- Like all other molecules, there is a thermodynamic drive for water molecules to achieve an equivalent concentration across a permeable membrane. However, because our formal concept of "concentration" does not apply to water, the concept of "Osmolarity" has been developed to describe the thermodynamic tendency of water molecules to achieve equivalence across a permeable membrane.
- The first challenge in understanding osmolarity is appreciating how differences in water "concentration" can be achieved across a water-permeable membrane. Such differences can only occur if the membrane is semi-permeable, meaning that it is permeable to water but not to some type of solute dissolved within the water. Importantly, solute molecules, regardless of their chemical nature, take up space; consequently, any given unit of fluid volume is composed of the summed volumes of the water and solute molecules. If the concentration of the solute is very high, in consequence the amount of water molecules per unit volume is lower, meaning that the water "concentration" is low. However, if the concentration of solute is very low, in consequence the amount of water molecules per unit volume is higher, meaning that the water "concentration" is high. Given these concepts, it is clear that a semi-permeable membrane separating a fluid with high solute concentration from a fluid with low solute concentration will in fact be separating two fluids with different water "concentrations".
- Because most ionic solute particles take up the same volume of space, their chemical identity is not important in describing how they change the water "concentration" when dissolved in a fluid. Consequently, the concept of "Osmolarity" was developed to describe the total concentration of solute particles, regardless of their chemical identity, within a fluid. Formally, the Osmolarity of a fluid is the number of moles of solute per liter of fluid. Consequently, for a mole of NaCl dissolved in a liter of fluid, the fluid will be considered to have an osmolarity of 2 Osmolar (Osm) since NaCl dissolves into separate Na+ and Cl- ions.
- The thermodynamic tendency of water to passively move across a semi-permeable membrane separating two fluids with different osmolarity is referred to as "Osmosis". In such a scenario, water will move across the membrane until the osmolarity of the relative fluid compartments becomes equilibrated. For example, if 1L of 2 Osm fluid is separated from 1L of 1 Osm fluid, then water will move from the 1 Osm fluid into the 2 Osm fluid, diluting the latter and concentrating the former until both fluids possess precisely the same osmolarity, although their relatives volumes will change in the process.
- Although water will passively move from a compartment of low osmolarity to a compartment of high osmolarity, this movement can be opposed by placing a pressure differential on the two fluids. The amount of pressure differential required to completely oppose the passive movement of water between two compartments of different osmolarity is referred to as the "Osmotic Pressure". As in the example above, water will naturally move from a compartment of 1 Osm fluid to a compartment of 2 Osm fluid. However, if a plunger is placed above the 2 Osm fluid and pressure is applied, then the passive osmosis of water can be prevented. As mentioned, the precise amount of pressure required to prevent the osmotic movement of water will be equivalent to the "Osmotic Pressure" between these two compartments.
- Oncotic Pressure refers to the osmotic pressure exerted by proteinacious solutes. As explained in Microcirculatory Physiology, the vascular wall prevents movement of proteinacious solutes within the vasculature from moving into the extracellular fluid. Because proteinacious solutes are selectively withheld within the plasma, they exert an osmotic pressure which is referred to as "Oncotic Pressure". As described in Starling Forces, this oncotic pressure is a key feature of preventing excessive egress of fluid from the vasculature into the interstitial space.