MRCP Part 1 MCQ question #23

A 24 year old in-patient has chronic diarrhea as well as a large draining fistula. She complains of nausea and anorexia and of marked weakness. On physical examination her blood pressures is 120/80 mmHg. When she stands up, she feels very dizzy and her blood pressure is 110/70. Her heart rate is 100. She weighs 50 kg. When the intern pinches her forehead skin, it stands up like a tent. Her eyes are sunken. Her laboratory tests are pending. The intern suspects that she has extracellular fluid volume depletion. What volume of water would be expected in the extracellular compartment of a healthy 50 kg female?

a) 30 liters
b) 2.5 liters
c) 10 liters
d) 20 liters
e) 7.5 liters

Answer:
 c) 10 liters

Explanation
Body water distribution can be calculated using the rule of thirds. 60% of an average persons weight is water. Of this, 2/3 is intracellular and 1/3 is extracellular. Of the extracellular water, 2/3 is interstitial and 1/3 is plasma. In a 50 kg person, there will be 30 kg of water (60%). This will be approximately 30 liters. Of this 1/3 is extracellular or 10 liters.