ILA 2: Water and Salt balance

Question 1

Total body water (TBW) is distributed into three compartments (intracellular/interstitial/intravascular). How is the TBW distributed by volume (in a healthy 70kg male)?
A.) 28L/14L/3L
B.) 42L/11L/3L
C.) 24L/14L/3L
D.) 28L/11L/3L
E.) 24L/11L/3L

Answer 1

D: 28L/11L/3L

Question 2

3.) Osmolarity can be calculated from blood results. Which of the following IS a factor in calculated osmolarity?
A.) Bicarbonate
B.) Urea
C.) Chloride
D.) Magnesium
E.) Phosphate

Answer 2

B: Urea

Why:
- Osmolarity: osmotic activity per litre of the whole situation. Expressed as mOsml/L of total solution
- Osmolality: osmotic activity per kg of solvent (water). Expressed as mOsm/kg of H20
- Calculated osmolarity:
2x Na+ = Glucose(mg/dL)/18 + BUN (mg/dl)/2.8
- Normal Plasma Osmolarity > 275-295 mOsm/kg
- BUN (Blood Urea Nitrogen) = Blood Urea (mg/dL)/2.14

Question 3

Osmolarity can be calculated from blood results. Which of the following IS a factor in calculated osmolarity?
A.) Bicarbonate
B.) Urea
C.) Chloride
D.) Magnesium
E.) Phosphate

Answer 3

B: Urea

Why:
- Osmolarity: osmotic activity per litre of the whole situation. Expressed as mOsml/L of total solution
- Osmolality: osmotic activity per kg of solvent (water). Expressed as mOsm/kg of H20
- Calculated osmolarity:
2x Na+ = Glucose(mg/dL)/18 + BUN (mg/dl)/2.8
- Normal Plasma Osmolarity > 275-295 mOsm/kg
- BUN (Blood Urea Nitrogen) = Blood Urea (mg/dL)/2.14

Question 4

Which of the following is an example of insensible fluid loss?
A.) Water from respiration
B.) Vomiting
C.) Diarrhoea
D.) Fluid in abdominal surgical drain
E.) Urine

Answer 4

A: Water from respiration

Why:

• Insensible fluid loss = loss that is not easily measured e.g. sweating/water lost from respiration/evaporation during abdominal surgery
• Sensible fluid loss = loss which is easily measured e.g. urine output, vomit, fluid in surgical drains

Question 5

You examine a patient and note he has lower leg pitting oedema. You also note that his blood albumin level is low. Which statement correctly describes the relationship between albumin and oedema?
A.) A low albumin causes an increase in oncotic pressure and water diffuses out of the blood into interstitial fluid
B.) A low albumin causes a decrease in oncotic pressure and water diffuses from the interstitial fluid into the blood
C.) A low albumin causes an increase in oncotic pressure and water diffuses from the interstitial fluid into the blood
D.) A low albumin causes a decrease in oncotic pressure and water diffuses from the blood into the interstitial fluid
E.) Low albumin is not related to oedema

Answer 5

D: A low albumin causes a decrease in oncotic pressure and water diffuses from the interstitial fluid into the blood

Why:

• A low albumin causes a DECREASE in oncotic pressure
• Means water will move FROM the blood INTO interstitial fluid