Regulation of Plasma Sodium and ECF Volume

Question 1

what does the hematocrit describe? what is it normally?

Answer 1

volume of red blood cells compared to total blood volume

normally about 45%

Question 2

changes in fluid volume and composition of extravascular fluid reflects what?

Answer 2

changes in intravascular fluid (they are in equilibrium)

Question 3

where is extra cellular fluid osmolarity and volume sensed? what types of cells are these?

Answer 3

osmolarity- hypothalamic osmoreceptors
volume- stretch receptors in the atria and vasculature (carotid sinus, aortic arch, renal afferent arterioles)
these cells are neuronal

Question 4

what 4 “effector agents” (organs) cause the necessary changes to correct for change in ECF volume and osmolarity? which are short and which are long term?

Answer 4

the posterior pituitary (long), autonomic nervous system (short), the heart (short) and the kidney (long)

Question 5

what is the role fo the autonomic nervous system in maintaining ECF volume and osmolarity?

Answer 5

it increases or decreases peripheral vascular resistance and heart rate to maintain blood pressure at a normal range

Question 6

what does the renin-angiotensin-aldosterone axis regulate?

Answer 6

peripheral vascular resistance and renal Na excretion

Question 7

what parameters are used to regulate ECF osmolarity? volume?

Answer 7

osm: renal free water excretion and water consumption
vol: blood pressure and sodium excretion

Question 8

what determines the ECF volume?

Answer 8

total amount of sodium in the ECF

high sodium= larger ECF volume

Question 9

how much water is in 1 kg? a 1 kg increase in weight reflects a positive Na balance of what?

Answer 9

1 L of water

140 mEq NaCl

Question 10

expansion of ECF volume results in what in the kidney? volume contraction?

Answer 10

expansion- increase in output of sodium and water in urine

contraction- decrease in output of sodium and water

Question 11

the initial absorption of Na after a salty meal has what effect?

Answer 11

induces increased release of ADH into the circulation
increases thirst and absorption of free water in the kidney
(maintains osmolarity and increases plasma volume)

Question 12

how is the increase in plasma volume from a high salt diet reduced over time?

Answer 12

by gradually decreasing renal sodium resorption and increasing excretion

Question 13

what is the initial effect in decrease of sodium consumption?

Answer 13

decrease in plasma osmolarity which decreases ADH

decrease in thirst and resorption of free water in the kidney (concentrates and decreases plasma)

Question 14

how is the volume contraction of a low sodium diet reduced over time?

Answer 14

gradual regulatory increase in renal Na absorption and decreased excretion

Question 15

an excess of salt in the diet can lead to what phenomenon? why is this important with increasing age?

Answer 15

has clinical effect of raising blood pressure

important because off loading excess sodium is more difficult with age

Question 16

what is the kidney’s response to ECF volume expansion? volume contraction? what is the change in GFR?

Answer 16

expansion- decreasing Na and water resorption
contraction- increasing sodium and water resorption
no change in GFR unless severe volume contraction

Question 17

how long does a change in sodium in the diet take to balance with sodium output? what is the detector for this correction?

Answer 17

4-5 days–very closely regulated

detected by baroreceptors that detect the secondary volume change that accompanies the salt intake

Question 18

compare the osmolarity and volume that is achieved after a increased Na consumption with normal conditions.

Answer 18

there is an isosmotic expansion of ECF volume

Question 19

what circumstance does the kidney react to to increase Na excretion?

Answer 19

in response to increase in ECF volume, not an increase in sodium concentration

Question 20

what changes induce regulation of sodium excretion?

Answer 20

changes in effective circulating volume, not total ECF volume

Question 21

what is the effective circulating volume?

Answer 21

functional blood volume reflecting extent of perfusion where blood pressure is sensed

Question 22

when does the effective circulating volume not match total ECF?

Answer 22

may be less in disease states that cause edema (like congestive heart failure)

Question 23

decrease in renal perfusion pressure does what? what does this lead to in the context of edema?

Answer 23

activates the renin- angiotensin- aldosterones system

further increases sodium retention and edema

Question 24

name four edematous disease states.

Answer 24

conesive heart failure, pulmonary edema, liver disease and nephrotic syndrome

Question 25

what do liver disease and nephrotic syndrome have in common?

Answer 25

both reduce plasma oncotic pressure by either failing to produce (liver) or allowing excretion of (kidney) albumin

Question 26

what do diuretic drugs do? how does this impact edema?

Answer 26

decrease plasma volume by forcing the kidney to increase sodium and water excretion in the urine favors absorption of edematous fluid

Question 27

which central vascular sensors sense low pressure? which sense high pressure? which are more important?

