Sketchy Path: Renal - "Osmolality & Sodium Disorders"

Question 1

Most of the water in the body is in which compartment, the intracellular or extracellular?

Answer 1

Intracellular

Think of the larger part of the window frame –about 2/3 of the window –representing the intracellular compartment.

Question 2

Of the extracellular fluid, which compartment has the majority of fluid?

Answer 2

The interstitial space

The intravascular portion accounts for only about 1/4 of the extracellular fluid (which is why only one of the four window panes representing the extracellular space is red).

Question 3

The main intracellular cation is ____________.

Answer 3

potassium (like the bananas in the left portion of the pane)

Balanced by proteins and phosphate.

Question 4

The main extracellular cation is ______________.

Answer 4

sodium (like the kid eating peanuts in the right portion of the pane)

Question 5

Why is extracellular osmolarity an acceptable measure of total body osmolarity?

Answer 5

All membranes in the body are permeable to water, so the blood osmolarity will equal the intracellular and interstitial osmolarities.

Question 6

Give the formula for blood osmolarity.

Answer 6

(2 •Na) + (glucose/18) + (BUN/2.8) = ~ 290 mOsm normally

Question 7

What is the driving force in body osmolarity?

Answer 7

2 x Na (like the condiments box with “2x the peaNAts”)

Think about it: glucose/18 should be ~ 5 and BUN/2.8 is usually 3-7, so sodium accounts for above 95% of the serum osmolarity.

Question 8

_____________ causes a hypovolemic, normonatremic state.

Answer 8

Secretory diarrhea

Secretory diarrhea leads to isotonic loss of ions normally in the serum, so blood osmolarity remains the same.

(Think of the guy SECRetly dumping a poop bucket out the left window –in the hypovolemic column –in the central row –the normonatremic row.)

Question 9

List ways to tell if someone is volume-depleted.

Answer 9

• Hypotension
• Tachycardia
• Dry mucous membranes
• Skin tenting
• Decreased urine output

Question 10

What causes hypervolemic, normonatremic states?

Answer 10

Excessive administration of isotonic IV fluids

(Think of the lady in the middle row –the normonatremic row –in the right column –the hypervolemic column –who is next to the ISO IVy plant. Also, the IVy plant is on an extension of the volume axis.)

Question 11

By what mechanism does hydrochlorothiazide lead to hyponatremia?

Answer 11

It causes volume depletion that leads to excess release of ADH (like the thigh guy lowering the sodium screen with his ADH water tank in the background).

Question 12

Review the causes of hypovolemic, hyponatremia.

Answer 12

Think of the guy in the lower (hyponatremic) row in the left column (hypovolemic):
•He’s got a beanie (Addison’s)
• He has amazing thighs (HTCZ)

Question 13

SIADH expands the volume of which compartments?

Answer 13

Both the extracellular and intracellular (like the drying towels that expand beyond the left and right sides of the window pane)

Question 14

What syndrome can mimic SIADH?

Answer 14

Excess water intake –both are hyponatremic, euvolemic disorders

The kidneys respond to the increased water by disposing of water and sodium, thus maintaining water balance but lowering the sodium levels.

Question 15

______________ causes hypervolemic, hyponatremia (three possible answers).

Answer 15

CHF, nephrotic syndrome, and cirrhosis

The loss of intravascular volume – in each of these disorders – leads to an excess release of ADH.

(Think of the sunken heart ship in the lower – hyponatremic –right –hypervolemic pane: there is an ADH tank in the window.)

Question 16

What can cause hypovolemic, hypernatremia?

Answer 16

• Vomiting, because the loss of water exceeds the loss of sodium
• Hyperglycemia, because sugar pulls water out, not sodium)
• Osmotic diarrhea
• Decreased water intake
• Sweating

(Think of the guy in the upper –hypernatremic –left –hypovolemic pane who is vomiting his Capri Sun and throwing his chocolate bowl with candy in it. Also, there is a cactus in the desert bowl behind him, symbolizing decreased water intake)

Question 17

Why does osmotic diarrhea lead to hypernatremia and secretory diarrhea leads to normonatremia?

Answer 17

Osmotic diarrhea pulls water out without pulling sodium out, so the sodium is concentrated.

Secretory diarrhea is caused by isotonic pumping of water and sodium out of the gut.

Question 18

What two things can cause euvolemic hypernatremia?

Answer 18

Diabetes insipidus and hyperaldosteronism

(Think of the guy in the upper –hypernatremic –center –euvolemic –pane who is bathing in the cherub symbol of DI wearing the flaming adrenal gland beanie.)

Question 19

_______________ is the only cause of hypernatremic hypervolemia.

Answer 19

Administration of hypertonic saline

Think of the birds eating the salty peanuts on the trellis by the window in the upper right pane.

Question 20

Why does sodium drop in severe hyperglycemia?

Answer 20

The glucose in the serum pulls water from the intracellular space to the extracellular space.