Final answer:
Adding isotonic saline (0.9%) to the body maintains the volume balance between the intracellular fluid (ICF) and the extracellular fluid (ECF) without causing a net movement of water, due to the osmolarity of the saline matching that of the bodily fluids.
Step-by-step explanation:
Adding isotonic saline (0.9%) to the body will not change the volume of the intracellular fluid (ICF) or the extracellular fluid (ECF) because the osmolarity remains balanced. Isotonic saline has the same osmolarity as body fluids, which leads to no net movement of water into or out of cells.
The ICF is the fluid within cells and makes up about 60 percent of the human body's total water content. The stability of ICF is important for cellular functions, and imbalances can result in cellular damage. On the other hand, the ECF includes the interstitial fluid (IF) and plasma, which contains a different composition of ions compared to the ICF. The osmolarity and composition of ECF are important in maintaining the balance of fluids in and out of cells.
When isotonic saline is introduced into the ECF, it matches the concentration of solutes in the ICF, maintaining fluid equilibrium without causing cells to swell or shrink. As a result, isotonic saline is often used in medical practices for hydration and restoring fluid balance without disrupting cellular homeostasis.