Most people are aware that salty foods have the property of inducing thirst. Perhaps you've also noticed that very sweet foods tend to do the same thing. This is because salt (as sodium and chloride ions) and sugars (as glucose molecules) function as ‌active osmoles‌ when dissolved in body fluids, primarily the serum component of blood. This means that, when dissolved in aqueous solution or the biological equivalent, they have the potential to influence the direction in which nearby water will move. (A solution is simply water with one or more other substances dissolved in it.)

"Tone," in the sense of muscles, means "tautness" or otherwise implies something that is fixed in the face of competing pulling-style forces. ‌Tonicity‌, in chemistry, refers to the tendency of a solution to pull in water compared to some other solution. The type of solution in question may be ‌hypotonic‌, ‌isotonic‌ or ‌hypertonic‌ compared to the reference solution.

Before discussing the implications of relative and absolute concentrations of solutions, it is important to understand the manners in which we quantify and describe the osmolarity, or concentration, of solutions.

Often, the concentration of solids dissolved in water (or other fluids) is expressed simply in units of mass divided by volume. For example, serum glucose is usually measured in grams of glucose per deciliter (tenth of a liter) of serum, or g/dL. (This use of mass divided by volume is similar to that used to calculate density, except that in density measurements, there is only one substance under study, e.g., grams of lead per cubic centimeter of lead.) Mass of solute per unit volume of solvent is also the basis for "percent mass" measurements; for example, 60 g of sucrose dissolved in 1,000 mL of water is a 6 percent carbohydrate solution (60/1,000 = 0.06 = 6%).

When dealing with concentration gradients that affect the movement of water or particles, it is important to know the total number of particles per unit volume, regardless of their size. It is this, not total solute mass, that influences this movement, counterintuitive though this may be. For this, scientists most commonly use ‌molarity‌ ‌(M)‌, which is the number of moles of a substance per unit volume (usually a liter). This in turn is specified by the molar mass, or molecular weight, of a substance. By convention, one mole of a substance contains 6.02 × 10²³ particles (Avogadro’s Number), derived from this being the number of atoms in precisely 12 grams of elemental carbon. The molar mass of a substance is the sum of the atomic weights of its constituent atoms. For example, the formula for glucose is C₆H₁₂O₆ and the atomic masses of carbon, hydrogen and oxygen are 12, 1 and 16 respectively. Therefore, the molar mass of glucose is

Thus, to determine the molarity of 400 mL of solution containing 90 g of glucose, you first determine the number of moles of glucose present:

Divide this by the number of liters present to determine molarity:

Concentration Gradients and Fluid Shifts

Particles that are free to move about in solution collide with each other at random, and over time, the directions of individual particles resulting from these collisions cancel each other out so that no net change in concentration results. The solution is said to be in equilibrium under these conditions. On the other hand, if more solute is introduced into a localized portion of the solutions, the increased frequency of collisions that follows results in a net movement of particles from areas of higher concentration to areas of lower concentration. This is called diffusion and contributes to the ultimate achievement of equilibrium, other factors held constant.

The picture changes drastically when semi-permeable membranes are introduced to the mix. Cells are enclosed by just such membranes; "semi-permeable" means simply that some substances can pass through while others cannot. In terms of cell membranes (in animal cells) and cell walls (in plant cells), small molecules such as water, oxygen and carbon dioxide gas can move into and out of the cell via simple diffusion, dodging the proteins and lipid molecules forming most of the membrane. Most molecules, however, including sodium (Na⁺), chloride (Cl^-), sodium chloride (NaCL), and glucose cannot, even when there is a concentration difference between the interior of the cell and the exterior of the cell.

Osmosis, the flow of water across a membrane in response to differential solute concentrations on either side of the membrane, is one of the most important cellular physiology concepts to master. Around three-quarters of the human body consists of water, and similarly for other organisms. Fluid balance and shifts are vital for literal survival on a moment-to-moment basis.

The tendency of osmosis to occur is called osmotic pressure, and solutes that result in osmotic pressure, which not all of them do, are called active osmoles. To understand why it happens, it is helpful to think of water itself as a "solute" that moves from one side of the semipermeable membrane to the other as a result of its own concentration gradient. Where solute concentration is higher, "water concentration" is lower, meaning that water will flow in a high-concentration-to-low-concentration direction just like any other active osmole. Water simply moves to even out concentration distances. In a nutshell, this is why you get thirsty when you eat a salty meal: Your brain responds to the increased sodium concentration in your body by asking you to put more water into the system – it signals thirst.

