What is a hypertonic solution?
A hypertonic solution is one that contains a total concentration of osmolarly active solutes greater than that of another reference solution . Osmolarly active means that the solutes cannot pass through a special type of membrane (similar to a cloth) that acts as a strainer or filter, that is, it lets the water through, and not the solute particles.
This high concentration of solutes causes hypertonic solutions to have a high osmotic pressure . This pressure is what practically sucks the water through the membrane, which is called osmosis.
The word hypertonic comes from the Greek prefix hyper-, which means “over, or above” and “tones”, which means tension, or pressure, so hypertonic literally means “having a greater pressure or tension”.
It is important to keep in mind that, in biology and medicine, whenever we speak of a hypertonic solution, it refers to a solution that is more concentrated than the solution contained in blood, which is called blood plasma.
Characteristics of hypertonic solutions
Hypertonic solutions have some basic characteristics:
They are made up of a solvent and at least one solute
Not just any solution can be hypertonic, even if it is highly concentrated. The solute must be unable to cross the membrane, while the water must be able to pass it without problem, otherwise, the osmotic pressure is not generated.
They have high concentrations of solutes
This is what makes them hypertonic solutions. As mentioned just now, hypertonic solutions are more concentrated than blood.
A hypertonic solution can contain a single solute like salt, or it can contain a mixture of solutes like salts and sugars, for example.
Solutes can be ionic, neutral, or both
The salts contain ions with electrical charges that do not allow them to pass through the membrane, even if they are small. Other solutes do not have ions, but they are large and fat and will not fit through the gaps.
They have a high osmotic pressure
As mentioned above, hypertonic solutions have an osmotic pressure higher than that of blood.
They can dehydrate cells
Hypertonic solutions draw water out of cells, leaving them like raisins. That is why our fingers and toes wrinkle when we bathe on the beach.
What are hypertonic solutions for?
Hypertonic solutions have many uses in daily life, and also in medicine. Some of the most common uses are:
They are used to preserve food
- Example: peaches in syrup do not rot because the syrup has a lot of dissolved sugar and is a very hypertonic solution. By dehydrating cells, hypertonic solutions do not allow fungi and bacteria to grow in food, because they die of thirst. This allows the food to last longer.
They serve to reduce inflammation
- Example : when someone has an accident and hits their head, inflammation often occurs in the brain. To cure the patient, doctors inject a 7.5% salt-in-water solution. This solution practically sucks up the water, reducing inflammation in the brain.
They serve to replenish electrolytes.
- Example : sodium is an electrolyte that we need to live, and when a patient has low sodium in the blood, a 7.5% saline solution is injected to replace the sodium that was lost.
They are used for parenteral feeding
- Example : when people cannot eat through their mouth or through a tube, they feed themselves by injecting them with a hypertonic solution that contains a type of sugar called glucose.
Differences with hypotonic and isotonic solutions
Just as there are hypertonic solutions, there are also hypotonic and isotonic solutions. The difference between the latter and hypertonic solutions is their concentration. Hypotonic solutions are those that are less concentrated than blood, and isotonic solutions have the same concentration as blood.
For example, pure water, since it has no solute at all, is hypotonic, while coconut water has salts and sugars with a concentration almost equal to that of blood, so it is isotonic.
In addition to differing by their concentration, hyper, hypo, and isotonic solutions differ in the way they affect cells.
The following image shows the effect of different types of solutions on red blood cells (the small cells that make blood red).
If the red blood cells are placed in a hypertonic solution, this removes the water from them, dehydrating them and leaving them wrinkled like a raisin.
In contrast, if they are placed in a hypotonic solution, the red blood cells absorb water and swell, and may even burst like a balloon that is over-inflated. Finally, if they are introduced into an isotonic solution, the water inside and outside the red blood cells will be in equilibrium so nothing happens.
Examples of hypertonic solutions
The syrup in which peaches and other fruits are preserved contains very high concentrations of sugar, making it a hypertonic solution.
Brine is a solution of water and salt with very high concentrations of between 3.5% and 25%, which can be almost 30 times more concentrated than blood (that is, it is very hypertonic).
A common vinegar is a hypertonic solution, since it has a concentration of between 3% and 5% of acetic acid. The balsamic vinegar reductions are even more concentrated.
The same blood from our veins, after sweating a lot.
By exercising a lot and sweating, the body eliminates water and salts, but it eliminates more water than salts (sweat is hypotonic) so the blood becomes concentrated and hypertonic.
Seawater contains many dissolved salts, with a total concentration of about 3.5%, which makes it hypertonic. That is why seawater wrinkles our fingers.
Cola drinks and other sugary drinks can contain up to 10% sugar, as well as salts and other solutes with concentrations higher than blood.
Solution of 5% dextrose and 0.45% sodium chloride (D5 ½ NS)
This is an example of a hypertonic solution used to feed people who cannot eat normally by mouth.
This is a frequently used solution to treat blows to the head. It is four times more concentrated than blood.
7.5% saline solution
It is a sterile, highly concentrated salt solution that is also injected into the veins of some patients with certain diseases.
All juices that are bought in the supermarket and that are not natural, have very high concentrations of sugar that make them hypertonic.
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