Which electrolyte is the chief plasma cation whose main function is maintaining osmotic pressure?

Which electrolyte is the chief plasma cation whose main function is maintaining osmotic pressure?

• A. Chloride
• B. Calcium
• C. Potassium
• D. Sodium

Answer: (D)

The main factor that sets plasma osmotic pressure is the extracellular cation sodium. Sodium is the predominant positively charged ion outside cells, and its salts (such as NaCl and NaHCO3) largely determine plasma osmolality. Osmotic pressure reflects the number of solute particles that cannot freely cross cell membranes; in blood, sodium salts contribute the majority of these particles, helping to retain water in the extracellular space and regulate fluid balance and volume. Potassium, by contrast, is the major intracellular cation and is more involved in maintaining resting membrane potential and intracellular osmolarity. Calcium has important roles in signaling, muscle contraction, and coagulation, but it is not the primary determinant of plasma osmotic pressure. Chloride is an extracellular anion that accompanies sodium to maintain electroneutrality and acid-base balance, but it is not a cation and does not define osmotic pressure on its own. So the electrolyte best described as the chief plasma cation responsible for maintaining osmotic pressure is sodium.

The main factor that sets plasma osmotic pressure is the extracellular cation sodium. Sodium is the predominant positively charged ion outside cells, and its salts (such as NaCl and NaHCO3) largely determine plasma osmolality. Osmotic pressure reflects the number of solute particles that cannot freely cross cell membranes; in blood, sodium salts contribute the majority of these particles, helping to retain water in the extracellular space and regulate fluid balance and volume.

Potassium, by contrast, is the major intracellular cation and is more involved in maintaining resting membrane potential and intracellular osmolarity. Calcium has important roles in signaling, muscle contraction, and coagulation, but it is not the primary determinant of plasma osmotic pressure. Chloride is an extracellular anion that accompanies sodium to maintain electroneutrality and acid-base balance, but it is not a cation and does not define osmotic pressure on its own.

So the electrolyte best described as the chief plasma cation responsible for maintaining osmotic pressure is sodium.