The human body is approximately 50-60% water (on the higher end for men, on the lower end for women). This water is distributed in two major compartments - the intracellular fluid compartment and the extracellular fluid compartment, in a ratio of roughly 2:1.
The extracellular fluid is further divided into interstitial fluid and plasma, in a ratio of about 3:1. Together, therefore, the breakdown looks something like this:
There are barriers between these compartments. Between the intracellular and extracellular compartments is the cell wall, and between the interstitial and plasma compartments lies the blood vessel wall (at its thinnest, just a single layer of endothelial cells).
Water can move freely between these compartments without difficultly, but many other substances, some of them osmotically active, can't. Therefore, the amount of water in each compartment depends on the relative number of osmolytes in each compartment - water will always flow to the compartment with a higher osmolality. The major division is between the intracellular and extracellular portions:
- The major extracellular determinants of osmolality are sodium and its accompanying anions (chloride and bicarbonate, mostly).
- The major intracellular determinants of osmolality are potassium and organic phosphates (ATP, creatinine phosphate and phospholipids).
Lastly, a word about the so-called ineffective osmoles. Osmolytes that (like water but unlike sodium) can move easily between compartments tend to increase the osmolality of both compartments without ever inducing water to shift compartments. Glucose and urea are two examples of such osmolytes. If I injected your extracellular fluid with extra urea, the osmolality of the compartment would increase temporarily, but since urea can move with ease to the intracellular compartment too, it would wouldn't have the opportunity to cause fluid shifts. It would merely increase the osmolality of both compartments simultaneously and there would be no net movement of water from any compartment. This is not the case with sodium, for example, which is effectively stuck in the extracellular compartment. Injecting your extracellular fluid with sodium would cause a rise in the osmolality of only this compartment, and fluid would flow out of the intracellular compartment until the osmolalities of both compartments were equal again (though both would be higher than before).
You can estimate the osmolality of your body's fluids by the following equation (all concentrations in mmol/L):
osmolality = 2 ⨯ [Na+] + [urea] + [glucose]
If your glucose and BUN measurements are made in mg/dL (typical in the US), then the equation becomes:
osmolality = 2 ⨯ [Na+] + [glucose]/18 + [BUN]/2.8