Question at position 16 The primary cation in extracellular fluid is:PotassiumSodiumMagnesiumCalcium单项选择题
A
Potassium
B
Sodium
C
Magnesium
D
Calcium

Question

Question at position 16 The primary cation in extracellular fluid is:PotassiumSodiumMagnesiumCalcium

BlackTom AI · Accepted Answer

Reframing the question to compare common extracellular and intracellular ions helps set the stage for evaluating each option.
Option 1: Potassium. This ion is mainly the principal intracellular cation, not extracellular, so saying it is the primary extracellular cation is inaccurate. The extracellular space has far lower potassium concentration than intracellular, which is why shifts in K+ are so clinically significant.
Option 2: Sodium. This is the correct choice. In extracellular fluid, the dominant cation is sodium (Na+). It contributes most of the positive charge in the ECF and is central to osmolality, fluid balance, and nerve/muscle function. Its high concentration in ECF contrasts with its much lower intracellular level.
Option 3: Magnesium. Magnesium is present in the extracellular space, but it is not the major cation there; it exists in smaller concentrations compared with sodium and potassium. It plays roles in enzyme activity and neuromuscular function, but not as the principal extracellular cation.
Option 4: Calcium. Calcium ions are important as extracellular signaling molecules and for bone health, but they are not the major extracellular cation. The extracellular calcium concentration is relatively high compared to intracellular, yet far less than sodium in terms of overall cation abundance in the ECF, so this choice is incorrect as the primary extracellular cation.
In summary, understanding the distribution of major ions across compartments clarifies why sodium sits at the top of the extracellular cation list while potassium is predominantly intracellular, and magnesium and calcium serve other critical but non-primary roles in the extracellular milieu.

Question at position 16 The primary cation in extracellular fluid is:PotassiumSodiumMagnesiumCalcium单项选择题

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