Seawater appears to be a source of liquid refreshment, but consuming it causes severe dehydration. The danger lies in the high concentration of dissolved salts, primarily sodium chloride, which the human body cannot process without rapidly losing its existing water reserves. Instead of quenching thirst, drinking ocean water accelerates the body’s water deficit, leading to a net loss of fluid. The salt load actively draws water out of cells and tissues to attempt to restore balance.
The Salinity Imbalance and Osmosis
The fundamental problem with drinking seawater stems from a significant difference in salt concentration compared to the human body’s internal environment. Typical ocean water contains approximately 3.5% salt, which is about four times the concentration found in human blood, maintained at 0.9% salinity. This creates a concentration gradient that the body attempts to equalize.
When this concentrated salt solution enters the bloodstream from the digestive tract, it creates a hypertonic environment, meaning the blood has a much higher concentration of solutes than the surrounding body cells. Osmosis then takes over to correct this imbalance. Osmosis is the passive movement of water across a semipermeable membrane, like the walls of your cells, from an area of lower solute concentration to an area of higher solute concentration.
To dilute the salt-laden blood, water is pulled directly out of the body’s internal water stores, including the fluid inside the cells. This cellular water leaves the cells and enters the saltier blood, causing the cells to shrink. This process causes cellular dehydration, as the body sacrifices its internal fluid balance to lower the blood’s salt concentration.
The Kidney’s Salt Overload Dilemma
The kidney, the body’s main regulatory organ, filters the blood and excretes excess salt and waste in the urine. The kidneys are highly efficient at concentrating waste products, but they have a physiological limit to how much salt they can remove. A healthy human kidney can produce urine with a maximum salt concentration far less than that of seawater.
Because the ingested seawater is much saltier than the strongest urine the kidneys can manufacture, the organs cannot excrete the salt load without using additional water. To flush out the sodium and chloride, the kidneys must use a large volume of water to dilute the salt to their maximum excretable concentration. This necessary water must be drawn from the body’s existing reserves, which have already been depleted by the initial osmotic shift.
This forced dilution results in a net negative gain of water. For every liter of seawater consumed, the body must excrete a greater volume of water—perhaps 1.5 liters or more—to clear the associated salt. The body uses its limited fresh water to process the saltwater, leading to an accelerated spiral of dehydration.
Systemic Effects of Severe Dehydration
As the kidneys fail to keep pace with the salt intake, the sodium concentration in the blood rises sharply, a dangerous condition known as hypernatremia. Hypernatremia severely impacts the central nervous system, with symptoms manifesting rapidly. The brain is particularly vulnerable to this hypertonic state.
The elevated sodium concentration outside the brain cells draws water out of them via osmosis, causing the brain tissue to shrink. This rapid reduction in brain volume can lead to neurological symptoms such as confusion, lethargy, and delirium, and may progress to seizures and coma. In severe cases, the shrinkage can stretch and tear the blood vessels surrounding the brain, potentially causing hemorrhage.
The electrolyte imbalance also affects the cardiovascular system and muscle function. The extreme loss of fluid volume and high salt levels disrupt the electrical signaling required for normal heart rhythm and muscle contraction. If the process continues unchecked, the systemic failure resulting from severe dehydration and hypernatremia leads to organ collapse and is fatal.