The ocean contains water, but consuming it is dangerous because it is incompatible with the body’s internal chemistry. Average seawater contains approximately 3.5% dissolved salts, or about 35 grams of salt per liter. The concentration of salt in human bodily fluids, such as blood plasma, is much lower, typically around 0.9%. This high salt concentration means drinking ocean water does not provide hydration but instead causes dehydration.
The Principle of Hypertonicity and Osmosis
The danger of saltwater lies in hypertonicity, which drives osmosis. Osmosis is the natural movement of water across a semipermeable membrane, such as cell walls, seeking to equalize solute concentration. Water moves from an area of low solute concentration to an area of high solute concentration.
When saltwater is absorbed into the bloodstream, the high concentration of sodium chloride makes the blood hypertonic relative to the cells. The body attempts to dilute this salt by drawing water directly out of the surrounding cells and into the bloodstream. This movement causes cells to shrink, disrupting their function.
This rapid cellular dehydration affects the entire body and triggers an intense feeling of thirst. Drinking saltwater results in a net loss of water from the body’s tissues because the high salt load is counterproductive to cellular hydration.
The Kidney’s Desalination Limits
The body’s primary system for maintaining fluid and electrolyte balance is the renal system, where the kidneys function as regulators of salt concentration. Under normal circumstances, the kidneys produce urine with a concentration close to that of blood plasma, which is roughly 300 milliosmoles per liter (mOsm/L). To manage a salt overload, the kidneys can concentrate urine to eliminate excess sodium, but this ability has a physical limit.
The maximum concentration the healthy human kidney can achieve is approximately 1,200 mOsm/L. Seawater, however, has an osmotic concentration that typically ranges around 1,000 to 1,200 mOsm/L. While the kidney can technically produce urine as concentrated as or slightly more concentrated than seawater, the salt in the urine must be balanced by water.
To excrete the salt load ingested from ocean water, the kidneys must use a specific volume of water to dissolve and flush out the excess sodium and chloride ions. Because the salt concentration of seawater is so high, the volume of water required to excrete that salt load is greater than the volume of seawater originally consumed. This creates a net water deficit, which accelerates dehydration.
This physiological struggle places stress on the renal system as it attempts to regulate the body’s internal environment. The kidney’s inability to produce urine concentrated enough to eliminate the salt without sacrificing more water confirms that drinking saltwater worsens the body’s overall hydration status.
Acute Symptoms and Systemic Failure
The accelerating dehydration and electrolyte imbalance caused by saltwater consumption lead quickly to acute physical symptoms. The high salt concentration irritates the lining of the stomach and intestines, triggering severe nausea and vomiting. This gastrointestinal distress is worsened by diarrhea, as the undigested salt draws additional water into the bowel through osmosis, resulting in a significant loss of fluids.
The excessive sodium in the blood, a condition called hypernatremia, severely affects neurological function. As brain cells lose water, the resulting cellular shrinkage can lead to confusion, disorientation, and delirium. If the consumption of saltwater continues, the hypernatremia progresses to cause muscle twitching, seizures, and ultimately, coma.
The attempt to excrete the salt load leads to renal shutdown, or acute kidney injury. This failure causes toxic waste products to build up in the body, and the inability to regulate fluid volume results in circulatory collapse. Without the introduction of fresh water to restore the proper balance, this systemic failure will rapidly lead to death.