Control of Extracellular Osmolarity and Sodium Concentration

Question 1

Sodium most abundant ion

Answer 1

extracellular fluid  Range 140 to 145 mEq/Liter  Average 142 mEq/Liter

Question 2

osmolarity  Average

Answer 2

300 mOsm/Liter [282 mOsm/Liter – corrected for

interionic attraction]  Range 291 to 309 mOsm/Liter [± 2% to 3%]

Question 3

precise control of sodium and osmolarity important because

Answer 3

they control distribution of water between intracellular and extracellular compartments

Question 4

Sodium and associated anions (chloride and bicarbonate) account for

Answer 4

94% of all extracellular solute

Question 5

Glucose and urea contribute

Answer 5

3 to 5% of total osmolarity  Urea able to permeate cells easily so exerts little effective osmotic force

Question 6

Sodium not very permeable so

Answer 6

has big effect on fluid movement between extracellular and intracellular compartments

Question 7

Plasma osmolarity =

Answer 7

2.1) x (Plasma concentration sodium)  Posm = (2.1) x (142 mEq/L) = 298 mOsm/L

Question 8

Two systems control / regulate extracellular osmolarity and sodium concentration

Answer 8

 Osmoreceptor – ADH system  Thirst mechanism

Question 9

Osmoreceptor Cells location, response, impulses, release, which does what?

Answer 9

 Located in anterior hypothalamus  Cells shrink in response to increased ECF [Na+] (i.e.
increased osmolarity)
 As cells shrink, number of impulses sent to other nerve cells in supraoptic nuclei
 Impulses passed to posterior pituitary  Impulses stimulate release of AHD stored in secretory
granules within nerve endings
 Increased [ADH] of blood stimulates increased water permeability in late distal tubules, cortical collecting tubules, and medullary collecting tubules

Question 10

Osmoreceptor Cells

 Increased osmolarity results in increased water permeability which allows

Answer 10

water to be reabsorbed (conserved) while sodium continues to be excreted at normal rate

Question 11

ADH release is tied into

Answer 11

the arterial baroreceptor reflexes (which respond to changes in blood pressure) and the cardiopulmonary reflexes (which respond to changes in blood volume)

Question 12

Reflex pathways tied into

Answer 12

hypothalamic nuclei that control ADH production and release

 Decreased blood pressure and/or decreased blood volume results in an increase in ADH release

Question 13

A 15 to 20% reduction in circulating volume produces a

Answer 13

HUGE increase in [ADH]

Question 14

Circulating volume must decrease approximately 10% before

Answer 14

appreciable change in [ADH]

Question 15

factors that Increase ADH Release

Answer 15

Increased plasma osmolarity
Decreased blood volume
Decreased blood pressure
Nausea
Hypoxia
Morphine, Nicotine, Cyclophosphamide

Question 16

factors that decrease ADH release

Answer 16

increase BV, increase BP, decrease plasma osmolarity, alcohol, clonidine, haloperidol

Question 17

Thirst center

Answer 17

Anteroventricular region of third cerebral ventricle (AV3V region) (Also promotes ADH release)
 Upper portion contains subfornical organ  Inferior portion contains organum vasculosum of the lamina terminalis

Question 18

thirst center located Anterolaterally in

Answer 18

preoptic nucleus

Question 19

thirst center neurons within area respond to changes in

Answer 19

osmolarity (function like

osmoreceptors)

Question 20

thirst mechanism stimulated by Na conc. of

Answer 20

2 mEq/Liter higher than normal  Thresholdfordrinking

Question 21

increase thirst

Answer 21

Increased plasma osmolarity
Decreased blood volume**
Decreased blood pressure**
Increased angiotensin II**
Dryness of mouth

Question 22

decrease thirst

Answer 22

Decreased plasma osmolarity
iNCREASED BLOOD VOLUME
Increased blood pressure
Decreased angiotensin II
Gastric distention

Question 23

With both osmoreceptor-ADH and thirst mechanism intake, able to prevent

Answer 23

though sodium intake has increases 6-fold

 Even if one system is not functional, other system still maintain the sodium concentration

Question 24

If both systems fail, there is no other system that can

Answer 24

regulate sodium concentration so sodium concentration will show large swings depending on sodium intake

Question 25

Angiotensin II and aldosterone play an important role controlling

Answer 25

SODIUM REABSORPTION

Question 26

angiotensin II and aldosterone DO NOT play a role in controlling

Answer 26

SODIUM CONCENTRATION

Question 27

increased levels of angiotensin II and aldosterone

Answer 27

will increase sodium reabsorption AND water reabsorption (Change in total amount of sodium and total amount of water, but no change in concentration)

Question 28

Extremely high levels of aldosterone will only produce an increase in sodium concentration of

Answer 28

3 to 5 mEq/Liter

Question 29

Complete loss of aldosterone secretion can lead to a

Answer 29

significant decrease in sodium concentration

Question 30

Sodium depletion leads to

Answer 30

volume depletion and decreased blood pressure which activates thirst reflex and the cardiopulmonary reflex which results in further decrease in sodium concentration as volume is ingested and/or reabsorbed