Science & Tech

Why can't humans drink saltwater?

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Fresh water is a life necessity - drinking saltwater results in fatal dehydration
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Drinking saltwater can be deadly.

Water is one of the most important natural resources available to man; yet much of the earth's water cannot be ingested due to its salt content. Why can't humans drink saltwater? Here is a simple explanation.

Water is considered highly saline (containing significant amount of salt) if it contains between 10,000 to 35,000 PPM (parts per million) of dissolved salts. While it is possible to desalinize water, it is an expensive process. While some salt is necessary for human health, fresh water is a life necessity. The success of entire civilizations is based on its supply of fresh water for human consumption, livestock survival and crop irrigation.

While humans can safely ingest small amounts of salt, the salt content in seawater is much higher than what can be processed by the human body. In addition, when we consume salt in our daily diet, we also drink liquids, which help dilute the salt and keep it at a healthy level. While living cells depend on sodium chloride to maintain the chemical balances and reactions that take place at the cellular level, too much sodium can be deadly.

The simplest answer to the question, "why can't humans drink saltwater?" is this: the kidneys can only make urine that is less salty than saltwater. To get rid of the excess salt taken in by drinking saltwater, you have to urinate more water than you drank you will die of dehydration even as you become thirstier.

A more scientific answer is:

Diffusion refers to the process in which molecules or ions spontaneously move from regions of higher concentration to regions of lower concentration. While the molecules move, they bump into one another causing a mixing action. The molecules are always trying to reach a state of equilibrium, or a uniform distribution.

Osmosis is the diffusion through a permeable membrane in response to a concentration gradient, or a situation in which different areas have varying levels of concentration. Osmosis is special diffusion and occurs whenever water molecules diffuse from a higher region of concentration to a region of lower concentration across a selectively permeable membrane.

Osmoregulation refers to the fact that cell membranes are usually permeable to water. Water is equilibrated throughout the body by osmosis, which means that the water concentration inside the cell and around the cell is essentially the same. This means it's an isotonic solution.

An isotonic solution is a solution that has the same concentration of dissolved particles as the solution with which it is compared. Therefore water enters and leaves the cell in equal amounts and the cell remains unchanged.

A hypertonic solution is one that contains a greater concentration of dissolved particles as the solution with which it is compared. This means that water moves out of the cells and into the fluid surrounding them, causing the cell to shrink.

A hypotonic solution is a solution that contains a lesser concentration of dissolved particles as the solution with which it is compared. Cells gain water through osmosis and therefore, swell. Even though cell membranes are elastic, it is possible for them to become so swollen that they burst.

The swelling or shrinking of cells can damage them. When you drink saltwater it creates a situation in where the cells in your mouth are in a hypertonic solution. In order to reach equilibrium, a net water movement out of the cells takes place. Therefore the cells have ever less water than before and you feel thirstier. Not only does the saltwater dry out the cells in your mouth, it also dries out the cells in your body. As it enters your system, the cells near it release water in order to reach equilibrium with the surrounding fluid. The cells shrink and become dehydrated.

Dehydration is dangerous business. As your body begins to sense that its water levels are low, it panics and holds onto the water it has and will try to hydrate the body with whatever water molecules can be found. This shuts down the body and seizures, pain, convulsing, coma and brain damage can result. Eventually, the kidneys will shut down as they cannot handle the overflow of salt. Death follows shortly after.

Humans are not equipped to drink saltwater and it should be avoided at all costs.

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