Drinking seawater for hydration is counterproductive because it leads to immediate and severe dehydration. The human body requires pure, low-solute water to maintain the delicate balance of fluids necessary for proper cell function. Ocean water contains a salt concentration far exceeding what the body can handle. Consuming it forces the body to expend its own water reserves to expel the excess salt, accelerating thirst and leading to crisis.
The Role of the Kidneys in Salt Management
The kidneys function as the body’s filtration and fluid regulation system, maintaining a precise balance of electrolytes like sodium. They constantly filter the blood and produce urine to excrete waste and excess salts. The average salinity of ocean water is approximately 3.5%, containing about 35 grams of dissolved salts per liter.
The human kidney has a maximum biological limit for concentrating salt in urine, which is significantly lower than the concentration found in seawater. While the total solute concentration in human urine can reach about 1200 milliosmoles per liter (mOsm/L), seawater contains a salt concentration equivalent to roughly 1000 mOsm/L.
To excrete the high salt load from ingested seawater, the kidneys must use more water than was initially consumed to dilute the excess sodium. This process creates a net loss of water from the body’s existing fluid reserves, directly worsening dehydration. The kidneys pull water from the bloodstream and surrounding tissues to flush out the salt.
Cellular Dehydration Through Osmosis
The core physical reason for dehydration lies in osmosis, which governs the movement of water across cell membranes. Osmosis is the passive movement of water across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration, aiming to equalize the concentration on both sides.
When concentrated saltwater enters the body, it rapidly increases the sodium concentration in the bloodstream and surrounding extracellular fluid. This makes the fluid outside the body’s cells, including red blood cells and brain cells, hypertonic—meaning it has a higher solute concentration than the fluid inside the cells.
To dilute this hypertonic environment, water is pulled out of the cells and into the bloodstream. This net outflow causes the body’s cells to shrink and shrivel (plasmolysis). This effect is particularly damaging to delicate tissues like the brain, where cellular shrinkage can cause significant neurological problems.
Immediate Physiological Consequences
The rapid onset of high sodium levels in the blood, known as hypernatremia, triggers a cascade of serious symptoms. The body’s immediate response is intense thirst, signaling that the brain is detecting the dangerously high salt concentration.
The digestive system often reacts with nausea and vomiting, which further accelerates fluid loss. As hypernatremia progresses and cellular dehydration intensifies, particularly in the brain, the symptoms become life-threatening.
Confusion, restlessness, and lethargy can set in quickly as brain cells struggle to function without adequate water. In severe cases, high sodium levels can lead to muscle twitching, seizures, coma, organ failure, and death.