Some definitions are necessary first to help us in our discussion of fluid and electrolyte conditions.
Osmolality is a measure of the number of particles present in solution and is independent of the size or weight of the particles. It can be measured only by use of a property of the solution that is dependent solely on the particle concentration. These properties are collectively referred to as the colligative properties.
The osmotic pressure is the hydrostatic (or hydraulic) pressure required to oppose the movement of water through a semipermeable membrane in response to an 'osmotic gradient' (ie differing particle concentrations on the two sides of the membrane).
Serum osmolality can be measured by use of an osmometer or it can be calculated as the sum of the concentrations of the solutes present in the solution. The value measured in the laboratory is usually referred to as the osmolality. The value calculated from the solute concentrations is reported by the laboratory as the osmolarity. The Osmolar gap is the difference between these two values. The two values usually don't match exactly for various: there are a number of formulas that can be used and they all give slightly different results; the formulas typically use the concentrations of only 3 solutes (Na, glucose, urea) in the calculation so contributions from abnormal small MW uncharged substances will be missed so the calculated value will be low; use of osmometers that use the vapour pressure method are unreliable in the presence of volatile chemicals.
Tonicity is a term used frequently in a medical context. It is also a term which is frequently misunderstood as it is defined in at least three different ways. The most rigorous & useful definition is:
Tonicity is the effective osmolality and is equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across the membrane.
The key parts are effective and capacity to exert. The implication is that tonicity is less then osmolality. How much less? Its value is less then osmolality by the total concentration of the ineffective solutes it contains. Why are some solutes effective and others ineffective?
Consider this experiment: Imagine a glass U-tube which contains two sodium chloride solutions which are separated from each other by a semipermeable membrane in the middle lowest part of the U-tube (see figure below). The membrane is permeable to water only and not to the solutes (Na+ and Cl-) present. If the total particle concentration (osmolality) of Na+ and Cl- on one side of the membrane was higher than the other side, water would move through the membrane from the side of lower solute concentration (or alternatively: higher H2O concentration) to the side of higher solute concentration. Water (the solvent) moves down its concentration gradient.
If the water levels were different in the two limbs of the U-tube at the start of the experiment then:
- What would be the equilibrium situation as regards particle (ie solute) concentration on the two sides of the membrane?
- What would the difference (if any) be in the heights of the water levels on the two sides of the membrane?
- Is the equilibrium condition reached when the particle concentration (ie osmolality) is equal on the two sides of the membrane?
The answer to the last question is no because this neglects the probable difference in the height of the water columns in the two limbs. This height difference is a hydrostatic (or hydraulic) pressure difference and this provides an additional force which must be accounted for in the balancing of forces needed to reach an equilibrium. (An equilibrium is present when there is no net water movement across the membrane.)
The equilibrium would occur when this net hydrostatic pressure is balanced by the remaining difference in osmolality between the two solutions. This osmolality difference results in an osmotic force which tends to move the water in the opposite direction to the hydrostatic pressure gradient. Equilibrium is when these opposing forces are equal.
Now consider what would happen in the above situation if the membrane was changed to one which was freely permeable both to the water and to the ions (sodium & chloride) present. Now none of the particles present has the capacity to exert an osmotic force across the membrane. At equilibrium there is no difference in the fluid levels in the two limbs of the U-tube because the particles present will move across the membrane until the concentration gradients for Na+ or Cl- are eliminated. The osmolality is now the same on both sides of the membrane. At equilibrium there will be no hydrostatic gradient either.
The conclusion is that if the membrane allows certain solutes to freely cross it, then these solutes are totally ineffective at exerting an osmotic force across this membrane and this must be corrected for when considering the particle concentrations across the membrane. Tonicity is equal to the osmolality less the concentration of these ineffective solutes and provides the correct value to use.
It is strictly wrong to say this or that fluid is isotonic with plasma - what should be said is that the particular fluid is isotonic with plasma in reference to the cell membrane (ie the membrane should be specified.) By convention, this specification is not needed in practice as it is understood that the cell membrane is the reference membrane involved.
