What is tonicity and how does it impact cells in the body?

Question

This question needs a through grasp of the terms isotonic, hypertonic, hypotonic, osmosis and selectively permeable membrane.

Answer 1

To understand tonicity, imagine the following scenario: you have a tank of water that has a selectively permeable (i.e. semi-permeable) membrane, which, for the purposes of this example, allows water (not solute) to flow freely. If you fill one side of the tank with some kind of dissolvable solute - in this case, let's say table salt - the water with the freshly added solute will now have a greater tonicity than the other side of the tank. In other words, the side of the tank that contains the table salt has more solute than the other, and therefore is considered "hypertonic" relative to the side of the tank that just contains water and no solute.

The concept of tonicity is rooted in comparing the solute concentrations of solutions, which are often separated by a semi-permeable membrane and allows for the free movement of water. A common physiological example is that of a cell (as described above), which has a cell membrane that allows for water to move inside or outside of the cell depending on the intracellular and extracellular concentrations of solutes. The following terms compare how "tonic" one solution is compared to another:

Hypertonic: one side of the semi-permeable membrane has a higher concentration of solute than the other

Hypotonic: one side of the semi-permeable membrane has a lower concentration of solute than the other

Isotonic: both sides of the semi-permeable membrane have an equal concentration to each other

In our example above, the side of the tank that has table salt is considered to be "hypertonic" to the other side that only contains water. Moreover, the side that only contains water is considered to be "hypotonic" to the side that contains the table salt.

Now, what happens in this example since water is able to move freely between the membrane? Water always flows from a high concentration of solute to a low concentration, so it will move to the side that contains the table salt to dilute the concentration relative to the side with no table salt, generally until both solutions on either side of the membrane are isotonic (i.e. equal in concentration) to each other.

With all this in mind, how does tonicity impact cells in the body? If a cell is "hypertonic" relative to the extracellular environment, since the cell membrane is permeable to water (but generally not solute, at least in this case) water will begin to move into the cell and the cell will swell. Conversely, if a cell is "hypotonic" relative to the extracellular environment, or the extracellular environment is "hypertonic" compared to the intracellular environment, water will move out of the cell and into the extracellular space - the cell will shrivel. If the cell is isotonic compared to its extracellular environment, all is well and there is no water movement observed.

These concepts are very important when it comes to considering how dehydration effects the cells in your body - for example, dehydration affects multiple organ systems as the lack of water in your blood increases the "drive" for water to exit the cells and enter the bloodstream - this is, water moves from the intracellular environment to the extracellular environment, as the cells shrivel, which can have negative consequences on organ function.

I hope this helps!

Answer 2

Hypertonic solution: More Solute/higher concentration in the fluid outside the cell

In a Hypertonic solution, you have excess solute on the outside of the cell, Since the fluid outside the cell is more concentrated than the fluid inside the cell, the water inside the cell flows outside (aka towards the more concentrated environment).

Isotonic cell:

Here the concentration of the solute (ex. salt) is present in equal concentration in the fluid outside and inside the cell. Therefore there is no movement of fluid needed to balance things out.

Hypotonic: Less solute/lower concentration in the fluid outside the cell

Here the concentration of the solute is greater in the cell when compared to its environment, therefore we need to get rid of fluid so the concentration will become greater outside of the cell. So the solute decides to go into a cell which results in the cell expanding/ swelling up.

Answer 3

Every time you eat, drink or get dehydrated the balance between the amount of water to dissolved solids suspended in the tissue fluids changes. In order to protect the cells of your body from dying, there exist many mechanisms to restore balance. To keep this answer as simple as possible let's follow the water! Osmosis is defined as the net movement of water from a region of high water molecule concentration towards an area of lower water molecule concentration. Knowing this fact, you can then apply this to a body cell and various liquid environments where the amount of water varies. If I take a red blood cell and put it in a beaker of 100% freshwater, what will happen to the cell? First you need to know where the water concentration is the greatest; inside the red blood cell or in the beaker because osmosis says water moves from and area of high concentration... Red blood cells are say 90% water. 90% is less that 100%, so water will pass through the cells membrane into the cell expanding the inner volume (intracellular fluid) leading to a ruptured membrane (hemolysis.) Think of a balloon which is blown up to normal size. If you continue to blow air past the normal limit, the balloon will burst. Conversely, if the red blood cell is put in a beaker of sea water what would happen? Again start with where the high concentration of water is located. Sea water is only 75% water and a red blood cell is 90% water. Since the high water concentration is inside the red blood cell, it will pass through the cell membrane out of the cell. This causes the cell membrane to collapse or shrivel (crenation.) Using the example of the balloon, if it is filled with air, the molecules of air hold the walls of the balloon maintaining the shape of the balloon. If you slowly allow the air to escape, the balloon shrivels.

Either too much water in the environment (hypotonic solution) or too little water (hypertonic solution) can kill body cells. Isotonic solutions have the same amount of water as the cells so there will be no net change in the cell membrane.