Sodium balance is important for which of the following functions?
A. Transmitting impulses in nerve and muscle fibers via the calcium-potassium pump.
B. Exchanging for magnesium and attracting chloride.
C. Combining hydrogen and chloride for acid-base balance.
D. Exchanging for potassium and attracting chloride.
Correct Answer: D. Exchanging for potassium and attracting chloride.
Sodium influences the levels of potassium and chloride by exchanging for potassium and attracting chloride. Sodium, which is an osmotically active anion, is one of the most important electrolytes in the extracellular fluid. It is responsible for maintaining the extracellular fluid volume, and also for regulation of the membrane potential of cells. Sodium is exchanged along with potassium across cell membranes as part of active transport.
Option A: The sodium-potassium pump acts to transport sodium and potassium ions across the cell membrane in a ratio of 3 sodium ions out for every 2 potassium ions brought in. In the process, the pump helps to stabilize membrane potential, and thus is essential in creating the conditions necessary for the firing of action potentials.
Option B: Magnesium is an intracellular cation. Magnesium is mainly involved in ATP metabolism, contraction and relaxation of muscles, proper neurological functioning, and neurotransmitter release. Most of the chloride, which is filtered by the glomerulus, is reabsorbed by both proximal and distal tubules (majorly by proximal tubule) by both active and passive transport.
Option C: The acid-base status of the blood drives bicarbonate levels. The kidneys predominantly regulate bicarbonate concentration and are responsible for maintaining the acid-base balance. Kidneys reabsorb the filtered bicarbonate and also generate new bicarbonate by net acid excretion, which occurs by excretion of both titratable acid and ammonia.
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