REB 23. Countercurrent Mechanism

Question 1

What type of urine is produced when an individual is dehydrated?

Answer 1

highly concentrated, low volume of urine

Question 2

What type of urine is produced when an individual is over-hydrated?

Answer 2

dilute, large volume of urine

Question 3

What is the osmotic concentration of 1 mmol of glucose?

Answer 3

1 mOsm/L

Question 4

What is the osmotic concentration of 1 mmol of NaCl?

Answer 4

NaCl

• can be split into Na and Cl
• therefore, 1 mmol of Na+ and 1 mmol of Cl-
• in total, 2 mOsm/L

Question 5

What is the definition of Osmotic Pressure?

Answer 5

the pressure that must be applied to a solution to prevent the movement of solvent into it when solution and solvent are separated by a semi-permeable membrane

Question 6

What is reabsorbed in the proximal convoluted tubule?

Answer 6

• Na+ [part of 65%]
• HCO3-
• glucose
• amino acids
• H2O [part of 65%]

basically all constituents of blood minus cells + proteins
(returns most of filtrate back to blood)
- around 70% of the filtrate is reabsorbed

Question 7

What is the osmolarity of the proximal convoluted tubule?

Answer 7

around 300 mOsm/L

Question 8

Which part of the renal tubules has NO regulatory role?

Answer 8

the proximal convoluted tubule

Question 9

What is mainly absorbed in the early Proximal Convoluted Tubule?

Answer 9

• HCO3- (90%) is a major anion reabsorbed with Na+

- due to counter transport of H+ (?)

Question 10

What is mainly absorbed in the Distal Convoluted Tubule?

Answer 10

• some Na+ is reabsorbed by simple diffusion and bulk flow

- Cl- lags behind and is helped by bulk flow so it catches up

Question 11

What percentage of filtrate is reabsorbed in the loop of Henle? What mechanism is this done through?

Answer 11

• 20% of the filtrate is reabsorbed through the loop of henle
• reabsorption is part of the countercurrent mechanism

Question 12

What percentage of filtrate is reabsorbed in the distal tubules and collecting duct?

Answer 12

• around 10% of filtrate is left
• this region deals with VARIABLE salt and water reabsorption

Low Water Intake = High Water Reabsorption
- to ensure enough water in body

High Water Intake = Low/No Water Reabsorption
- to get rid of excess water

Question 13

What is the part of the tubule which has VARIABLE resorption?

Answer 13

distal tubules + collecting duct

Question 14

What is sodium reabsorption controlled by in the distal tubules and collecting duct?

Answer 14

Aldosterone

Question 15

What is water resorption controlled by in the distal tubules and collecting duct?

Answer 15

Anti-Diuretic Hormone (ADH)

- aka Vasopressin

Question 16

What is the normal urinary output?

Answer 16

1 mL/min

Question 17

If there is a high water intake, what does this lead to? How does this effect urinary output?

Answer 17

• high water intake leads to no water reabsorption

- the urinary output is around 12 mL/min

Question 18

If there is a low water intake, what does this lead to? How does this effect urinary output?

Answer 18

• low water intake leads to high (maximal) water reabsorption
• the urinary output is around 0.5 mL/min

Question 19

What are the 2 requirements for adjustable water reabsorption in the collecting duct?

Answer 19

[1] Presence of ADH

• ADH inserts aquaporins
• without ADH, collecting duct is impermeable to water

[2] Hypertonic Interstitium

• surrounds the collecting duct
• this provides the osmotic gradient to reabsorb water

Question 20

What is the function of the hypertonic interstitium in the collecting duct?

Answer 20

it provides the OSMOTIC GRADIENT to reabsorb the water

Question 21

What is the main function of the Loop of Henle?

Answer 21

it generates and maintains the hypertonic interstitium
- this allows water to leave the tubules through osmosis!

aka it creates a concentration gradient (countercurrent) in the medulla of the kidney

Question 22

What are the characteristics of the Loop of Henle? [3]

Answer 22

[1] Countercurrent Flow

• filtrate flows down in descending limb
• filtrate flows up in ascending limb

[2] Descending Limb Permeable to Water

[3] Ascending Limb Impermeable to Water

• lined with salt pumps which deposit salt into the interstitium
• there is a difference of around 200 mOsm/L between the ascending limb + surrounding interstitium

Question 23

What are the 2 parts of the Countercurrent Mechanism?

