Major ions in ECF are:
**Core Concept**
The major ions in the extracellular fluid (ECF) play a crucial role in maintaining proper bodily functions, including nerve and muscle excitability, fluid balance, and acid-base homeostasis. The major ions in ECF include sodium (Na+), chloride (Cl-), calcium (Ca2+), magnesium (Mg2+), and potassium (K+).
**Why the Correct Answer is Right**
The correct answer is sodium (Na+), as it is the most abundant ion in ECF, making up approximately 90% of the total cations. Sodium is essential for maintaining fluid balance, nerve and muscle excitability, and blood pressure regulation. The sodium-potassium pump (Na+/K+-ATPase) helps to maintain the concentration gradient of sodium across cell membranes, which is crucial for proper cellular function.
**Why Each Wrong Option is Incorrect**
* **Option A:** While chloride (Cl-) is an important ion in ECF, it is not the most abundant ion. Chloride helps to maintain fluid balance and is a major contributor to the anion gap.
* **Option B:** Calcium (Ca2+) is an essential ion, but it is not the most abundant ion in ECF. Calcium plays a critical role in muscle contraction, nerve transmission, and bone mineralization.
* **Option D:** Potassium (K+) is an important ion, but it is primarily found inside cells. Potassium helps to regulate heart rhythm, muscle contraction, and nerve excitability.
**Clinical Pearl / High-Yield Fact**
The sodium-potassium pump is an example of a cotransport mechanism, where the transport of one ion (sodium) is coupled with the transport of another ion (potassium) against their concentration gradients.
**Correct Answer: A. Sodium (Na+)**