Answer 27

low- atria and pulmonary vasculature (more important) | high- carotid sinus, aortic arch and juxtaglomerular apparatus in the renal afferent arteriole

Question 28

how do receptors sense the ECF volume?

Answer 28

by sensing vascular pressure changes that occur with changes in volume

Question 29

what four systems are activated with decrease in effective circulating volume?

Answer 29

renin-angiotensin-aldosterone system, increased renal sympathetic nerve activity, ADH release by the posterior pituitary and decrease in release of atrial natriuretic peptide

Question 30

how does angiotensin II affect the kidney? aldosterone?

Answer 30

ang- promotes sodium retention by stimulating Na/H exchange in proximal tubule and decreases renal plasma flow promoting increased Na rebsorption ald- increased sodium resorption by late distal tubule and collecting duct

Question 31

what does renal sympathetic nerve activity do during decreased intravascular pressure?

Answer 31

induces renal vasoconstriction and increased Na reabsorption

Question 32

what is the function of atrial natriuretic peptide?

Answer 32

it induces sodium excretion with high atrial pressures

Question 33

where is renin synthesized and stored? what is its function?

Answer 33

synthesized and stored in granular cells of the juxtaglomerular apparatus of the kidney converts angiotensinogen to angiotensin I

Question 34

where is ACE located and what does it do?

Answer 34

on luminal surface of vascular endothelium, particularly in lungs. it converts angiotensin I to angiotensin II

Question 35

describe the deactivation of angiotensin II

Answer 35

has a short half life due to rapid degredation by aminopeptidases cleaving it into angiotensin III (less activity)

Question 36

what is the most important factor controlling angiotensin II levels?

Answer 36

renin release

Question 37

what are the three renal mechanisms that regulate renin release?

Answer 37

local renal baroreceptors in afferent arterioles decreased systemic pressure influences sympathetic stimulation of the juxtaglomerular apparatus cells of the macula densa sense tubular fluid sodium concentration

Question 38

what are the 5 actions of angiotensin II?

Answer 38

induces aldosterone release from adrenal cortex hypothalamus- increases thirst and induces ADH release vasoconstricts renal and systemic vessels enhances Na/H exchange in proximal tubule induces hypertrophy of renal tubule cells

Question 39

what is the purpose of renal vasoconstriction induced by angiotensin II?

Answer 39

constricts efferent more than afferent increasing GFR | increases starling forces favoring resorption of tubular fluid by peritubular capillaries

Question 40

what is the primary long term regulator of salt balance and ECF volume? where is it produced

Answer 40

aldosterone (a mineralcorticoid) | produced in the adrenal cortex

Question 41

what two effects does aldosterone have on the kidney?

Answer 41

increases resorption of sodium and therefore water | promotes potassium secretion by the distal nephron

Question 42

in what cells does aldosterone increase Na absorption and where are they? what distinguishes these cells?

Answer 42

principal cells in the late distal tubule and early collecting duct in the renal cortex express of intracellular aldosterone receptors

Question 43

in what three ways does aldosterone increase sodium absorption?

Answer 43

increases basolateral Na/K pumps, increases apical membrane Na channels and increases mitochondrial enzymes to make more ATP

Question 44

what happens to sodium absorbed in the late distal tubule? how does it change the cortico medullary solute concentration gradient?

Answer 44

since it is in the cortex, it is returned to circulation and does not participate in the gradient

Question 45

how is cortical Na resorption separated from the counter current multiplication system?

Answer 45

there is separate venous circulation in the cortex and in the vasa recta of the medulla

Question 46

where is most filtered sodium absorbed in the nephron? where is most of the remaining sodium absorbed?

Answer 46

proximal tubule | thick ascending limb of the loop of henle

Question 47

how is the loop of henle split functionally? what is the difference?

Answer 47

into the cortical and medullary portions | only medullary portion contributes to the counter current multiplication

Question 48

how much of Na is reabsorbed in Na balance? what segments account for the variability in that number?

Answer 48

99% of filtered Na is reabsorbed | late distal tubule and early collecting tubule (aldosterone)

Question 49

what mechanism is not included in the renal handling of Na?

Answer 49

secretion of Na does not occur

Question 50

describe resorption of NaCl in principal cells of the cortical collecting tubule.

Answer 50

Na is passively absorbed into the cell and actively pumped into the blood via a Na/K pump. Cl follows transcellularly

Question 51

how is Na absorption coupled to K secretion in the late distal tubule and collecting duct?

Answer 51

increase in Na resorption occurs simultaneously with increase in transcellular K secretion through luminal channels

Question 52

in what two ways does aldosterone increase K secretion?

