A 50 year-old male weighing 80 kg has an estimated intravascular space of about how many liters?

• A. 8 L
• B. 4 L
• C. 16 L
• D. 12 L

Answer: (B)

Intravascular space corresponds to plasma volume, the fluid inside the vessels. A practical bedside rule is that plasma volume in adults is about 50 mL per kilogram of body weight. For this 80 kg man, that calculation gives 80 × 0.05 L = 4 L. So the estimated intravascular space is around 4 liters. This sits a bit below total blood volume, which is roughly 70 mL/kg (about 5.5–5.6 L for this patient). The larger numbers (8, 12, 16 L) would imply a plasma or blood volume far beyond normal for an adult, so 4 L is the appropriate estimate.