases. ACEIs decrease GFR, increase RPF, FF decreases.

Question 34

What effect does an increase in plasma protein have on GFR, RPF, and FF?

Answer 34

Decreases GFR and FF (increased oncotic pressure keeping things in the capillaries). No change on RPF.

Question 35

What effect does a decrease in plasma protein have on GFR, RPF, and FF?

Answer 35

Increases GFR and FF. No change in RPF.

Question 36

What does GFR + Px represent?

Answer 36

Filtered load

Question 37

What does Ux * V represent?

Answer 37

Excretion rate

Question 38

What does Filtered-excreted =

Answer 38

Reabsorption

Question 39

What does Excreted - filtered =

Answer 39

Secretion.