FEN: Osmolality

Question 1

Define osmolality

Answer 1

Osmolality is a measure of the osmoles of solute per Kg of solvent (Osm/kg)

Question 2

Define osmolarity

Answer 2

Osmolarity is a measure of osmoles per liter of solution (Osm/L)

Question 3

What is normal range for plasma osmolality?

Answer 3

275-290 mOsm/kg

Question 4

How to convert from plasma osmolality to plasma osmolarity?

Answer 4

Plasma osmolarity (mOsm/L) = plasma osmolality x 0.995 (i.e. plasma osmolarity is about 1% lower than plasma osmolality)

Question 5

List two ways normal plasma osmolality is maintained

Answer 5

1. Thirst

2. Secretion of arginine vasopressin (i.e. antidiuretic hormone [ADH[) from the posterior pituitary

Question 6

List three determinants of plasma osmolality from most to least impactful

Answer 6

1. Sodium
2. Glucose
3. BUN

Question 7

What is a common equation used to calculate plasma osmolality?

Answer 7

2*Na + Glucose/18 + BUN/2.8

Question 8

What is the physiologic consequence of an increase in plasma osmolality?

Answer 8

1. osmotic shift of fluid into the plasma

2. cellular dehydration and shrinkage

Question 9

What is the physiologic consequence of a decrease in plasma osmolality?

Answer 9

1. osmotic shift of fluid into cells
2. cellular overhydration and swelling
3. red blood cell swelling can cause cell rupture (i.e. hemolysis)
4. Brain cells can swell, causing cerebral edema and herniation. Most likely to occur with acute hyponatremia (occuring in less than 2 days)

Question 10

Describe isotonic, hypertonic and hypotonic fluids

Answer 10

1. Isotonic do not cause fluid shift
2. Hypertonic cause fluid shift from IC to the EC (i.e. IS, IV) compartment, causing dehydration and shrinkage
3. Hypotonic shift fluid from EC to the IC, causing overhydration and swelling

Question 11

Define equivalent weight

Answer 11

Equivalent weight = MW divided by valence

Question 12

What is the MW, valence and equivalent weight of Sodium?

Answer 12

1. MW: 23
2. Valence: 1
3. Equivalent weight: 23 g

Question 13

What is the MW, valence and equivalent weight of Potassium

Answer 13

1. MW: 39
2. Valence: 1
3. Equivalent weight: 39 g

Question 14

What is the MW, valence and equivalent weight of Chloride

Answer 14

1. MW: 35.5
2. Valence: 1
3. Equivalent weight: 35.5 g

Question 15

What is the MW, valence and equivalent weight of Magnesium

Answer 15

1. MW: 24
2. Valence: 2
3. Equivalent weight: 12 g

Question 16

Define an osmole

Answer 16

Osmole is number of particles in a solution assuming complete dissociation

Question 17

What is a milliequivalent?

Answer 17

1000 mEq per 1 equivalent

Question 18

What is a milliosmole?

Answer 18

1000 mOsm per 1 osmole

Question 19

Convert 23.4% NaCL to mEq/mL

Answer 19

1. 23.4 grams per 100 mL
2. MW of NaCl = 58.5
3. Equivalent = 58.5 grams

(23.4 g/100 mL) *(1 equivalent/58.5 g) * (1000 mEq/1equivalent) = 4 mEq/mL

Question 20

In general, how do you calculator the osmolarity of a fluid in mOsm/L?

Answer 20

1. Convert a substance from grams or equivalents into moles.

2. Convert from moles into osmoles assuming complete dissociation or using osmotic coefficient

Question 21

Calculate osmolarity of normal saline using the osmotic coefficient

Answer 21

1. NS = 0.9 g/100mL
2. 1 mol = 58.5 g
3. 1 mol NaCl = 2 Osm
4. 93% dissociation in fluid

(0.9 g/100 mL)(1 mol/58.5g)(2 Osm/1mol)(1000 mOsm/1 Osm)(1000mL/1L) *0.93 = 287 mOsm/L

Question 22

Calculate the osmolarity of D5W

Answer 22

1. D5W = 5 g/100 mL
2. 1 mol = 198 g
3. 1 mol = 1 Osm
4. complete dissociation

(5 g/100 mL)(1 mol/198g)(1 Osm/1mol)(1000mOsm/1Osm)(1000 mL/1L) = 252.5 mOsm/L

Question 23

Calculate the osmolarity of D5W/NS using the osmotic coefficient

Answer 23

1. Osmolarity of NS w/ coefficient = 287 mOsm/L
2. Osmolarity of D5W = 252.g mOsm/L

252.5 m/Osm/L + 287 mOsm/L = 539.5 mOsm/L

Question 24

Calculate osmolarity of normal saline + potassium chloride 20 mEq/L

Answer 24

1. 1 equivalent KCl = 74.5 g
2. 1 mol KCl = 2 Osm
3. 1 mEq = 0.0745 g; 20 mEq = 1.49 g
4. (1.49 g/L)(1 mol/74.5g)(2 Osm/1mol)(1000 mOsm/1Osm)= 40 Osm/L
5. Add osmolarity of NS + KCl = 287 mOsm/L + 40 mOsm/L = 327 mOsm/L