We have all heard the human body is 60% water and we have all heard all the jokes generated from this information (1). It’s more than just a joke; dosing a drug depends on the amount of water in the body and that amount changes depending on age and gender. Water is able to flow freely through cells and distributes itself until the water concentration is even everywhere. The two terms we use to describe the movement of water are osmosis and tonicity. Osmosis and tonicity are often confused, but I’ll explain them in a way that you will never get them mixed up again.
Osmosis is the movement of water across a semipermeable membrane to compensate for changes in solute concentration (2). Your body needs a certain concentration of certain molecules for the body to function. Sodium, for example, needs to be at 140 millimoles per liter in your plasma (2). Plasma is the water in your blood so for every liter of blood you need 140 millimoles of sodium. Water will flow in and out of your blood vessels so that the concentration of sodium stays within range.
Let’s compare osmosis to the dodgeball. The center line is like the semipermeable membrane. The people can’t cross, but the dodgeballs can, hence semipermeable. The dodgeballs are like the water, they can freely pass into all areas of the court. The people are like sodium; they have to stay in their designated area. In order for the game to work, the players need dodgeballs. The more players, the more dodgeballs. Once the game starts, both teams run up to the center line and each team gets an even number of dodgeballs. Equal players, equal dodgeballs or equal sodium, equal water. As the game progresses and one side loses players the dynamics shift. The side with more players needs more balls to throw, they want more dodgeballs. Just like if a solution has more sodium it will need more water to compensate so it will take it from another solution.
The easiest trick to osmosis is to remember that water moves from areas of low concentration to areas of high concentration. [Low] → [High] It’s all about concentration. If you know the concentration of the two solutions, you can figure out which way the water moves then you can compare the two solutions and know which is hyperosmotic and which is hypoosmotic. These terms are used to compare the two solutions. They tell us which solution has more solutes and where the water will flow.
The following diagram, Figure 1, shows the movement of water to compensate for…