Na / Water Balance

Question 1

What percentage of water is intra / extracellular

Where is most of the Na?

Answer 1

2/3 water intracellular. 1/3 extracellular

Most Na is extracellular.

Question 2

Osmolality equation

Normal range for osmolality

Answer 2

2xNa + glucose/18 + BUN/2.8

Normal range is 280-300

Question 3

Where are osmol receptors?

Answer 3

Hypothalamus

Question 4

Where are baroreceptors?

Answer 4

Carotid sinus and aortic arch

Question 5

3 non-osmotic factors that can trigger ADH

Answer 5

Pain, nausea, meds

Question 6

Causes of hypovolemic hypoosmolar states (5)

Answer 6

• Renal losses (urine Na is inappropriately high [>20 mEq/L]) – kidney disease, diuretics, aldosterone deficiency
• Non-renal losses (urine Na is appropriately low [less than 10 mEq/L]) – GI losses, skin / burns.

Question 7

3 causes of hypervolemic hypo-osmolarity

Answer 7

CHF, cirrhosis, nephrotic syndrome

Question 8

9 causes of SIADH

Answer 8

SIADH caused by excess pituitary secretion (tumor, infection trauma), ectopic ADH production (small cell lung cancer), drugs, pulmonary infection, COPD, nausea, or severe pain.

Question 9

2 hormonal causes of euvolemic hypo-osmolarity that mimic SIADH

Answer 9

• Hypothyroidism – decreased cardiac output → decreased intravascular volume → decreased GFR → increased proximal reabsorption of water / Na → less delivery to diluting segment of nephron. Stimulation of ADH via baroreceptors also occurs.
• Cortisol deficiency – cortisol normally feeds back negatively on ADH production, so deficiency → more ADH than normal. May mimic SIADH.

Question 10

Beer drinker’s potomania

Answer 10

Poor diet → low solute intake → low solute excretion → limits how much water can be excreted w/ maximally dilute urine

Question 11

4 causes of hypernatremia

Answer 11

Osmotic diarrhea / diuresis, inadequate replacement of insensible water losses, diabetes insipidus

Question 12

3 causes of central diabetes insipidus

Answer 12

Caused by CNS abnormalities (pituitary / sellar mass, head trauma, infiltrating disease of hypothalamus)

Question 13

4 causes of nephrogenic diabetes insipidus

Answer 13

Caused by inherited / congenital receptor dysfunction, meds (lithium, demeclocycline), or hypercalcemia

Question 14

5 effects of Ang II

Answer 14

Stimulates Epi / Norepi and promotes release of ADH. Constricts efferent arteriole to maintain GFR. Increases proximal tubular Na resorption via Na/H exchange. Triggers thirst

Question 15

2 things that trigger aldosterone release

Answer 15

Ang II and hyperkalemia

Question 16

Primary hyperaldosteronism
Presentation (3)
Diagnosis (2)
Treatment (3)

Answer 16

• Presents w/ MILD volume expansion, HTN, and possibly hypokalemia
• Confirmed by salt-loading or administration of normal saline.
• Treat w/ surgery or drugs that block mineralocorticoid receptor: spironolactone, eplerenone

Question 17

Treating hypoaldosteronism

Answer 17

Fludrocortisone

Question 18

Syndrome of apparent mineralocorticoid excess
Presentation
Cause

Answer 18

• Presents similarly to primary hyperaldosteronism but renin and aldosterone are both low.
• Cortisol can activate mineralocorticoid receptor, but normally, 11b hydroxysteroid dehydrogenase type 2 in kidney metabolizes cortisol → cortisone to prevent this.
• This enzyme may be inactivated by a mutation or by licorice (glycyrrhiazic acid).
• This is why HTN is common in Cushing’s. Too much cortisol overwhelms 11b hydroxysteroid dehydrogenase

Question 19

Pheochromocytoma
Sxs (6)
Treatment (4)

Answer 19

• Sxs – HTN (due to vasoconstriction) and paroxysmal headache / palpitations / sweating / pallor / tremor.
• Treat w/ surgery, but BP must be controlled to avoid hypertensive crisis during surgery.
• Alpha blockers are first line, which block α1 mediated vasoconstriction. Subsequent vasodilatation can lead to reflex tachycardia, so volume repletion + beta blockers can be used to control the HR.
• If BB is given first, it will block β2 mediated vasodilatation → “unopposed α1” vasoconstriction → severe hypertension.

Question 20

What is considered polyuria

Diff Dx for polyuria (5)

Answer 20

• Polyuria is defined as >3L / day

* Diff Dx of polyuria: psychogenic polydipsia, diabetes insipidus, DM, hyperglycemia / calcemia (osmotic diuresis)