What is meant by volume?
Physicians use the terms “volume” and “extracellular fluid volume” interchangeably. Because sodium is largely restricted to the extracellular fluid (ECF), total body sodium determines the ECF volume.
Therefore changes in total body sodium lead to changes in ECF volume. A typical Westerner consumes about 150 mmol of sodium chloride per day. Let’s consider the hypothetical case of adding 150 mmol of sodium chloride to the ECF of a normal human. The resultant rise in the plasma sodium concentration and plasma osmolality will stimulate thirst and antidiuretic hormone (ADH) secretion. Ingestion of water and reabsorption of water by the collecting ducts will restore the plasma sodium concentration and plasma osmolality to normal. The end result is that 1 L of an isosmotic solution has been added to the ECF compartment; an increase in total body sodium has led to an increase in ECF volume.
