What percentage of total body water is extracellular fluid?

The correct choice indicates that extracellular fluid constitutes approximately one-third of total body water. To understand this, it’s essential to grasp the distribution of fluids within the human body. Body fluids are typically divided into two main compartments: intracellular fluid (ICF) and extracellular fluid (ECF).

In adults, total body water makes up about 60% of body weight, and extracellular fluid, which includes interstitial fluid and blood plasma, generally represents about 20% of body weight. Since total body water is approximately 60%, this means that the extracellular fluid makes up around one-third of that total, specifically calculated as 20% of body weight in relation to the 60% total body water.

So, referring to the percentage as one-third of total body water accurately reflects the typical distribution of fluid compartments in the body, thereby making it the correct answer.