Passive Renal reabsorption and renal clearance

Question 1

What affects the passive reabsorption of substances?

Answer 1

1.Electrochemical gradient force (helps with diffusion across a membrane)
2. Permeability of the membrane
3. Time (how long a substance stays in tubules)

Question 2

Water reabsorption

Answer 2

-passive in the kidneys
-due to high intercellular concentration of solutes and high permeability of cells

Question 3

Where does water reabsorption occur?

Answer 3

-through cells transcellularly and through tight junctions paracellularly

Question 4

Solvent drag

Answer 4

-mechanism for the reabsorption of some solutes along water osmosis transfer. Basically water drags solutes in with it

Question 5

Water permeability

Answer 5

-variable in distal tubules, collecting tubules, and collecting ducts due to ADH

Question 6

Chloride Reabsorption methods

Answer 6

1.Na reabsorption causes a positive electrical charge gradient between the lumen and intercellular space. Leads to passive diffusion of Cl though paracellular pathway
2. Reabsorption of water after Na causes the Cl concentration gradient to increase and allows for passive Cl diffusion
3. Cl reabsorption also occurs by secondary active mechanism (co-transport with Na)

Question 7

Urea reabsorption

Answer 7

-some reabsorption can occur due to concentration gradient but cells are not very permeable to urea
-water reabsorption causes an increase in luminal urea concentration which results in some passive reabsorption of urea

Question 8

Na reabsorption and gradient-time transport pattern reasons

Answer 8

1.High capacity of Na/K pumps
-keeps low Na inside the cells, creating favorable concentration gradient
2.Leak of Na back into tubules because interstitial fluid Na levels is high due to ATP pump so Na wants to return back.

Question 9

Renal clearance

Answer 9

-rate of clearing substance from plasma
-expressed as the volume of plasma that is completely cleared of a substance per time (mL/min)

Question 10

What is renal clearance used to assess?

Answer 10

-assessment of excretory function of the kidney

Question 11

Renal clearance example

Answer 11

-Have 60mL of plasma running through kidney. Every 10mL has 1mg of substance X
-Only 30mL of plasma is cleared of substance X in 10 minutes. Therefore 3mg of substance X is cleared, and the other 3mg of substance X continues in renal vein

Question 12

How can we determine the concentration of substance X within the renal vein?

Answer 12

-Cannot take blood sample from renal vein to calculate substance X or renal clearance
-Need to take urine sample and calculate the amount of substance X present

Question 13

Excretion rate

Answer 13

-how much substance X is excreted per time
-determined though flow rate and concentration of substance X in urine

Question 14

Clearance Rate equation

Answer 14

Clearance rate=(urine concentration of substance)(urine flow rate)/ Plasma concentration of substance

Question 15

Substance is freely filtered, none reabsorbed

Answer 15

-all substance in the filtrate is excreted
-since all plasma is reabsorbed clear from substance, the clearance rate=GFR

Question 16

What substances are freely filtered?

Answer 16

-inulin (a plant polysaccharide)
-can be injected intravenously and then measured in urine to calculate clearance rate
-can also be used to calculate GFR

Question 17

Creatinine

Answer 17

-produced by body and can be used to estimate GFR

Question 18

Substance is freely filtered and all is reabsorbed

Answer 18

-none of the substance in the filtrate is excreted
-since almost all plasma is reabsorbed, there is no volume of plasma cleared from that substance and its clearance rate is 0

Question 19

Example of a substance that is freely filtered and all is reabsorbed

Answer 19

glucose

Question 20

Substance is freely filtered, none reabsorbed, and excess is secreted

Answer 20

-amount excreted is filtered amount plus secreted amount of that substance
-Therefore clearance of the substance is more than the GFR
-Example: H+

Question 21

Para-aminohippuric acid, PAH

Answer 21

-almost completely cleared (90%) from the blood that enters the kidney (filtration, no reabsorption, high secretion)
-Therefore clearance is similar to renal plasma flow (more than GFR) and its clearance can be used as an estimate of renal/blood flow

Question 22

Pressure natriuresis

Answer 22

-increase in Na excretion due to increases in arterial pressure

Question 23

Pressure diuresis

Answer 23

-increase in water excretion due to increases in arterial pressure

Question 24

Why do changes in arterial pressure between 80-170 mm Hg not significantly affect renal blood flow and GFR?

Answer 24

-Because when arterial blood pressure increases, there will be increased water and Na excretion to bring blood pressure back to normal.

Question 25

Autoregulatory mechanisms

Answer 25

-maintains GFR
- pressure natriuresis and pressure diuresis can both help if autoregulatory mechanisms are impaired

Question 26

Impaired GFR autoregulation leads to:

Answer 26

1.Significant increase in GFR and urine output
2.Reduced reabsorption of Na and water
3.Reduced Ang II formation (important for enhancing Na reabsorption and if reduced, more Na and water is excreted)

Question 27

Factors needed to reduce reabsorption of Na and Water occurs (pressure natriuresis and pressure diuresis)

Answer 27

1.renal arterial pressure increases
2.hydrostatic pressure of peritubular capillaries increases
3.hydrostatic pressure of renal interstitial fluid increases
4.Increases leak of Na and water back into tubules (instead of going to capillaries)