The normal plasma osmolality (mOsmol/kg) is
**Core Concept**
Normal plasma osmolality is the balance between the concentration of solutes and water in the plasma, which is crucial for maintaining proper cellular function. The normal osmolality range is tightly regulated by the hypothalamic-pituitary-adrenal axis and is essential for preventing cellular swelling or shrinkage.
**Why the Correct Answer is Right**
The normal plasma osmolality is approximately 285-295 mOsmol/kg, which is maintained by the balance between sodium, glucose, and urea. The kidneys play a crucial role in regulating osmolality by adjusting the amount of water reabsorbed or excreted based on the body's needs. The hypothalamic-pituitary-adrenal axis also regulates osmolality through the release of antidiuretic hormone (ADH) and aldosterone, which affect water reabsorption and sodium retention, respectively.
**Why Each Wrong Option is Incorrect**
* **Option A:** This value is too low and may indicate hyponatremia or a condition of excessive water intake.
* **Option C:** This value is too high and may indicate hypernatremia or a condition of excessive sodium intake.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that a plasma osmolality of less than 280 mOsmol/kg is generally considered to be hyponatremic, while a value greater than 300 mOsmol/kg is generally considered to be hypernatremic.
**Correct Answer: B. 295 mOsmol/kg**