Final answer:
The principal solute used to regulate cell volume in mammals is sodium (Na+) (option A) , which is the major cation of the extracellular fluid and is crucial for creating the osmotic pressure gradients that are essential for various physiological processes.
Step-by-step explanation:
The principal solute used to regulate cell-volume in mammals is sodium (Na+). Sodium is the major extracellular cation that contributes to osmotic pressure gradients, affecting not only the cell volume but also impacting blood pressure. These pressure gradients between the interior of cells and their external environment are essential for various physiological processes.
Potassium (K+), on the other hand, is the primary intracellular cation and is crucial for establishing resting membrane potential and action potentials in neurons and muscle fibers. While it does play a role in cellular functions, potassium has a lesser effect on osmotic pressure compared to sodium. The sodium-potassium pump, which actively transports sodium out of the cell and potassium into the cell using ATP, helps maintain the high levels of potassium and low levels of sodium inside the cell, which is key for cell function and volume regulation.