Sun28November0628PM 47

In a normal adult woman weighing 75 kg which of the following are true and which false regarding the extracellular fluid (ECF)?

True / False

Forms a greater proportion of the total body weight in an obese than in a lean woman Correct

Contains no protein Correct

Is isotonic throughout the body Correct

Has a sodium concentration of 135-145 mmol/L Correct

Has a total volume of 12-15 litres Incorrect answer selected

Explanation

Total body water is approximately 37.5 litres (0.5 × 75), of which 1/3 is ECF (13 L) and 2/3 (22 L) intracellular fluid.

In the obese, ECF is relatively contracted.

Normal sodium concentration is approximately 135-145 mmol/L.

ECF is composed of intravascular fluid and extravascular fluid. Both contain plasma proteins.

The ECF compartment consists of:

Interstitial fluid (ISF) has the compositional characteristics of ECF (as mentioned above) but in addition it is distinguished by its usually low protein concentration (in comparison to plasma)

Plasma

Transcellular fluids (CSF, gastric juice, urine, aqueous humor)

These compartments are distinguished by different locations and different kinetic characteristics. The ECF compositional similarity is in some ways, the opposite of that for the ICF (ie low in potassium & magnesium and high in sodium and chloride). Transcellular fluids are much less likely to be isotonic.

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Times answered: 241