S1B4 - Body Fluid Compartments

Question 1

What is the 60-40-20 rule of body fluid compartments?

Answer 1

The 60-40-20 rule of body fluid compartments:

• 60% of body weight is water
• 40% of body weight is intracellular fluid (ICF)
• 20% of body weight is extracellular fluid (ECF)

Question 2

What substance is used to measure plasma volume?

Answer 2

Plasma volume is measured by radiolabeled albumin.

Question 3

What is the first line treatment for patients with a hypoosmotic volume expansion due to syndrome of inappropriate ADH secretion (SIADH)?

Answer 3

The first line treatment for patients with a hypoosmotic volume expansion due to SIADH is fluid restriction.

Note: Patients with SIADH can also present with euvolemic hyponatremia. Total body water is increased, but near-normal blood volume is maintained due to the body’s compensatory response (decreased thirst, suppressed aldosterone release). In this presentation, peripheral edema does not develop.

Question 4

The extracellular fluid (ECF) is divided into what 2 volume compartments?

Answer 4

The ECF is further divided into two compartments: the interstitial fluid and the plasma.

Question 5

Interstitial fluid is normally what percentage of total body weight?

Answer 5

Interstitial fluid is normally 15% of total body weight.

Question 6

Patients given an infusion of isotonic saline experience what type of volume expansion?

Answer 6

Patients given an infusion of isotonic saline is an example of an isosmotic volume expansion.

Question 7

What substance is used to measure extracellular fluid (ECF) volume?

Answer 7

ECF volume is measured by inulin.

Question 8

Patients with adrenocortical insufficiency (e.g., Addison’s disease) experience what type of volume contraction?

Answer 8

Patients with adrenocortical insufficiency (e.g., Addison’s disease) experience a hyposmotic volume contraction.

Question 9

What is the range of values for normal blood osmolality?

Answer 9

Normal blood osmolality is 285 - 295 mOsm/kg H2O.

270-290 mOsm in handout

Question 10

What type of fluids are given to patients with a hypoosmotic volume expansion? Why is the hypoosmotic volume expansion in these patients corrected slowly?

Answer 10

Patients with hypoosmotic volume expansion that require fluids should be given IV hypertonic saline. It is important to note that the patient’s osmolarity should be corrected slowly to prevent osmotic demyelination syndrome (formerly known as central pontine myelinolysis).

Question 11

What are the major cations within the intracellular fluid (ICF)?

Answer 11

The major cations within the ICF are potassium and magnesium.

Question 12

Plasma volume is normally what percentage of total body weight?

Answer 12

Plasma volume is normally 5% of total body weight.

Question 13

What type of fluid is given to a patient experiencing volume contraction due to severe diarrhea?

Answer 13

Patients experiencing a significant isosmotic volume contraction due to diarrhea are treated with isotonic saline.

Question 14

Patients with diarrhea experience what type of volume contraction?

Answer 14

Patients with diarrhea experience an isosmotic volume contraction since the osmolarity of fluid from the GI tract is approximately equal to that of the ECF.

Question 15

Excessive consumption of salty foods results in what type of volume expansion?

Answer 15

Excessive dry sodium intake (e.g. salty foods) is an example of a hyperosmotic volume expansion.

Question 16

What is the major cation within the extracellular fluid (ECF)? What are the 2 major anions within the ECF?

Answer 16

The major cation within ECF is sodium; the major anions are chloride and bicarbonate.