Answer 52

by increasing synthesis of the Na/K pump in the basolateral membrane and by increasing synthesis of K channels in the luminal membrane

Question 53

what is the driving force for paracellular resorption of Cl?

Answer 53

because 3 Na+/2K+ are transported, the transport of Na and K is electrogenic. the negative luminal voltage difference drives Cl movement

Question 54

how does aldosterone produce effects in the cell?

Answer 54

it binds to an intracellular receptor and induces transcription for membrane transport proteins

Question 55

what do hormone specific secretogogues refer to?

Answer 55

agents that increase release of hormones into circulation

Question 56

what secretogogues are there for aldosterone?

Answer 56

angiotensin II (induces ACTH release by anterior pituitary), increased plasma K+ and ACTH

Question 57

what is the normal plasma sodium concentration?

Answer 57

135-145 mEq/L

Question 58

what are the symptoms of hypernatremia? at what levels do they become severe?

Answer 58

lethargy, weakness and irritability | >158 mEq/L- seizures and coma may occur

Question 59

what can cause hypernatremia?

Answer 59

caused by loss of free water from plasma due to inadequate consumption in excess of solute or inappropriate renal excretion of water

Question 60

what are the possible causes of hypovolemic hypernatremia?

Answer 60

inadequate water consumption, extreme sweating, severe diarrhea and polyuria (like in diabetes mellitus and insipidus)

Question 61

how does diabetes mellitus cause hypernatremia?

Answer 61

glucose transport is maximized and there is still excess in tubular fluid. this opposes reabsorption in distal nephron segments and prevents water resorption

Question 62

what is central diabetes insipidus?

Answer 62

inadequate release of ADH from the posterior pituitary to an increase in plasma osmolarity. free water is not reabsorbed and is excreted in the collecting duct

Question 63

what is nephrogenic diabetes insipidus?

Answer 63

inability of the collecting tubule to respond to ADH despite adequate release

Question 64

when does hypervolemic hypernatremia occur?

Answer 64

with consumption of hypertonic NaCl solution (seawater) or from inappropriate IV infusionof hypertonic saline may also occur from hyperaldosteronism

Question 65

in the case of hyperaldosteronism, how do central osmoreceptors change? how does this effect ADh release?

Answer 65

central osmoreceptors decrease sensitivity and for given increase in plasma osmolarity there is less ADH release

Question 66

what are the symptoms of hyponatremia? at what level do they get more severe?

Answer 66

nausea, vomiting, headache, lethargy, fatigue, loss of appetite, muscle weakness, spasm and cramps <125 mEq/L neurological deficis, brain swelling, seizures and coma occur

Question 67

what are some causes of hyponatremia?

Answer 67

extreme excessive consumption of water and inappropriate increase in free water reabsorbed secondary to dysregulation of ADH (syndrome of inappropriate ADH release)

Question 68

what is SIADH?

Answer 68

syndrome of inappropriate ADH release when levels are elevated with plasma osmolarity is reduced

Question 69

is hyponatremia typically the result of water imbalance or salt imbalance?

Answer 69

water

Question 70

hypervolemic hyponatremia may result from what 5 conditions?

Answer 70

congestive heart failure, kidney failure, liver failure, syndrome of inappropriate anti diuretic hormone and polydypsia

Question 71

how does congestive heart failure lead to hypervolemic hyponatremia?

Answer 71

inadequate pumping of blood effectively decreases circulating volume, causing the release of ADH

Question 72

how does hypervolemic hyponatremia result from kidney failure?

Answer 72

Na reabsorption is not sufficient and there is an over compensation for plasma volume depletion by increasing free water reabsorption in excess of solutes

Question 73

how does liver failure cause hypervolemic hypernatremia?

Answer 73

effective circulating volume is decreased due to edema and ADH is released

Question 74

what is psychogenic polydipsia?

Answer 74

compulsive water consumption verging on intoxication. imbalance in consumption and excretion dilutes plasma and increases volume

Question 75

in what settings does hypovolemic hyponatremia occur?

Answer 75

severe vomiting or diarrhea, hemorrhage, prolonged exercise in heat, diuretic drug therapy and in addison's disease

Question 76

how does diuretic drug use cause hypovolemic hyponatremia?

Answer 76

diuretics inhibit sodium reabsorption in the kidney but low plasma volumes increase water resorption in excess of solute

Question 77

what is addison's disease? how does this lead to hypovolemic hyponatremia

Answer 77

the adrenal gland does not respond to angiotensin II. kidney is unable to increase sodium resorption in response to volume depletion but water may still be retained

Question 78

why does plasma volume trump plasma osmolarity?

Answer 78

plasma volume will be protected at the expense of osmolarity becuase adequate circulation must be maintained to deliver oxygen and nutrients