Which statement describes extracellular fluid (ECF)?

Which statement describes extracellular fluid (ECF)?

• A. Fluid within cells, comprising two-thirds of total body fluid, rich in potassium.
• B. Fluid outside cells, comprising one-third of total body fluid, rich in sodium.
• C. Fluid in bone marrow.
• D. Fluid in the intracellular spaces.

Answer: (B)

Understanding how body fluids are distributed helps you see why this describes extracellular fluid. Extracellular fluid is the fluid outside cells and makes up about one-third of total body water. It is rich in sodium (and its accompanying chloride and bicarbonate ions), which supports the function of nerves, muscles, and circulating blood plasma. So the statement that best describes extracellular fluid is the one describing fluid outside cells, about one-third of total body fluid, rich in sodium. Fluid within cells, with high potassium, is intracellular fluid. Fluid in bone marrow isn’t a defined extracellular compartment, and fluid in intracellular spaces is, by definition, intracellular fluid.

Understanding how body fluids are distributed helps you see why this describes extracellular fluid. Extracellular fluid is the fluid outside cells and makes up about one-third of total body water. It is rich in sodium (and its accompanying chloride and bicarbonate ions), which supports the function of nerves, muscles, and circulating blood plasma.

So the statement that best describes extracellular fluid is the one describing fluid outside cells, about one-third of total body fluid, rich in sodium. Fluid within cells, with high potassium, is intracellular fluid. Fluid in bone marrow isn’t a defined extracellular compartment, and fluid in intracellular spaces is, by definition, intracellular fluid.