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Urinary System > Lec 9 - Control of Plasma Osmolarity > Flashcards

Lec 9 - Control of Plasma Osmolarity Flashcards

1
Q

What is the osmolarity of most bodily fluids?

A
  • Isotonic

- osmolarity is 280 - 310 (300) mOsm/ Kg

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2
Q

What happens to the plasma osmolarity if water intake is less than water excretion?

A

Plasma osmolarity increases.

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3
Q

What happens to plasma osmolarity if water intake is more than water excretion?

A

Plasma osmolarity decreases

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4
Q

How much do most people on average urinate?

A

1 - 1.5L/d

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5
Q

How much do most people on average ingest?

A

600 - 1000 mOsm/d

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6
Q

What can urine osmolarity vary between?

A

50 - 1200 mOsm/Kg

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7
Q

What is the major cation of the ECF?

A

Na+

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8
Q

What are the sensors that regulate the changes in plasma osmolarity?

A

Hypothalamic osmoreceptors

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9
Q

What are the two efferent pathways that the hypothalamic osmoreceptors act on?

A
  1. ADH

2. Thirst

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10
Q

What is the effector in the ADH pathway and what is affected?

A

effector —> kidney

affected —> renal water excretion.

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11
Q

What is the effector in the thirst pathway and what is affected?

A

effector —> brain: drinking behaviour

affected —> water intake

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12
Q

In which conditions is ADH release increased?

A

Under conditions of predominant loss of water, osmoreceptors in the hypothalamus increase the release of ADH from the posterior pituitary.
- increase of 1% in osmolarity increases ASH.

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13
Q

What inhibits ADH secretion?

A

decreased osmolarity.

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14
Q

What happens to osmolarity when ECV is decreased?

A

The set point is shifted to lower osmolarity values and the slope of the relationship is steeper.

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15
Q

What do the kidneys do when faced with circulatory collapse?

A

The kidneys continue to conserve H20 even though this will reduce the osmolarity of body fluids.

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16
Q

What is the analogue of thirst?

A

Salt ingestion.

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17
Q

How are large deficits in water or an increase in salt compensated for?

A
  • only partially compensated for in the kidney.
  • ingestion is the ultimate compensation. –> this is stimulated by an increase in fluid osmolarity also by reduced ECF volume.
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18
Q

How is drinking induced?

A

it is induced by increases in plasma osmolarity or by decreases in ECF volume.

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19
Q

What does ADH do?

A
  1. acts on the kidney to regulate the volume and osmolarity of the urine.
  2. It increases the permeability of the collecting duct to water and urea.
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20
Q

What happens when there is a low plasma ADH?

A

diuresis

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21
Q

What happens when there is a high plasma ADH?

A

anti-diuresis

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22
Q

When does central diabetes insipidus occur?

A
  • It results when plasma ADH levels are too low.
23
Q

What are some examples that cause plasma ADH to be low?

A
  1. damage done to the hypothalamus or pituitary gland.
  2. A brain injury, particularly a fracture of the base of the skull.
  3. A tumour
  4. sarcoidosis or tuberculosis
  5. An aneurysm.
  6. Some forms of encephalitis or meningitis
  7. The rare disease langerhans cell histiocytosis.
24
Q

When does nephrogenic diabetes insipidus occur?

A
  • it is from an acquired insensitivity of the kidney to ADH.
25
Q

What does central diabetes insipidus and nephrogenic diabetes insipidus have in common?

A
  • In both water is inadequately reabsorbed from the collecting ducts.
  • –> This means a large quantity of urine is produced.
  • They are both managed clinically by ADH injections or by ADH nasal spray treatments.
26
Q

What does SIADH stand for and what is it characterised by?

A
  • Syndrome of inappropriate antidiuretic hormone secretion.

- characterised by excessive release of ADH from the posterior pituitary gland or another source.

27
Q

What is dilution hyponatremia characterised by?

A
  • plasma sodium levels are lowered.

- total body fluid is increased.

28
Q

Where are AQP2 found?

A
  • they are abundant in the apical plasma membrane and apical vesicles in the collecting duct principal cells that are sensitive to ADH.
29
Q

Where are AQP3 and AQP4 expressed?

A
  • they are expressed in cells in the collecting duct principal cells, where they are abundant in the basolateral plasma membranes.
  • They represent potential exit pathways from the cell for water entering via AQP2.
30
Q

What happens when plasma osmolarity decreases?

A
  1. There is no ADH stimulation which means that there are no AQP2 in the apical membrane and AQP3 and 4 on the basolateral membrane only of the latter DCT and collecting ducts.
  2. There is limited water reuptake in the latter DCT and limited in the collecting duct.
  3. Tubular fluid rich in water passes through the hyper osmotic renal pyramid with no change in water content.
  4. Loss of large amount of hypo-osmotic (dilute) urine.
  5. Get diuresis
31
Q

What happens when plasma osmolarity increases?

A
  1. The body needs to produce a hyperosmotic urine.
  2. The kidney must reabsorb as much water as possible from the kidney tubule.
  3. Water will move out of the collecting duct into a hyper osmotic environment if there are AQPs in both the apical and the basolateral epithelium of the tubule cells.
  4. The release of ADH causes insertion of AQP2 channels into apical membrane.
  5. Needs a hypertonic interstitium.
32
Q

What is the concentration of urine due to? The medullary counter current mechanism.

