Control of Extracellular Osmolarity/[Na+]-Exam 4 Flashcards
What is the most abundant ion in extracellular fluid?
Sodium
What is the range in concentrations of sodium?
140-145 mEq/L
Avg: 142 mEq/L
What is average osmolarity?
300 mOsm/L
What is average osmolarity corrected for interionic attraction?
282 mOsm/L
What is the range in osmolarity?
291-309 mOsm/L [+/- 3%]
Why is precise control of osmolarity and sodium important?
They both control distribution of water between intracellular and extracellular compartments
Sodium and associated anions account for what percent of extracellular solute? What are the associated ions?
94%; chloride and bicarbonate
Glucose and urea contribute what percent of total osmolarity?
3 to 5%
Why does urea exert little effective osmotic force?
Able to permeate cells easily
Why does sodium have a big effect on fluid movement between extracellular and intracellular compartments?
Not very permeable
Equation for Plasma Osmolarity
(2.1) x (Plasma concentration sodium)
How do you calculate the plasma osmolarity of sodium?
Psm= (2.1) x (142 mEq/L) = 298 mOsm/L
What two systems control/regulate extracellular osmolarity and sodium concentration?
- Osmoreceptor - ADH system
2. Thirst mechanism
Where are osmoreceptor cells located?
Anterior hypothalamus
When would osmoreceptor cells shrink?
In response to increased ECF [Na+] (i.e. increased osmolarity)
What happens with impulses as osmoreceptor cells shrink? Where are impulses passed?
Number of impulses sent to other nerve cells in supraoptic nuclei; impulses passed to posterior pituitary
What do impulses from shrunken osmoreceptor cells stimulate?
Release of ADH stored in secretory granules within nerve endings
Water deficit leads to _________(increase/decerease) in extracellular osmolarity.
Increase
Increased Extracellular osmolarity leads to increased secretion of what?
ADH secretion (posterior pituitary)
ADH secretion leads to increased water reabsorption leading to ____________(increase/decrease) in water excreted.
Decrease
ADH is tied to what reflexes?
Arterial baroreceptor reflexes and cardiopulmonary reflexes
What do arterial baroreceptor reflexes response to?
Changes in blood pressure
What do cardiopulmonary reflexes response to?
Changes in blood volume
Reflex pathways tied to ________________ (2) that control ADH production and release.
Hypothalamic nuclei
Decreased blood pressure and/or decreased blood volume results in ____________ (increased/decreased) ADH release.
Increased
Small increase of what percent of osmolarity will trigger increase [ADH]?
1%
Circulating volume must decrease by what percent before appreciable change in [ADH]?
10%
What percent reduction in circulating volume produces a HUGE increase in [ADH]?
15-20%
What causes increased ADH release?
Increased plasma osmolarity Decreased blood pressure Decreased blood volume Nausea Hypoxia
What drugs cause increased ADH release?
Morphine, Nicotine, Cyclophophamide
What causes decrease in ADH release?
Decreased plasma osmolarity
Increased blood volume
Increased blood pressure
What drugs cause decreased ADH release?
Alcohol, Clonidine (Antihypertensive) Haloperidol (dopamine blocker)
What mechanism controls fluid intake?
Thirst
Why is the thirst mechanism important?
needed to replace fluid loss via sweating, breathing GI tract
Where is the thirst center?
Anteroventricular region of third cerebral ventricle (AV3V region); anterolaterally in preoptic nucleus
What does the thirst cneter also do?
Promotes ADH release
Upper portion of thirst center contains what region?
Subfornical organ
INferior portion of thirst center contains what?
Organum vasculosum of the lamina terminalis
How long does the drive to drink continue?
As long as thirst center is stimulated
Neurons within thirst center respond to changes in what? They function like what?
Osmolarity; function like osmoreceptors
What is the thirst mechanism stimulated by? (threshold for drinking)
Sodium concentration 2mEq/L higher than normal
What causes increased thirst?
Increased plasma osmolarity Decreased bp Decreased blood volume Increased angiotensin II dryness of mouth
What causes decreased thirst?
Decreased plasma osmolarity Increased bp increased blood volume Decreased angiotensin II gastic distension
What does increased angiotensin II probably act on?
Organum vasculosum of lamina terminalis
How long does it take to absorb and distribute digested fluid?
30-60 minutes
Why would we get overhydrated if thirst drive wasn’t suppressed?
We would be driven to continue drinking until osmolarity was returned to normal
With our regulatory systems, we’re able to prevent large changes in sodium concentration even though sodium intake increases how much?
6-fold
What would happen if one system of sodium regulation wasn’t functional?
Other systems would still maintain the sodium concentration
If both sodium regulation methods fail, what happens?
There is no other system that can regulate sodium concentration so sodium concentration will show large swings depending on sodium intake
What two hormones play an important role in controlling sodium reabsorption?
Angiotensin II and aldosterone
Angiotensin II and aldosterone do not play a role in controlling what?
Sodium Concentartion
Increased levels of angiotensin II and aldosterone will result in what?
INcreased sodium reabsorption and water reabsorption
How would you block the aldosterone system?
Removing adrenal glands and infusing aldosterone to maintain plasma concentration
Extremely high levels of aldosterone will only produce an increase in sodium of what?
3 to 5 mEq/L
Complete loss of aldosterone secretion can lead to what?
Significant decrease in sodium concentration
Sodium depletion leads to ________ depletion.
Volume
Sodium depletion leads to decreased _____________ (2), which activates the thirst reflex and cardiopulmonary reflex.
Blood pressure
Increase reflex stimulation results in further decrease in _____________ (2) as volume is ingested and/or reabsorbed.
Sodium concentration