From a cell's viewpoint, it is net osmolar gradient across the cell membrane at any moment that is important. Tonicity (and not osmolality) is important for predicting the overall final outcome (the equilibrium state) of a change in osmolality because it allows for those solutes which will cross the membrane. All the cells in the body (with a few exceptions eg cells in the hypertonic renal medulla) are in osmotic equilibrium with each other. Movement of water across cell membranes occurs easily and rapidly and continues until intracellular and extracellular tonicities are identical. If water can cross the membrane faster than the ineffective solute can cross then the effect of an abrupt change in extracellular osmolality may be initially and temporarily different from that predicted from the acute tonicity change alone.
If a hyperosmolar solution was administered to a patient, this would tend to cause water to move out of the cell. However if the solute responsible for the hyperosmolality was also able to cross cell membranes it would enter the cell, increase intracellular osmolality and prevent this loss of intracellular fluid. This is the situation with hyperosmolality due to high urea concentrations as urea crosses cell membranes relatively easily.
Hyperglycaemia in untreated diabetics results in ECF which is both hyperosmolar and hypertonic (as compared to the normal situation) as glucose cannot easily enter cells in these circumstances. Water moves out of the cells until the osmolar gradient is abolished.
In some situations, a more operational definition of tonicity is used to explain the term: though not incorrect this explanation is less versatile and rigorous than the one discussed above. This is based on the experiment of immersing red cells in various test solutions and observing the result. If the red cells swell and rupture, the test solution is said to be hypotonic compared to normal plasma. If the red cells shrink and become crenated, the solution is said to be hypertonic.
If the red cells stay the same size, the test solution is said to be isotonic with plasma. The red cell membrane is the reference membrane. Red cells placed in normal saline (ie 0.9% saline) will not swell so normal saline is said to be isotonic. Haemolysis does not occur until the saline solution is less then 0.5%. The point about this definition of tonicity is that it is qualitative and not quantitative. It does imply that permeant solutes will be ineffective because it is essentially a test against a real membrane.
A major physiology text (Ganong 16th ed., 1993) defines tonicity as a term used to describe the osmolality of a solution relative to plasma (as in hypotonic, isotonic or hypertonic). This less rigorous definition is wrong as it does not cover the full sense in which the term tonicity is used. Ganong argues that an infusion of 5% dextrose is initially isotonic but that when the glucose is taken up and metabolised by cells, the overall effect is of infusing a hypotonic solution. This is really a problem with his definition. More correctly, one would say that the 5% dextrose is initially isosmolar with plasma (and this avoids haemolysis). Glucose is a permeant solute in the non-diabetic and can easily enter cells. When infused, the 5% dextrose is very hypotonic (with reference to the cell membrane) despite being isosmolar. Water does not leave the cells initially (and haemolysis does not occur) because there is no osmolar gradient across the cell membrane. The solution is however hypotonic and when the glucose enters cells water does also. If insulin is not present, this movement of glucose does not occur. In this latter case, the solution is isosmolar before infusion and can be considered isotonic after infusion as well.
The particular problem with this definition is that it does not distinguish tonicity from osmolality as it makes no recognition of whether the available solutes are permeant (and thus 'ineffective') or non-permeant (and thus 'effective') with respect to a particular membrane as in the example of the 5% glucose which is isosmolar but hypotonic. The definition really doesn't add much more then could be achieved by the terms hypo- and hyper-osmolality. Of course, the referencing to actual plasma osmolality means this definition is effectively the same as the 'red cell test' definition, while obscuring the fact that tonicity is referenced to the cell membrane.
Note that tonicity is defined in several ways which don't all have exactly the same meaning. This is confusing. The definition based on tonicity as the effective osmolality is best.
A final point here regarding the meaning of the term "osmotic pressure".
Consider again the U-tube experiment but pure water on one side and a test solution of unknown osmolality on the other side of a semipermeable membrane which is permeable only to water. Water will move into the test solution. What would happen if further amounts of the test solution were added before any movement of water had occurred? An equilibrium situation would be reached at which the hydrostatic pressure (ie difference in fluid heights in the two limbs of the U-tube) on the test solution side of the membrane would balance the osmotic tendency for water to move across the membrane into the test solution.
At this equilibrium point, the hydrostatic pressure is a measure of the osmotic tendency in the test solution: indeed the opposing hydrostatic pressure needed to balance the osmotic forces is usually referred to as the osmotic pressure.
There would be practical difficulties in performing this experiment with body fluids as the test solution as the osmotic pressure to be measured is over 7 atmospheres and an extremely long-limbed U-tube would be necessary! Alternatively, the pressure could be supplied from a piston or a compressed gas source rather than a column of fluid.