Answer 23

[1] Countercurrent Exchange System

[2] Countercurrent Multiplier

Question 24

What is the countercurrent exchange system?

Answer 24

it is the anatomical arrangement of vessels so that flow in one vessel is in the opposite direction from flow in the adjacent vessel

Question 25

What is the countercurrent multiplier?

Answer 25

it is the anatomical arrangement of the loop of Henle that concentrates solute in the renal medulla

Question 26

What are the 3 distinct regions of the Loop of Henle?

Answer 26

[1] Thin-Walled Descending [2] Thin-Walled Lower Portion of Ascending [3] Thick-Walled Upper Portion of Ascending

Question 27

What is the thin-walled descending loop of henle permeable to?

Answer 27

- highly permeable to H2O, but NOT solutes | - H2O will flow from the loop into the surrounding medium via osmosis

Question 28

What is the thin-walled lower portion of ascending loop of henle permeable to?

Answer 28

- highly permeable to Na+ and Cl- - moderately permeable to urea - almost completely impermeable to H2O

Question 29

What is the thick-walled upper portion of the ascending loop of henle permeable to?

Answer 29

- Na+ and Cl- are actively pumped out of the filtrate into the surrounding medium

Question 30

*** What is the countercurrent mechanism? List and explain the steps. ***

Answer 30

[1] the active salt pump in the ascending lumb transports NaCl out of the lumen - this happens until the surrounding interstitium is 200 mOsm/L more concentrated than tubular fluid [2] water moves passively out of the descending limb until the osmolarities become equal (equilibrium) [3] the entire column of fluid then goes around the loop of henle [4] ascending limb again transports salt out while water passively diffuses from the descending limb until a 200 mOsm/L difference is re-established between ascending limb and interstitium at each level [5] now advance the entire column of fluid around the loop of Henle [6] again, active extrusion of salt from ascending limb coupld with diffusion of water out of descending limb re-establishes the 200 mOsm/L gradient at all levels [7] as the process continues, the fluid in the descending limb becomes increasingly hypertonic until it reaches a max. of 1200 mOsm/L at bottom of loop [8] interstitial fluid achieves equilibrium in medulla [9] concentration of tubular fluid in ascending limb progressively decreases as salt is pumped out

Question 31

What may the osmotic concentration of urine be as it leaves the collecting tubule during LOW water intake?

Answer 31

it can go up to 1200 mOsm/L | - there is only a small volume of concentrated urine excreted

Question 32

What may the osmotic concentration of urine be as it leaves the collecting tubule during HIGH water intake?

Answer 32

- it can go down to around 100 mOsm/L - there is a large volume of dilute urine - there is barely any H2O reabsorption (water is eliminated)

Question 33

What is the salt pump in the thick ascending loop of Henle? What is the carrier protein involved? What is it inhibited by? What does inhibition do?

Answer 33

- carrier protein (NKCC2) transports 1Na+, 1K+ and 2Cl- - inhibited by loop diuretics (e.g. furosemide + bumetanide) - inhibition reduces hypertonic interstitium - -- reduces ability to reabsorb by osmosis - -- salt and water excretion increases - -- aka ions do not move into the interstitium

Question 34

How is urea handled in the nephron?

Answer 34

- urea is mainly absorbed by the thin descending loop of henle - thick ascending + DT is highly impermeable to urea - so tubular [urea] increases - in the final collecting duct/loop, the urea concentration increases - ADH also increases urea permeability by promoting UT-A1 and UT-A3 urea transporters

Question 35

How does ADH increase urea permeability?

Answer 35

- it increases urea permeability by promoting UT-A1 and UT-A3 urea transporters

Question 36

How much urea is left remaining as the urine leaves the tubules?

Answer 36

around 20%

Question 37

Why isn't the hypertonic interstitium "washed away"? ****

Answer 37

[1] blood flow to the medulla interstitium is very low - only about 2% of flow in the inner medulla [2] vasa recta capillaries "hairpin" operates as a countercurrent flow

Question 38

What are the pressures in the vasa recta?

Answer 38

- net inward pressure of 15 mmHg in the vasa recta (as in the other peritubular capillaries) [[ NaCl and water in the interstitium is therefore drawn into the vessels ]] Blood Pressure: 20 mmHg Oncotic Pressure = 35 mmHg Net Pressure = 15 mmHg - flow is directed INTO the vessel