Where is most of the body's potassium found?

A. It is metabolized to produce ammonium and new bicarbonate ions: In the proximal tubule cells, glutamine deamination yields ammonium ions and alpha-ketoglutarate, which is further metabolized to bicarbonate. The ammonium is excreted to safely eliminate protons, while the "new" bicarbonate is reabsorbed. This renal response provides a long-term correction for systemic acidosis.

B. It is converted to glucose for energy: While the kidneys can perform gluconeogenesis during prolonged fasting, the specific metabolic pathway for glutamine during acidosis is prioritized for pH regulation. The carbon skeleton is used to generate bicarbonate rather than just providing cellular fuel. It serves a primary homeostatic function in acidemia.

C. It is excreted unchanged: Glutamine is a valuable nitrogen source and is generally reabsorbed by the renal tubules rather than wasted in the urine. During acid-base disturbances, its active metabolism is required to generate the buffer needed to neutralize excess acidity. Passive excretion would fail to support pH recovery.

D. It is used to produce sodium and chloride: Sodium and chloride are inorganic electrolytes that cannot be synthesized from organic amino acids like glutamine. While the metabolism of glutamine may involve sodium-coupled transporters, the amino acid itself is not a precursor for these ions. Its primary products are ammonium and bicarbonate.

A. 300 mL: This value roughly corresponds to the amount of water lost daily through metabolic processes or expiration alone. It is far below the total daily output required to eliminate nitrogenous wastes via the kidneys. Such low output would indicate severe oliguria.

B. 2200 mL: While 2200 mL is close to the average intake, the total daily loss usually reaches a higher threshold when factoring in insensible losses. It underestimates the combined volume of urine, sweat, feces, and respiratory evaporation. 2500 mL is the standard physiological estimate.

C. 2500 mL: An average adult loses approximately 1500 mL through urine, 600 mL via skin and lungs, and 400 mL through other secretions. This total must be balanced by oral intake and metabolic water production. It represents the standard daily turnover for euvolemic homeostasis.

D. 1800 mL: This volume is often cited as a minimum requirement for fluid intake but does not account for the full spectrum of physiological output. An output of only 1800 mL would not balance the typical 2500 mL intake. It fails to include all insensible loss pathways.