In the body, water moves only from plasma to intracellular fluid, not in the reverse direction.

True: Sodium is the primary extracellular cation, while chloride is the major extracellular anion, maintaining osmotic balance outside the cell. Active transport by the sodium-potassium pump ensures these ions remain at low concentrations within the cytoplasm. This distribution is critical for cellular hydration and electrical signaling.

A. In the digestive tract: While potassium is ingested through the diet, it is rapidly absorbed into the bloodstream and then sequestered into cells. The lumen of the gut does not serve as a primary storage site. Most potassium is located within the soft tissues.

B. In the intracellular fluid (ICF): Approximately 98% of total body potassium is sequestered within the cells, primarily in skeletal muscle. This high internal concentration is maintained by the active transport of the Na+/K+-ATPase pump. It is essential for maintaining the resting membrane potential.

C. In the extracellular fluid (ECF): Plasma potassium represents only a tiny fraction of the total body store, usually measured between 3.5 and 5.0 mmol/L. Even minor shifts in this ECF concentration can have lethal effects on cardiac rhythm. The ECF is not a reservoir.

D. In the renal filtrate: The kidneys filter potassium, but the vast majority is reabsorbed in the proximal tubule and Loop of Henle. The amount present in the filtrate at any given time is minimal. The renal system manages the narrow margin of potassium excretion.