5 – Passive Renal Absorption and Renal Clearance

Question 1

Passive reabsorption of substances is based on gradient transport rules and is affected by:

Answer 1

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

Question 2

How is water reabsorbed?

Answer 2

-passively
>through cells AND tight junctions

Question 3

Why is water reabsorbed passively?

Answer 3

-high intercellular concentration
-high permeability of cells

Question 4

Solvent drag:

Answer 4

-mechanism for reabsorption of some solutes along water osmosis transfer
>some solutes will be transported with water due to the high water reabsorption

Question 5

Water permeability in distal tubules, collecting tubules and collecting ducts is:

Answer 5

-variable
>affected by ADH

Question 6

What are the 2 Na+ related mechanisms that affect chloride reabsorption?

Answer 6

1. Na+ reabsorption causes a positive electrical charge between lumen and intercellular space
   a. Passive diffusion of chloride through paracellular pathways
2. Reabsorption of water after Na+ causes a concentration gradient for passive chloride diffusion
   *some is also reabsorbed by secondary active mechanism (co-transport with Na+)

Question 7

Reabsorption of water after Na+ causes a concentration gradient for passive chloride diffusion:

Answer 7

-lots of water is reabsorbed from lumen of tubule
>when volume of water is reducing in lumen of tubule=concentration of Cl in tubule increases
>provides concentration gradient for it to diffuse into interstitial space

Question 8

How is urea reabsorbed?

Answer 8

-some due to concentration gradient
>but cells are NOT permeable to urea

Question 9

What are the possible reasons for Na following a gradient-time transport pattern?

Answer 9

-high capacity of Na/K pumps creates a large concentration gradient in the interstitial space and the lumen of the tubule
>leads to some Na leaking back into tubule
*also have Na coming in from the tubule through secondary active transport (down concentration gradient but relies on Na/K ATPase)

Question 10

Renal clearance:

Answer 10

Renal clearance:
-rate of clearing a substance from plasma
-used for assessment of excretory function of the kidney

Question 11

How is renal clearance expressed?

Answer 11

-volume of plasma that is completely cleared of a substance per time (mL/min)

Question 12

If we know urine flow rate we can determine:

Answer 12

-excretion rate
>=urine concentration x urine flow rate

Question 13

Renal clearance rate is calculation:

Answer 13

=(urine concentration of substance) x (urine flow rate(V)) divided by (plasma concentration of substance)

Question 14

When a substance is freely filtered and none of it is reabsorbed:

Answer 14

-all of the substance in filtrate is excreted
>almost all plasma is reabsorbed, so clearance rate equals GFR
Ex. inulin and creatinine

Question 15

Inulin:

Answer 15

-plant polysaccharide
-freely filtered and none is reabsorbed
*can be injected intravenously and then measured in urine to detect clearance rate=used to evaluate GFR

Question 16

Creatinine:

Answer 16

-freely filtered and none is reabsorbed
-produced in body
*clearance can be used to estimate GFR

Question 17

When a substance is freely filtered and ALL of it is reabsorbed:

Answer 17

-no substance in filtrate is excreted
>almost all plasma is reabsorbed, so no volume of plasma is cleared (clearance rate=0)
Ex. glucose

Question 18

When a substance is freely filtered and none of it is reabsorbed and more is secreted:

Answer 18

-amount excreted=filtered amount PLUS secreted amount
-clearance is MORE than its GFR
Ex. hydrogen and para-aminohippuric acid (PAH)

Question 19

Para-aminohippuric acid (PAH):

Answer 19

-almost completely cleared from blood (90%) that enters kidney
-no reabsorption, high secretion
*clearance of PAH is similar to renal plasma flow (way more than GFR)
>clearance can be used to estimate renal plasma/blood flow

Question 20

Pressure natriuresis:

Answer 20

-increase in Na excretion due to increases in arterial pressure

Question 21

Pressure diuresis:

Answer 21

-increase in water excretion due to increases in arterial pressure

Question 22

Increased renal arterial pressure leads to: ‘steps’

Answer 22

-increased hydrostatic P of peritubular capillaries
-increased hydrostatic P of renal interstitial fluid
-increased leak of Na and water back into tubules (nephrons)

Question 23

Impaired GFR autoregulation (kidney) disease) leads to:

Answer 23

1. Significant increase in GFR and urine output
2. Reduced reabsorption of Na and water
3. Reduced Ang II formation

Question 24

Reduced Ang II formation:

Answer 24

-usually enhances Na reabsorption
*if reduced=more Na (and water) is excreted