Regulation of extracellular osmolarity

Question 1

Regulation of extracellular fluid

Answer 1

-critical for cells
-Na is most common ion in ECF that needs to be regulated

Question 2

What mechanisms are involved in regulation of ECF osmolarity?

Answer 2

1.Osmoreceptor-ADH system
2. The thirst mechanism
**failure of either can be compensated for by the other but failure of both would cause significant changes in plasma Na concentration

Question 3

Why are plasma and ECF considered together?

Answer 3

-their osmolarity and Na concentrations are very close to each other

Question 4

How is plasma osmolarity changes sensed?

Answer 4

-Through specialized neuronal cells called osmoreceptors that are located in the hypothalamus at AV3V (anteroventral wall of 3rd ventricle)

Question 5

Osmoreceptor-ADH feedback

Answer 5

1.Osmolarity of ECF/plasma increases, the Na in the ECF will increase resulting in the osmoreceptors shrinking
2.Shrinkage of osmoreceptors fire signal to supraoptic and paraventricular neurons which triggers ADH release
3.ADH produced by neurons are stored in vesicles that are accumulated at nerve endings in posterior pituitary gland
4. ADH is released in the blood and travels to the kidneys and increases permeability of late distal tubules, cortical collecting tubules and collecting ducts to water.
5.ADH increases water reabsorption so that the osmolarity of plasma and ECF is adjusted

Question 6

Blood brain barrier and osmolarity detection

Answer 6

-less significant barrier in regions involved in sensing the osmotic pressure of plasma which allows solutes to enter the ECF

Question 7

What signals affect ADH release?

Answer 7

-osmolarity changes
-decreased arterial pressure
-decreased blood volume

Question 8

Stimuli affecting ADH

Answer 8

Increase ADH:
-nausea
-nicotine
-Morphine

Decrease ADH:
-alcohol
-caffeine

Question 9

Blood volume effects on ADH

Answer 9

-reduced blood volume stimulates ADH release but if extracellular osmotic pressure is constant, the ADH release in response to reduced volume is slow
-if blood volume stays constant and osmolarity of ECF increases, the ADH release is fast

Question 10

Why is water intake important?

Answer 10

-we are constantly losing water through evaporations, perspiration, urine and feces

Question 11

Thirst center

Answer 11

-located at AV3V and anterolateral wall of 3rd ventricle

Question 12

What causes sensation of thirst?

Answer 12

Methods:
1.increased osmolarity of ECF causes dehydration cells in the thirst center and lead to sensation of thirst
2.decreased arterial pressure also stimulates thirst through Ang II
3.Dryness of mouth and mucus membranes and esophagus stimulates thirst. Drinking can immediately reduce thirst through mechanism even before decreasing extracellular osmolarity

Question 13

Angiotensin II

Answer 13

-released when there is low arterial pressure and blood volume loss
-enters the thirst centre and directly stimulates the centre for restoring blood pressure and volume

Question 14

What can reduce thirst?

Answer 14

-gastrointestinal distension

Question 15

Threshold for drinking

Answer 15

-changes over 2mEq/L in Na concentration lead to activation of thirst signals

Question 16

Blockage of aldosterone effects

Answer 16

-aldosterone does not lead to significant changes in plasma Na because ADH and thirst mechanisms regulate this
-both aldosterone and angiotensin II increase Na reabsorption but they also increase water reabsorption. Therefore Na concentration does not change although more Na is being reabsorbed by kidneys

Question 17

Salt appetite

Answer 17

-we have a behavioural drive to ingest more salt when Na is low. Not very important in humans but it is important in herbivores because low Na diet

Question 18

Salt appetite center location

Answer 18

-located in AV3V

Question 19

How is salt appetite stimulated?

Answer 19

-by extreme Na deficiencies or decreased blood volume and blood pressure
Eg. Due to Addison’s disease (very low aldosterone that leads to Na excretion)

Question 20

What determines the osmolarity of the ECF?

Answer 20

-a balance between the intake and output of water and salt. Na excretion is usually adjusted based on intake

Question 21

Two main types of adjustments for Na and water excretion

Answer 21

1.Intrarenal adjustments (very effective for normal variations for Na intake)
2. Systemic adjustments (more significant under severe changes of Na intake and renal dysfunction)

Question 22

Intrarenal adjustments

Answer 22

1.changes in GFR
-from tubuloglomerular feedback and myogenic mechanism
2.changes in reabsorption
-glomerulotubular balance mechanism

Question 23

Systemic adjustments

Answer 23

1.Pressure natriuresis and diuresis
2.Sympathetic nervous system
3.Angiotensin II
4.Aldosterone
5. ADH

Question 24

Pressure natriuresis and diuresis effect on systemic adjustments

Answer 24

-powerful mechanism for excretion of excess Na and water
-more important with chronic increased BP due to sustained high Na intake
-with chronic high blood pressure, renal compensatory mechanisms are impaired, so natriuresis can help excrete excess Na and water

Question 25

Steps of pressure natriuresis and diuresis and increased fluid and Na excretion

Answer 25

1. increased Na intake
2. accumulation of water
3. increased in ECF volume
4. increase cardiac output
5. increase BP
6. natriuresis and diuresis= more Na excretion
7. decrease in ECF volume

Question 26

Sympathetic nervous system effect on Na and water excretion

Answer 26

1.reduced blood volume (eg.hemorrhage)
2.increased sympathetic NS
3. increased in arteriole constriction, tubular reabsorption of salt and water, and renin-angiotensin II
4. Leads to increased Na reabsorption and decrease in GFR

Question 27

Angiotensin II effect on Na and water excretion

Answer 27

1.increased Na intake
2.decrease in renin
3.decrease in angiotensin II
4. decrease in tubular Na reabsorption

Question 28

Aldosterone effect on Na and water excretion

Answer 28

1.increase Na intake
2.decrease aldosterone
3.increase in Na excretion

Question 29

Adrenal gland tumours

Answer 29

-can lead to excessive release of aldosterone
-results in increase in Na reabsorption but when excess Na causes increased arterial pressure, the kidneys escape from further reabsorption of Na under the effect of aldosterone

Question 30

ADH

Answer 30

*allows kidneys to concentrate urine without changing solute excretion

Question 31

Excessive ADH effect on Na and water excretion

Answer 31

1.excessive ADH released
2.increase arterial pressure
3.excretion of Na and water (natriuresis and diuresis)
4.extracellular fluid is adjusted but excessive Na excretion

Question 32

Problem with ADH release

Answer 32

Results in increase in urine (5-10 fold)
-needs to be adjusted by ingestion of more water