Osmolarity of plasma is
**Core Concept**
The osmolarity of plasma is a measure of the concentration of osmotically active particles, such as ions and small molecules, in the blood. This concept is crucial in understanding the balance of fluids and electrolytes in the body.
**Why the Correct Answer is Right**
The osmolarity of plasma is approximately 285-295 mOsm/L, which is slightly higher than the osmolarity of the cerebrospinal fluid (CSF). This is because the blood-brain barrier allows certain molecules to pass through, but restricts others, maintaining a delicate balance between the two fluids. The primary contributors to plasma osmolarity are sodium (Na+) and glucose, which are the most abundant osmotically active particles in the blood.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is too low. The osmolarity of plasma is not significantly lower than that of CSF.
* **Option B:** This option is incorrect because it is too high. The osmolarity of plasma is not significantly higher than that of CSF.
* **Option C:** This option is incorrect because it is not specific enough. While it is true that the osmolarity of plasma can vary depending on the concentration of certain molecules, the correct answer is a specific and well-established value.
**Clinical Pearl / High-Yield Fact**
In clinical practice, it is essential to remember that changes in plasma osmolarity can have significant effects on the body, including alterations in blood pressure, fluid balance, and even neurological function. Monitoring plasma osmolarity is crucial in patients with conditions such as diabetes insipidus or hyponatremia.
**Correct Answer:** C. 285-295 mOsm/L