When distilled water enters the bloodstream, it creates an immediate imbalance. Distilled water is pure water, stripped of dissolved salts and minerals (solutes). The fluid component of blood, plasma, is a carefully balanced saline solution with a specific concentration of solutes, known as its osmolarity. A healthy plasma osmolarity is maintained between 275 and 300 milliosmoles per kilogram of water (mOsmol/kg). Introducing a fluid with an osmolarity of zero directly into the circulation disrupts this balance, initiating a rapid chain of events driven by how water moves across biological membranes.
The Role of Osmosis
The immediate reaction to the introduction of distilled water is governed by the physical process of osmosis. Osmosis is the movement of water across a semi-permeable membrane, like a cell wall, from an area where the water concentration is higher to an area where it is lower. This movement occurs because the water seeks to equalize the solute concentration on both sides of the membrane.
When distilled water mixes with the blood, it creates a localized environment that is hypotonic compared to the interior of the body’s cells. A hypotonic solution has a lower concentration of solutes than the cell’s internal fluid. The red blood cells circulating in the blood are encased in a membrane that acts as a selective barrier, permitting water to pass freely but restricting the movement of larger solutes, such as salts and proteins, which are concentrated inside the cell.
Since the distilled water contains almost no solutes, its water concentration is much higher than the water concentration inside the red blood cells, which are rich in dissolved particles. This difference establishes a steep concentration gradient across the cell membrane. Water molecules rush inward across the cell membrane toward the higher solute concentration inside the cell. This osmotic influx of water is an unstoppable physical mechanism aiming to dilute the internal contents of the red blood cell.
Cellular Consequences of Hypotonicity
The influx of water causes the red blood cells to swell rapidly. Normally, red blood cells have a unique biconcave disc shape, which provides flexibility to squeeze through narrow capillaries and gives them a high surface area for oxygen exchange. As water rushes in, the cells lose this specialized shape and become increasingly spherical.
Cells lack the rigid cell wall found in plant cells, making them highly susceptible to volume changes. The cell membrane can only stretch so far before it reaches its physical limit. The continued, excessive pressure from the accumulating internal water eventually overcomes the structural integrity of the cell membrane, causing it to rupture. This destructive process is termed hemolysis, or the bursting of the red blood cell.
Hemolysis releases the cell’s internal contents, including hemoglobin, directly into the surrounding blood plasma. This free hemoglobin is toxic in high concentrations and must be rapidly cleared from the circulation. The ruptured cell fragments and the released hemoglobin are then processed by the body’s clearance mechanisms, which include the liver and spleen.
Volume Determines Systemic Severity
The overall outcome of distilled water entering the bloodstream depends on the volume and speed of the injection. A small, accidental injection is quickly diluted, which minimizes the hypotonic effect. This limits the hemolysis to a tiny, localized area that the body can easily manage.
However, the accidental intravenous infusion of a larger volume of distilled water can be medically disastrous. This large-scale infusion causes rapid, widespread hemolysis throughout the body, leading to a sudden loss of red blood cells and hemolytic anemia. The massive release of free hemoglobin overwhelms the body’s clearance systems, resulting in acute kidney injury or even failure.
The dilution of the blood plasma can also cause a severe electrolyte imbalance, most notably a drop in sodium levels known as hyponatremia. This systemic dilution and the associated electrolyte shock can affect the nervous system, potentially leading to cerebral edema, seizures, and coma. Drinking distilled water is not dangerous because the water is rapidly mixed and buffered by stomach acids and intestinal contents before it is absorbed into the bloodstream.