The phenomenon of osmosis compels the introduction of adjectives to describe the relative concentration of solutions. As touched on above, a substance that is less concentrated than a reference solution is called hypotonic ("hypo'" is Greek for "under" or "deficiency"). When the two solutions have equal concentrations, they are isotonic solutions ("iso" means "same"). When a solution is more concentrated than the reference solution, it is hypertonic ("hyper" means "more" or "excess").

Distilled water is hypotonic to sea water; salt water is hypertonic to fresh water. Two kinds of soda that contain exactly the same amount of sugar and other solutes are isotonic.

Imagine what might happen to a living cell or group of cells if the contents were highly concentrated compared to the surrounding tissues, meaning if the cell or cells are hypertonic to their surroundings (in other words they are surrounded by a hypotonic solution/hypotonic environment). Given what you have learned about osmotic pressure, you would expect water to move into the cell or group of cells to offset the higher concentration of solutes on the interior.

This is exactly what happens in practice. For example, human red blood cells, formally called erythrocytes, are normally disc-shaped and concave on both sides, like a cake that has been pinched. If these are placed in a hypertonic solution, water tends to leave the red blood cells through a process called plasmolysis, leaving them collapsed and shriveled. When the cells are placed in a hypotonic solution, water tends to move in and bloat the cells to offset the osmotic pressure gradient – sometimes to the point of not merely swelling but bursting the cells. Since cells exploding inside the body is not generally a favorable outcome, it is clear that avoiding major osmotic pressure differences in adjacent cells in tissues is critical.

Human red blood cells are not isotonic in distilled water, and in medical practice the high solute concentration of salt in human cells is an important consideration for IVs, fluids, vaccines, and other infusions. These liquids need to have a lower osmotic pressure relative to human cells for safe infusions, so they are often combined with saline solutions. This way there is not a lower concentration of solutes entering the bloodstream, and the body can continue appropriate osmoregulation to maintain the proper amount of water and pressure.

If you are engaged in a very long bout of exercise, such as a 26.2-mile running marathon or a triathlon (a swim, a bike ride and a run), whatever you have eaten beforehand may not be enough to sustain you for the duration of the event because your muscles and liver can only store so much fuel, most of which is in the form of chains of glucose called glycogen. On the other hand, ingesting anything besides liquids during intense exercise can be both logistically difficult and, in some people, nausea-inducing. Ideally, then, you would take in liquids of some form because these tend to be easier on the stomach, and you would want a very sugar-heavy (that is, concentrated) liquid so as to deliver maximum fuel to the working muscles.

Or would you? The problem with this very plausible approach is that when substances you eat or drink are absorbed by your intestine, this process relies on an osmotic gradient that tends to pull substances in food from the inside of the intestine to the blood lining your intestine, thanks to being swept up by the movement of water. When the liquid you consume is highly concentrated – that is, if it is hypertonic to the fluids lining the intestine – it disrupts this normal osmotic gradient and "sucks" water back into the intestine from the exterior, causing absorption of nutrients to stall and defeating the whole purpose of taking in sugary drinks on the go.

In fact, sports scientists have studied the relative absorption rates of different sports drinks containing varying concentrations of sugar and have found this "counterintuitive" result to be the correct one. Drinks that are hypotonic tend to be absorbed most quickly, while isotonic and hypertonic drinks are absorbed more slowly, as measured by the change in glucose concentration in blood plasma. If you have ever sampled sports drinks such as Gatorade, Powerade or All Sport, you have probably noticed that they taste less sweet than do colas or fruit juice; this is because they have been engineered to be low in tonicity. Many of these drinks also carry electrolytes and other important nutrients that are lost at an accelerated rate during exercise.

Consider the problem that marine organisms – that is, aquatic animals that specifically live in the Earth's oceans – face: They not only live in extremely salty water, but they must get their own water and food from this highly hypertonic solution of sorts; additionally, they must excrete waste products into it (mostly as nitrogen, in molecules such as ammonia, urea and uric acid) as well as derive oxygen from it.

The predominant ions (charged particles) in sea water are, as you would expect, Cl^- (19.4 grams per kilogram of water) and Na⁺ (10.8 g/kg). Other active osmoles of significance in sea water include sulfate (2.7 g/kg), magnesium (1.3 g/kg), calcium (0.4 g/kg), potassium (0.4 g/kg) and bicarbonate (0.142 g/kg).

Most marine organisms, as you might expect, are isotonic to sea water as a basic consequence of evolution; they do not need to employ any special tactics to maintain equilibrium because their natural state has allowed them to survive where other organisms have not and cannot. Sharks, however, are an exception, maintaining bodies that are hypertonic to sea water. They achieve this through two main methods: They retain an unusual amount of urea in their blood, and the urine they excrete is very dilute, or hypotonic, compared to their internal fluids.