Hormonal Control of Sodium and Water Balance

Question 1

Contrast the actions of ADH and Aldosterone in water and sodium regulation.

Answer 1

ADH- enhances water reabsorption in response to osmolarity (hypothalamus) and then ECV (baroreceptors of carotid sinus and aortic arch)
Aldosterone: increases sodium reabsorption in response to ECV or when sodium levels in filtrate are low

Question 2

What is characteristic of an “effective osmol.”? What does that mean effective. ineffective osmolality?

Answer 2

particles that are trapped in one compartment (cannot diffuse across the membrane) will pull water from another to dilute themselves— these are called effective osmosis

compartments with effective osmols is hypertonic, one with fewer effective osmols is hypotonic

Question 3

Give examples of ineffective and effective osmols. in cell dynamics.

Answer 3

ineffective: BUN, glucose (normally)
effective: Na

normally Na is the main determinant of ECF tonicity

Question 4

What is the range of “normal” osmolarity of the ECF?

Answer 4

280-300 mosmols/kg of water

Question 5

Where and how does ADH work?

Answer 5

acts at the distal nephron, ADH allows water reabsorption

binds to membrane receptor of principal cell in collecting duct, stimulates cAMP and insertion of water channels into the luminal membrane

Question 6

What is required for normal water excretion and dilution of urine?

Answer 6

adequate renal blood flow
reabsorption of Na and Cl in thick distal limb
dilute urine must be maintained by lack closed water channels
water excreted cannot exceed the amount that can be excreted given the osmolality of max. dilute urine

Question 7

Abnormalities in the serum sodium are almost always due to alterations in ______ ______ ____.

Answer 7

body water content: addition of water causing hyponatremia, loss of water causing hypernatremia

Question 8

Symptoms of high and low Na are primarily ______ and are due to?

Answer 8

symptoms are primarily neurologic and are due to shrinking or swelling of brain cells, related to the severity and rapidity of Na change

Question 9

What are the 3 kinds of hypnatremia?

Answer 9

isoosmolar (mostly a lab artifact due to elevated plasma proteins or lipids)
hyperosmolar
hyperoosmolar

Question 10

Describe the condition of hyperosmolar hyponatremia.

Answer 10

caused by accumulation of an effective osmol (other than Na)

most commonly in uncontrolled diabetes; excess sugar causes shift of fluid to ECF and dilutes the ECF sodium

** an individual with marked hyperglycemia and serum sodium concentration that is NOT LOW is in a state of severe water depletion (osmotic diuresis)

Question 11

What is the most important next step in a patient that has low sodium and low osmolality?

Answer 11

determine their volume status (3 types of hypoosmolar hyponatremia)

Question 12

What are causes of hypovolemic hypoosmolar hyponatremia?

Answer 12

usually not endocrine: due to loss of both water and salt, but more sodium than water (look for dry membranes, tacky and orthostatic hypotension)

renal losses in kidney disease, diuretic use, aldosterone deficiency (elevated urinary Na) or non renal GI losses, burns (urinary Na appropriately low)

Question 13

What are causes of hypervolemic hypoosmolar hyponatremia?

Answer 13

usually not endocrine: due to gain of both water and sodium but more water than sodium- usually total body volume overload but low effective intravascular volume (CHF, cirrhosis)

look for peripheral edema, increased JVP

Question 14

What are causes of euvolemic hypoosmolar hyponatremia?

Answer 14

caused by gain of water (constant input/output) without sodium- commonly due to SIADH, urine cannot be fully diluted; SIADH can be from pituitary secretion, ectopic ADH production or due to drugs (possibly hypothyroidisma, cortisol deficiency or psychogenic polydipsia)

leads to low [Na] and low serum osmolality, urine osmolality that is inappropriate (not maximally dilute)— tx with restriction of free water

Question 15

What is the cause of Beer drinker’s potomania?

Answer 15

poor diet leads to less than normal daily solute excretion which limits the amount of water that can be excreted with maximally dilute urine

Question 16

Hypernatremia is almost always due to what?

Answer 16

deficit of free water, rarely due to addition of salt w/o water

hyperosmolality triggers ADH secretion and thirst so hypernatremia is rarely seen in those who have free access to water (excludes those with altered mental status, ICU, infants)

Question 17

Describe the cause of diabetes insipidus (DI).

Answer 17

lack of ADH action either through deficiencies in production (central) or in resistance to ADH action (nephrogenic)

patients will produce large volumes of dilute urine leading to hyperosmolality— patients will often drink until osmolality normalizes if they have access

Question 18

How do you differentiate between central and nephrogenic causes of diabetes insipidus?

Answer 18

create a stimulus for ADH secretion by the “water deprivation test”

hyperosmolality should serve as a strong ADH secretion stimulus (occurs only successfully in nephrogenic), after administer ADH and look for evidence of urinary concentration (occurs successfully only in central)

Question 19

How do you treat nephrogenic and central DI?

Answer 19

central DI- give synthetic ADH

nephrogenic- partial (give high dose ADH), complete (give diuretics to induce volume depletion and induce aldosterone)

Question 20

What determines ECF volume?

Answer 20

sodium content- mechanisms to regulate ECF are actually regulating ECF volume

Question 21

How is Aldosterone release stimulated by RAAS?

Answer 21

renin is produced in JG apparatus and is produced when renal perfusion pressure is low or when sodium levels are low in the filtrate

renin convert angiotensiogen to AT1
ACE convert AT1 to AT2
AT2 stimulates aldosterone production

Question 22

How does aldosterone work to increase sodium reabsorption?

Answer 22

bound to renal cortical collecting duct causes reabsorption of Na in exchange for potassium and acid (H+) out

Question 23

What are the causes and presentation of someone with primary hyperaldosteronism?

Answer 23

state of aldosterone excess: either due to adrenal adenoma or hyperplasia (renin will be appropriately low)

presents with volume expansion, hypertension and +/-hypokalemia

Question 24

What are diagnostic tests used in primary hyperaldosteronism and possible treatments?

Answer 24

confirmed by salt-loading or admin. of normal saline to expand volume; abnormal sources will not suppress

treated surgically or medically with drugs that block mineralocorticoid receptor (spironolactone or eplerenone)

Question 25

What is the presentation of someone with hypoaldosteronism?

Answer 25

typically seen in primary adrenal insufficiency, aldosterone deficiency leads to an impairment in renal sodium reabsorption causing volume depletion and hyperkalemia

Question 26

What are treatments used in pheochromocytoma?

Answer 26

BP must be controlled to avoid hypertensive in surgery to remove tumor; alpha blockers are used first line- they block a1 mediated vasoconstriction; BB are given afterwards to reduce reflex tachycardia

(if BB are given first, they will block B2 mediated vasodilation leading to unopposed a1 vasocontriction and severe hypertension)