Producing a Concentrated or Dilute Urine Flashcards
1
Q
How can the kidneys regulate water excretion?
A
varying the relative proportions of ions and water in urine in response to the hydration status and salt intake
2
Q
How do osmolality levels vary along the nephron?
A
- plasma osmolality maintained at 300 mOsm/L throughout ECF of cortex and within proximal tubule
- osmolality reduces to 100 mOsm/L
- reduced further in DCT and collecting ducts due to additional resorption of sodium chloride
3
Q
How is dilute urine produced?
A
- in absence ADH, DCT and collecting ducts is impermeable to water and tubular fluid becomes further diluted (40-50 mOsm/L)
- failure to reabsorb water and continued reabsorption of ions leads to large volume of dilute urine
4
Q
What is needed for the production of concentrated urine?
A
- high level of ADH allowing distal tubules and collecting ducts to become permeable to water and water is reabsorbed
- establishes gradient due to high osmolality of renal medulla which is used to pull water out
5
Q
What factors contribute to a build-up of osmolality in the medulla?
A
- passive absorption of ions across epithelia of thin ascending limb of loop of Henle
- active transport of Na+ and co-transport of K+, Cl- and other ions out of thick portion of ascending limb of loop of Henle
- active transport of ions from collecting duct
- facilitated diffusion of urea from medullary portion of collecting ducts
- diffusion of small amounts of water from medullary tubules into medullary interstitium (creates osmotic imbalance)
6
Q
Describe the movement of water and ions at the descending limb of the nephron
A
- impermeable to salt
- water moves out passively by AQP1
7
Q
Describe the movement of water and ions at the thin ascending limb of the nephron
A
- passive Na+ movement paracellular out of tubule
- passive Cl- movement transcellular out of tubule
- no H20 movement
8
Q
Describe the movement of water and ions in the thick ascending limb of the nephron
A
- active pumping against Na+ gradient
- no H20 movement
9
Q
Describe the counter current mechanism of the nephron
A
- fluid enters loop of Henle from PCT at 300 mOsm/L (same as plasma)
- active transport of ions from thick ascending limb establishes 200 mOsm/L gradient between tubular fluid and intersitital fluid
- tubular fluid in descending limb now equilibriates with interstitial fluid as water moves out
- transport of ions but not water in ascending limb maintaining gradient
- fluid moving into loop of Henle from PCT moves fluid in the limbs, hyperosmotic fluid in descending limb moves to ascending limb
- additional ions pumped out of fluid from ascending limb until 200 mOsm/L gradient again established
- movement of water out of descending limb to reach osmotic equilibrium with medullary interstitial fluid
- this increases osmolarity in tubule in descending limb moving to ascending limb for processes of Na+ and other ions movement
- repeated over and over until deepest part of medulla rises to 1200 - 1400 mOsm/L
10
Q
Describe the formation of concentrated urine
A
- tubular fluid leaving loop of Henle into DCT
- DCT continues to remove ions due to active transport of sodium chloride so osmolarity of tubule fluid continues to fall
- cortical section of collecting ducts reabsorbs urine, the amount is dependent on ADH levels
- in presence of ADH, absorbed water rapidly transported out of kidney by large blood flow through kidney cortical peritubular capillaries
- water absorption in kidney cortex rather than the medulla helps preserve osmotic gradient in the medulla
- as fluid goes through collecting duct and medulla more water reabsorption occurs into medullary interstitium
- water carried away by vasa recta into venous supply
- high levels of ADH cause collecting ducts become permeable to water and dluid at end has same osmolarity as renal medulla
- by resorbing as much water as possible, kidneys can produce a concentrated urine
11
Q
Describe how urea can contribute to concentrating urine
A
- occurs in dehydration
- in descending loop urea concentration continues to rise due to further water reabsorption into medullary interstitium and passive moemvent of urea from medullary interstitial fluid into tubule
- thick ascending limb, DCT, and collecting ducts are all relatively impermeable to urea so does not move back into interstitium
- presence of ADH and further reabsorption of water from cortical tubule sections further increases urea concentration of urea already in tubule
- in medullary collecting ducts more water is absorbed and urea becomes even more concentrated
- high concentration causes it to diffuse out of collecting ducts into medullary intetstitial fluid
- moderate amount can move back into tubule at inner medullary section and can recirculate through tubule distal to this part of nephron several times
- recirculation can contribute to concentration of urea in distal tubular fluids in dehydration and provide another mechanism for formation of hyperosmotic renal medulla
- in full hydration where water is to be lost, tubular flow is greater and recirculation is less marked and contributes less
12
Q
Describe the blood flow through the vasa recta
A
- medullary blood flow is low and about 5% of total renal blood flow
- enough to supply metabolic needs of tissues but does not result in loss of solute from medullary interstitium
- looped structure serves as counter current mechanism to prevent washout of solutes from medullary interstitium
- blood becomes more hypertonic as it descends into medullary interstitium and becomes less hypertonic as it ascends back towards cortical regions
- solutes and water move to mirror concentrations in medulla
13
Q
What factors increase ADH?
A
- increased plasma osmolarity
- decreased blood volume
- decreased blood pressure
- nausea
- hypoxia
- drugs (morphine/nicotine/cyclophosphamide)
14
Q
What factors decrease ADH?
A
- decrease plasma osmolarity
- increased blood volume
- increased blood pressure
- drugs (alcohol/clonidine/haloperidol)
