06/20/2025
Water, Water—Everywhere
Most water in the body resides in two types of compartments: intracellular (within the cells) and extracellular (outside the cells). The two primary extracellular compartments are the intravascular compartment, which contains plasma (the fluid component of blood), and the interstitial compartment, which contains any fluid not located in the body’s cells or plasma. Intracellular fluid (ICF) refers to water inside cells, and extracellular fluid (ECF) refers to water outside of cells (in the interstitium or plasma).
Because cell membranes are permeable to fluid via aquaporins (specialized water channels), fluid moves freely between the three compartments (intracellular, intravascular and interstitial). One cause of this is osmosis: In osmosis, water moves from areas of high fluid concentration to areas of low concentration in an attempt to balance the levels on both sides of the cell membrane. This movement is driven, in part, by the quantity of solutes (substances dissolved in the fluid) in each compartment. Solutes cannot move through cell membranes, but fluid can. During osmosis, water moves from areas of lower solute concentration to areas of greater concentration, shifting the amount of water on each side of the membrane. An area with a higher solute concentration cannot help but pull water into it, even if this creates other problems.
When equilibrated, the three compartments—think of them as buckets—hold the appropriate amounts of fluid. However, when one bucket experiences a loss of water volume or an increase in solute concentration, water from another bucket is more likely to pour into balance things out. This difference between solute concentrations on the two sides of a semipermeable membrane is called an osmotic gradient and it drives water flow between compartments.
Water moving into, or out of, the ICF may cause cells to shrink or expand. A little change in size is a small problem, but large shifts can trigger undesirable signaling cascades affecting metabolism, transport, hormone release, cell proliferation and programmed cell death (Guelinckx et al., 2016; Lang, 2007; Lang et al., 2017; Nishiyama & Kobori, 2018). Cells get ticked when they shrink or swell. Shrinkage of cells in the ICF is the consequence of chronic hypohydration, and you will soon see why it has been accused of health crimes.
While the rules of osmosis may seem cut and dried (fluid shifts until balance is achieved), the body is more complex than that: Certain parts of the body do a more important job than others, so they take priority when it comes to allocation of resources, including water.
Case in point: Plasma accounts for only 7% of TBW, while most of the body’s water—about 60%–70%—is found in intracellular fluid. However, adequate blood volume is critical to maintaining whole-body homeostasis. Plasma is, after all, the body’s crucial transporter of nutrients, waste, oxygen, and carbon dioxide. Viscous blood doesn’t flow as nicely and tends to clump. Lower blood volume (and thicker blood) means each organ system (heart, lungs, kidneys, liver, etc.) has to make do with less, making its job more difficult. Thus, the body prioritizes the intravascular compartment (containing plasma) at the expense of other fluid compartments.
One demonstration of this prioritization is that blood osmolality the balance of water to dissolved substances—remains remarkably consistent in people with widely different levels of habitual water intake. Thus, the intravascular compartment’s volume is maintained, but if enough fluid for this purpose is not provided by an external source (i.e., food or drink), the water has to come from somewhere within the body. This need can arise, for example, when “ad libitum” intake (fluid intake based on sensations of thirst or desire for liquid) is subject to “unconscious, involuntary dehydration,” where the individual drinks to satiety but does not overcome a water deficit (Stookey, Hamer, & Killilea, 2017).
Source: AFAA
