4.4C. Control of body fluid volumes and extracellular fluid osmolality. (Hypothalamic regulation of water conservation and uptake.)

Question 1

I. What are the 2 important body fluid conditions?

Answer 1

Two important body fluid conditions involving water intake / loss:
- Hypoosmotic hypervolemia: fluid volume goes up, so fluid osmolarity goes down
- Hyperosmotic hypovolemia: fluid volume goes down, so fluid osmolarity goes up

Question 2

II. Hypoosmotic hypervolemia
1. How does our body respond to Hypoosmotic hypervolemia?

Answer 2

drank water [more fluid], [salt]↓

Question 3

II. Hypoosmotic hypervolemia
3. What are the 2 mechanisms that correct Hypoosmotic hypervolemia?

Answer 3

• Hypothalamic osmoreceptors
• Automatic renal response

Question 4

II. Hypoosmotic hypervolemia
4. How do hypothalamic osmoreceptors correct hypoosmotic hypervolemia?

Answer 4

• Hypothalamic osmoreceptors: receptor cells swell
  -> ADH secretion↓
  -> kidney water
  reabsorption↓
  -> urination↑ (diuresis)
• (Fun fact: ethanol inhibits ADH secretion = urine excretion↑)

Question 5

II. Hypoosmotic hypervolemia
5. How do Automatic renal response correct hypoosmotic hypervolemia?

Answer 5

↓plasma oncotic pressure⇒↑GFR⇒↑medullary circulation⇒↓water reabsorption (↑dilution)

1) Plasma oncotic pressure↓, so filtration pressure / GFR↑ and peritubular capillary H2O-reabsorption from PT↓

2) ↑ medullary circulation (due to ↑BP) through the juxtamedullary nephrons, washes out solutes from the inner medulla, thus ↓ the osmotic force driving collecting duct H2O-reabsorption (thus more water is urinated out)

Question 6

III. Hyperosmotic hypovolemia
1. How does our body respond to Hyperosmotic hypovolemia?

Answer 6

sweat (less fluid), [salt] ↑

Question 7

III. Hyperosmotic hypovolemia
2. What happen during Hyperosmotic hypovolemia?

Answer 7

• When fluids are lost via sweating, ECF/ICF compartments concentrate, volume↓ + osmolarity↑

Question 8

III. Hyperosmotic hypovolemia
3. How does Hypothalamic osmoreceptors correct Hyperosmotic hypovolemia?

Answer 8

1. receptor cells shrink -> ADH secretion↑ -> kidney H2O-reabsorption↑ -> urination↓
2. thirst feeling↑ -> drinking -> rapid correction

Question 9

III. Hyperosmotic hypovolemia
4. How does Volume receptors correct Hyperosmotic hypovolemia?

Answer 9

Volume receptors: also plays a role in stimulating ADH secretion at low volumes, can dominate the correction of this condition in the case of severe hypovolemia
- If more fluid is lost: BP↓, concentrated urine↑, painful feeling of thirst (=ADH secretion↑↑↑!)

Question 10

IV. Hyperosmotic isovolemia
1. What is the cause of Hyperosmotic isovolemia?

Answer 10

↑ salt-uptake, volume unchanged

Question 11

IV. Hyperosmotic isovolemia
2. What happen during Hyperosmotic isovolemia?

Answer 11

In the case of salt intake, osmolarity increases with a constant volume. This results in a redistribution of fluid volume ratios
1. Na+ and Cl- redistributed mostly to ECF volume (EC osmotic concentration↑)
2. That will drive water from ICF to ECF (ECF volume↑)
3. Osmotic concentration sensed by the osmoreceptors

Question 12

IV. Hyperosmotic isovolemia
3. How does our body correct Hyperosmotic isovolemia?

Answer 12

• Correction occurs via increased thirst/drinking and ADH-secretion which decreases the osmotic concentrations
• Rebalances the ECF/ICF distributionpartial osmotic correction
• Osmoreceptor does promote H2O-uptake
• Volume receptors sense the larger circulating blood volume, but do not prevent further H2O-uptake
• We still drink water until the osmotic equilibrium (290mOsm)perfect osmotic concentration
  => We go from a hyperosmotic isovolemia condition to isosmotic hypervolemia (volume↑)

Question 13

IV. Hyperosmotic isovolemia
4. How does Osmoreceptor correct Hyperosmotic isovolemia?

Answer 13

• Osmoreceptor does promote H2O-uptake

Question 14

IV. Hyperosmotic isovolemia
5. How does Volume receptors correct Hyperosmotic isovolemia?

Answer 14

Volume receptors sense the larger circulating blood volume, but do not prevent further H2O-uptake

Question 15

V. Extra
1. We still drink water until the osmotic equilibrium (290mOsm) -> perfect osmotic concentration
=> We go from a hyperosmotic isovolemia condition to isosmotic hypervolemia (volume↑)
=> What will happen?

Answer 15

Because we now have a hypervolemic condition, ANP will be released to increase excretion of Na+-rich urine, returning things to the conditions found before ingestion of the salty meal§