Filtration and Clearance

Question 1

Glomerular filtration

Answer 1

The process by which plasma is filtered across the glomerular capillaries to form a protein-free ultrafiltrate in Bowman’s space
- driven by differences in the oncotic and hydrostatic pressure across the glomerular capillaries
- with the exception of plasma protein, organic and inorganic anionic and cationic solutes are freely filtered across the glomerular capillaries and exist in the same concentration in plasma and ultrafiltrate
GFR= 180L/day

Question 2

Filtration rate

Answer 2

= Kf [(Pgc-Pbs)-(πgc-πbs)]
where Kf is filtration coefficient, (Pgc-Pbs) is the difference in hydrostatic pressure inside the glomerular capillary and Bowmans space, and (πgc-πbs) is the difference in oncotic pressure inside the globerular capillary and Bowmans space.

Question 3

What happens to Pgc during the length of the capillary?

Answer 3

At the beginning, Pgc is ~45-50 mmHg, and decreases to 41-47 mmHg at the end.
-occurs despite decrease in plasma volume and is due to post capillary efferent arteriole constriction

Question 4

What happens to πgc during the length of the capillary?

Answer 4

πgc is 25 mmHg at the beginning and increases to 35 mmHg at the end due to plasma filtration and concentration of the plasma protein.

Question 5

What are Pbs and πbs?

Answer 5

Pbs is 10 mmHg, πbs is 0 mmHg. In nephrotic syndrome, πbs is significantly increased due to filtration of plasma protein.

Question 6

What are the glomerular barriers to filtration?

Answer 6

1) Endothelial cells of glomerular capillaries (restrict passage of cellular elements into Bowman’s space)
2) Capillary basement membrane (Restricts filtration of solutes larger than 1 kDa. Restricts filtration of anionic proteins)
3) Podocytes in the visceral epithelial layer of Bowman’s capsule

Question 7

Filterability

Answer 7

Small molecules are more filterable than large molecules.

Cationic molecules are more filterable than neutral, which are more filterable than anionic.

Question 8

RBF/min

Answer 8

1-1.2 L

Question 9

RPF/min

Answer 9

600-700 mL

Question 10

GFR/min

Answer 10

125 mL

Question 11

FF

Answer 11

= GFR/RPF = 125/600 = .2

Question 12

Relationship between GFR and RPF

Answer 12

GFR increases with increasing RPF

Question 13

Relationship between FF and RPF

Answer 13

FF decreases with increasing RPF

Question 14

Effect of afferent arteriolar constriction on RPF, GFR

Answer 14

RPF decrease, GFR decrease

Question 15

Effect of efferent arteriolar constriction on RPF, GFR

Answer 15

RPF decrease, GFR increase

Question 16

Effect of afferent and efferent arteriolar constriction on RPF, GFR

Answer 16

RPF decreases a lot, GFR has no change

Question 17

Effect of increased plasma protein on RPF, GFR

Answer 17

RPF no change, GFR decrease

Question 18

Effect of decreased plasma protein on RPF, GFR

Answer 18

RPF no change, GFR increase

Question 19

Effect of obstructed ureter on RPF, GFR

Answer 19

RPF no change, GFR decrease

Question 20

Net fluid reabsorption or filtration

Answer 20

(Ppc-Pis)-(πpc-πis)
pc = peritubular capillary
is= interstitial space
negative = reabsorption
positive = secretion

Question 21

Forces favoring fluid reabsorption

Answer 21

Interstitial hydrostatic pressure, peritubular plasma oncotic pressure

Question 22

Forces opposing fluid reabsorption

Answer 22

Interstitial oncotic pressure, capillary hydrostatic pressure

Question 23

GFR =

Answer 23

(Us * V)/ Ps (assuming no reabsorption, no secretion, complete filtration)

Question 24

solutes used to measure GFR

Answer 24

exogenous: inulin
endogenous: creatinine

Question 25

Renal clearance =

Answer 25

Cs = (Us * V)/ Ps

Question 26

Fractional Excretion of Water =

Answer 26

Pin/Uin

Question 27

Autoregulation of renal blood flow is due to:

Answer 27

1) myogenic response to increased or decreased pressure resulting in a decrease or increase in the diameter of the blood vessel
2) tubuloglomerular feedback at the macula densa cells sensing an increase or decrease in GFR, resulting in a regulatory decrease or increase in constriction of the afferent arteriole