Osmolality of plasma in a normal adult:September 2005, March 2012, September 2012
**Core Concept**
The plasma osmolality in a normal adult is a critical parameter that reflects the balance between water and solutes in the extracellular fluid. It is essential to maintain plasma osmolality within a narrow range to ensure proper cellular function and prevent osmotic imbalances.
**Why the Correct Answer is Right**
Plasma osmolality is typically measured in milliosmoles per kilogram of water (mOsm/kg). In a normal adult, plasma osmolality ranges from 280 to 300 mOsm/kg, with an average value of approximately 290 mOsm/kg. This narrow range is maintained by the balance between sodium, glucose, and urea, which are the primary osmotically active particles in plasma. Sodium is the main contributor to plasma osmolality, accounting for approximately 95% of the total osmolality.
**Why Each Wrong Option is Incorrect**
* **Option A:** This value is too high and would indicate hyperosmolality, which can lead to dehydration and increased blood viscosity.
* **Option B:** This value is too low and would indicate hyposmolality, which can lead to water retention and potentially life-threatening complications such as cerebral edema.
* **Option C:** This value is within the normal range, but it is not the average value, which is typically around 290 mOsm/kg.
* **Option D:** This value is not a valid measurement unit for osmolality.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that plasma osmolality is a critical parameter that can be affected by various conditions, including diabetes mellitus, renal failure, and dehydration. It is essential to measure plasma osmolality in patients with these conditions to prevent and diagnose osmotic imbalances.
**Correct Answer: D. 285 mOsm/kg**