REB 23. Countercurrent Mechanism Flashcards
What type of urine is produced when an individual is dehydrated?
highly concentrated, low volume of urine
What type of urine is produced when an individual is over-hydrated?
dilute, large volume of urine
What is the osmotic concentration of 1 mmol of glucose?
1 mOsm/L
What is the osmotic concentration of 1 mmol of NaCl?
NaCl
- can be split into Na and Cl
- therefore, 1 mmol of Na+ and 1 mmol of Cl-
- in total, 2 mOsm/L
What is the definition of Osmotic Pressure?
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
What is reabsorbed in the proximal convoluted tubule?
- 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
What is the osmolarity of the proximal convoluted tubule?
around 300 mOsm/L
Which part of the renal tubules has NO regulatory role?
the proximal convoluted tubule
What is mainly absorbed in the early Proximal Convoluted Tubule?
- HCO3- (90%) is a major anion reabsorbed with Na+
- due to counter transport of H+ (?)
What is mainly absorbed in the Distal Convoluted Tubule?
- some Na+ is reabsorbed by simple diffusion and bulk flow
- Cl- lags behind and is helped by bulk flow so it catches up
What percentage of filtrate is reabsorbed in the loop of Henle? What mechanism is this done through?
- 20% of the filtrate is reabsorbed through the loop of henle
- reabsorption is part of the countercurrent mechanism
What percentage of filtrate is reabsorbed in the distal tubules and collecting duct?
- 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
What is the part of the tubule which has VARIABLE resorption?
distal tubules + collecting duct
What is sodium reabsorption controlled by in the distal tubules and collecting duct?
Aldosterone
What is water resorption controlled by in the distal tubules and collecting duct?
Anti-Diuretic Hormone (ADH)
- aka Vasopressin
What is the normal urinary output?
1 mL/min
If there is a high water intake, what does this lead to? How does this effect urinary output?
- high water intake leads to no water reabsorption
- the urinary output is around 12 mL/min
If there is a low water intake, what does this lead to? How does this effect urinary output?
- low water intake leads to high (maximal) water reabsorption
- the urinary output is around 0.5 mL/min
What are the 2 requirements for adjustable water reabsorption in the collecting duct?
[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
What is the function of the hypertonic interstitium in the collecting duct?
it provides the OSMOTIC GRADIENT to reabsorb the water
What is the main function of the Loop of Henle?
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
What are the characteristics of the Loop of Henle? [3]
[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
What are the 2 parts of the Countercurrent Mechanism?
[1] Countercurrent Exchange System
[2] Countercurrent Multiplier
What is the countercurrent exchange system?
it is the anatomical arrangement of vessels so that flow in one vessel is in the opposite direction from flow in the adjacent vessel
What is the countercurrent multiplier?
it is the anatomical arrangement of the loop of Henle that concentrates solute in the renal medulla
What are the 3 distinct regions of the Loop of Henle?
[1] Thin-Walled Descending
[2] Thin-Walled Lower Portion of Ascending
[3] Thick-Walled Upper Portion of Ascending
What is the thin-walled descending loop of henle permeable to?
- highly permeable to H2O, but NOT solutes
- H2O will flow from the loop into the surrounding medium via osmosis
What is the thin-walled lower portion of ascending loop of henle permeable to?
- highly permeable to Na+ and Cl-
- moderately permeable to urea
- almost completely impermeable to H2O
What is the thick-walled upper portion of the ascending loop of henle permeable to?
- Na+ and Cl- are actively pumped out of the filtrate into the surrounding medium
** What is the countercurrent mechanism? List and explain the steps. **
[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
What may the osmotic concentration of urine be as it leaves the collecting tubule during LOW water intake?
it can go up to 1200 mOsm/L
- there is only a small volume of concentrated urine excreted
What may the osmotic concentration of urine be as it leaves the collecting tubule during HIGH water intake?
- 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)
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?
- 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
How is urea handled in the nephron?
- 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
How does ADH increase urea permeability?
- it increases urea permeability by promoting UT-A1 and UT-A3 urea transporters
How much urea is left remaining as the urine leaves the tubules?
around 20%
Why isn’t the hypertonic interstitium “washed away”? **
[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
What are the pressures in the vasa recta?
- 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
