ATI Pathophysiology Practice Exam

Which of the following ions is most abundant in extracellular fluid?

1. Potassium

2. Calcium

3. Sodium

4. Chloride

The correct answer is: Sodium

Sodium is the most abundant ion in extracellular fluid because it plays a crucial role in maintaining the overall osmotic balance and fluid distribution in the body. In extracellular fluid, sodium is primarily found in the interstitial fluid and the plasma. Its concentration is significantly higher compared to other ions, which is essential for various physiological processes, including the regulation of blood volume, blood pressure, and the transmission of electrical impulses in nerve and muscle cells. Sodium's prominence in extracellular fluid is also tied to its function in the sodium-potassium pump, a vital mechanism that maintains cellular homeostasis by regulating the concentration of sodium and potassium ions across the cell membrane. This balance is essential for normal cellular function, nerve impulse conduction, and muscle contraction. While potassium, calcium, and chloride are also important ions within the body, they are not as abundant in the extracellular fluid as sodium. Potassium, for instance, is primarily found in the intracellular fluid, contributing to the resting membrane potential of cells. Calcium plays significant roles in muscle contraction and neurotransmitter release but exists in lower concentrations in extracellular fluid compared to sodium. Chloride, while present in extracellular fluid, primarily serves as an anion that balances the positive charge of sodium and is less abundant overall.