M04.03.018 Blood Volume versus Plasma Volume

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Learning Objective: By the end of this section, the learner should be able to distinguish between plasma volume and blood volume, calculate blood volume using hematocrit, and understand the distribution of various intravenous fluids in body compartments.

Blood Volume versus Plasma Volume

Blood volume (BV) represents the plasma volume (PV) plus the volume of red blood cells (RBCs). RBCs are usually expressed as hematocrit (Hct), the fractional concentration of RBCs. The formula to convert plasma volume to blood volume is:

$BV = \frac{PV}{1 - Hct}$

Example 1

Plasma volume (PV) = 3 L
Hematocrit (Hct) = 50% (0.50)

$BV = \frac{3 \, \text{L}}{1 - 0.50} = \frac{3 \, \text{L}}{0.5} = 6 \, \text{L}$

Explanation: If hematocrit is 0.5, half of the blood is RBCs and half is plasma, so blood volume is double the plasma volume.

Example 2

Blood volume by body weight:

Approximate BV = 7% of body weight in kg
For a 70 kg individual:

$BV \approx 0.07 \times 70 \, \text{kg} = 4.9 \, \text{L} \approx 5 \, \text{L}$

Distribution of Intravenous Fluids

Compartment	Example Fluids
Vascular compartment	Whole blood, plasma, dextran in saline
Extracellular fluid	Saline, mannitol
Total body water	D5W (5% dextrose in water – water distributes 2/3 ICF, 1/3 ECF after glucose metabolized)