A
  1. Juxtamedullary nephron
    - –> The long loop of Henle creates a vertical osmotic gradient.
  2. Vasa recta helps to maintain this osmotic gradient.
  3. The collecting duct of all nephrons use the gradient along with hormone ADH (vasopressin) to produce urine of varying concentration.
  4. Urea also helps in urine concentration mechanism.
33
Q

What is the diluting action of the thick ascending limb of the loop of Henle on the filtrate?

A
  1. It removes solute without water and therefore increases the osmolarity of the interstitium.
  2. Blocks NaK2Cl (NaKCC) transporters with a loop diuretic medullary interstitium.
    - –> so it becomes isosmotic and copious dilute urine is produced.
34
Q

What is the descending limb of the long loop of henle permeable to?

A
  • It is highly permeable to water due to AQP1 water channel which are always open.
35
Q

What is the descending limb of the long loop of Henle impermeable to?

A
  • impermeable to Na+
  • –> This means that Na+ remains in the descending limb of the loop of Henle and filtrate concentration (osmolarity) increases.
36
Q

Where is the maximum osmolality of the loop of Henle and what is it?

A
  • It is in the tip of the loop of henle.

- It is 1200 mOsm/Kg

37
Q

What is the osmolality of the fluid entering the DCT?

A
  • It is low

- 100 mOsm/Kg

38
Q

What does the ascending limb of the loop of Henle do?

A
  • It actively transports NaCl out of the lumen into interstitial fluid.
39
Q

What is the ascending limb impermeable to?

A
  • It is impermeable to water.
  • –> so as NaCl leaves water remains.
  • ——> This causes the osmolality of the ascending limb to decrease.
40
Q

What is counter current multiplication?

A

This is when there is an increased concentration in the interstitial fluid which is achieved by the loop of Henle.
- Created as Na+ is actively transported out of the ascending limb of the loop of henle, this causes the concentration of the interstitial fluid surrounding the loop of Henle to increase.

41
Q

Describe how osmolarity of fluid changes across the nephron.

A
  • At PCT is is around 300 mOsm/Kg.
  • Then down the descending limb of the loop of henle it increases, at the base it is 1200 mOsm/Kg.
  • Then it decreases up the ascending limb to 100 mOsm/ Kg at the DCT.
  • Then increases at the collecting duct.
42
Q

What is the vertical osmotic gradient established?

A
  • In the interstitial fluid of the medulla.
43
Q

What is the osmolarity at the corticomedullary border?

A

Isotonic - 300 mOsm/Kg

44
Q

What is the osmolarity in the medullary interstitium?

A

Hyperosmotic

- up to 1200 mOsm/Kg at papilla.

45
Q

What are essential mechanisms in the vertical osmotic gradient?

A
  1. active NaCl transport in thick ascending limb.
  2. Recycling of urea.
  3. Unusual arrangement of blood vessels in the medulla descending components in close opposition to ascending components.
46
Q

How is urea recycled?

A
  1. Urea is reabsorbed from the medullary collecting duct.
  2. The cortical collecting duct cells are impermeable to urea.
  3. Movement of urea into the interstitium and diffusion back into the loop of henle.
  4. Under the influence of ADH fractional excretion of urea decreases and urea re-cycling increases.
47
Q

How is the counter current established?

A
  1. The active salt pump in the ascending limb establishes a 200 mOsm gradient at each horizontal level.
  2. As the fluid flows forward, several frames a mass of 200 mOsm fluid exits into the distal tubule and a new mass of 300 mOsm fluid enters from the proximal tubule.
  3. The ascending limb pump and descending limb passive fluxes reestablish the 200 mOsm gradient at each horizontal level.
  4. Once again, the fluid flows forward several frames.
  5. The 200 mOsm gradient at each horizontal level is established once again.
  6. The final vertical osmotic gradient is established and maintained by the ongoing countercurrent multiplication of the long loop of Henle.
48
Q

How is the concentration gradient produced and maintained?

A
  • It is produced by the loop of henle acting as a counter current multiplier.
  • But it is maintained by the vasa recta acting as a counter-current exchanger.
49
Q

What are vasa recta?

A

They are straight vessels.

50
Q

How are the vasa recta able to act as a counter current exchanger?

A
  • This is because flow in the vasa recta is in opposite direction to fluid flow in the tubule.
  • –> This maintains the osmotic gradient.
51
Q

What happens in the descending limb of the vasa recta?

A
  1. Isosmotic blood in the vasa recta enters hyperosmotic Millieu of the medulla.
    - –> In here there is a high concentration of Na+ ions, Cl- ions and urea.
  2. Na+, Cl- and urea diffuse into the lumen of the vasa recta.
  3. This causes the osmolarity of the blood in the vasa recta to increase as it reaches the tip of the hairpin loop.
52
Q

What happens in the ascending limb of the vasa recta?

A
  1. Blood ascending towards the cortex will have higher solute content than surrounding interstitium.
  2. Water moves in from the descending limb of the loop of Henle.
53
Q

In conclusion what does the vasa recta do?

A

It preserves the hyperosmolarity of the renal medulla.

Urinary System (7 decks)

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