The human body maintains a precise balance of water and salts. When a person drinks seawater, which contains approximately 3.5% dissolved salts, this delicate internal balance is overwhelmed. The concentration of salt in human blood is a much lower 0.9%, making seawater about four times saltier than our internal fluids. This difference in concentration means seawater does not hydrate the body but instead triggers a severe state of internal drought. The danger of drinking salt water is the subsequent and paradoxical loss of the body’s existing fresh water reserves.
How Salt Water Disrupts Cellular Balance
When saltwater is absorbed from the digestive system into the bloodstream, it dramatically increases the concentration of solutes, primarily sodium, in the blood plasma. This elevated concentration immediately disrupts the normal movement of water across cell membranes, a process known as osmosis. Because the blood now has a significantly higher salt concentration than the fluid inside the body’s cells, the blood is considered hypertonic.
To dilute the excess sodium in the bloodstream, water is osmotically pulled out of every cell in the body. Water leaves the cells of muscles, organs, and even the brain, moving into the extracellular fluid and the blood plasma. This water loss causes the cells to shrink, a process called cellular dehydration or crenation. This rapid and widespread water theft across all tissues initiates the severe, life-threatening dehydration associated with drinking seawater.
Why Kidneys Cannot Process Seawater
The kidneys are the body’s primary regulators, constantly working to maintain the precise balance of electrolytes and fluid volume. When the bloodstream is flooded with the high sodium load from seawater, the kidneys must work to excrete the excess salt. The human kidney has a maximum renal concentrating capacity, which is the highest concentration of waste it can excrete in urine, typically around 1,200 milliosmoles per kilogram of water (mOsm/kg).
Seawater has an average osmolality of around 1,000 mOsm/L, a concentration very close to the kidney’s absolute limit. To package and excrete the salt from the ingested seawater, the kidneys must dissolve it in water. Even if the kidneys produced urine at their maximum concentration of 1,200 mOsm/kg, they would still need a volume of water greater than the volume of seawater originally consumed to flush out the entire salt load. This requirement is known as obligatory water loss.
The water needed for this excretion process must be drawn from the body’s existing fresh water reserves. This means that the body must expend a greater amount of its own water to process the salt, resulting in a net loss of fluid. The kidney’s effort to filter the salt accelerates the dehydration that was already started at the cellular level. This mechanism creates a fatal loop: drinking seawater increases dehydration, which further impairs the kidney’s ability to function, leading to a cascade of internal failure.
Systemic Failure and Hypernatremia
The ultimate consequence of this compounding dehydration and salt retention is a condition called hypernatremia, which is an abnormally high sodium concentration in the blood. This condition causes profound neurological symptoms because the brain is highly sensitive to changes in fluid balance. As water is pulled out of brain cells, they shrink rapidly, which can lead to confusion, restlessness, and lethargy.
The physical shrinkage of brain tissue can be so severe that it causes tearing of the cerebral blood vessels, potentially leading to internal bleeding. As the condition progresses, the neurological damage intensifies, resulting in seizures and eventually coma. Simultaneously, the severe loss of total body fluid causes a critical drop in blood volume, straining the cardiovascular system. Without sufficient fluid to maintain blood pressure and oxygen delivery, the heart and other vital organs fail, leading to death.