Living organisms are largely composed of water. In fact, approximately 70% of a cell’s weight is due to water. Additionally, most of the chemical reactions that occur in cells require an aqueous environment. Water is a critical molecule for living organisms due to its ability to act as a solvent for polar molecules such as sugars, DNA, RNA, and many proteins.
Controlling the levels of water inside and outside of cells is critical. Cell volume is dependent on the ratio of ions and solutes inside the cell to the levels outside of the cell. The concentration of solutes in a solution is called its osmolarity. Cells are permeable to water, that is water can pass into and out of a cell, while many ions and solutes cannot. Many animal cells even contain a family of water channels that help to regulate cellular osmolarity by allowing the free flow of water across the cell membrane. These water channels are called aquaporins. The permeability of cells to water means that water flows into or out of cells according to its concentration gradient (which is determined by the ratio of solutes). Therefore, if a cell is in an environment containing a high concentration of solutes, called a hypertonic environment, water will rush out of the cell, from high concentration (the dilute intracellular environment with a high concentration of water) to low concentration (the extracellular environment with a high concentration of solutes), causing the cell to shrink. Likewise, if the cell is exposed to an environment that is very dilute (called hypotonic, with a high water to solute ratio), water will rush into the cell, causing it to swell and possibly burst.
Animal cells have many mechanisms to regulate cell volume and prevent cell damage under most conditions. These mechanisms include ion channels, pumps, transporters, and various sensors that carefully monitor and regulate the solute concentration inside and outside the cell. Our bodies also control the solute concentration of the extracellular environment at the level of the whole organism through monitoring and controlling blood solute levels. This is one of the main functions of the kidneys.
Despite the necessity of water for life, drinking excessive amounts of water can actually be deadly. Excessive water intake is referred to as water intoxication or water poisoning. Water intoxication causes an electrolyte imbalance, which results in a rapid decrease in serum (blood) and extracellular sodium levels that can lead to death. The decrease in sodium levels causes a fall in extracellular osmolarity. As excessive amounts of water are consumed, the levels of sodium outside of cells decrease, creating acute systemic hypo-osmolarity (all cells are now exposed to a dilute, hypotonic extracellular environment). Low sodium level, termed ‘hyponatremia’ in the medical community, can cause confusion, disorientation, nausea, and vomiting. Severe hyponatremia causes seizures, coma, and death. The development of acute hyponatremia can cause neurological symptoms that mimic psychosis due to the movement of water from the low salt hypotonic environment into brain cells. This can lead to brain swelling and brain edema, which is an increase in brain volume due to increased water content. This swelling can manifest as neurological symptoms that progress as the sodium level continues to decrease from confusion to drowsiness to coma.
So who is at risk of water intoxication? Water intoxication has been observed in cases of overhydration after heat-related injuries or over exertion (as in marathon runners). During exercise and in hot environments, water and electrolytes are lost as sweat, depleting both water and salt levels. Replenishing just the water levels can lead to a drop in sodium levels in the blood, which can cause the symptoms described above. Secondly, water intoxication is associated with several psychiatric conditions. Psychogenic polydipsia is compulsive water drinking that has been observed in several cases of mental illness. In these cases, individuals compulsively drink excessive amounts of water and, despite how much they drink, they continue to feel the need to drink more water. Lastly, water intoxication has also been associated with fatalities and brain damage during forced water consumption in cases of child abuse and torture.
The danger of water intoxication is determined not only by the volume of water consumed, but also by the rate at which it is consumed. In order to be fatal, very large amounts of water must be consumed in a very short period of time. According to the Dietary Reference Intakes published by the Institute of Medicine (2005), the adequate water intake for an average adult is 9–13 cups (liquid beverage) per day. This accounts for 80% of the total water intake, as 20% of water intake is believed to come from food. Water intake should be increased in cases where individuals exercise, are exposed to excessive heat, are pregnant, or are breastfeeding. In cases of extreme heat or exercise (where excessive water is lost due to sweating), it is suggested that individuals take extra precautions and consider replenishing sodium and other electrolytes as well as water. Water toxicity has been observed when individuals consume quantities of water that greatly exceed the amount of water that can be excreted by the kidneys. The maximal excretion rate for the kidneys is approximately 0.7 to 1.0 Liters (about 3–4 cups) per hour. Thus greatly exceeding this amount of water under normal conditions is not recommended.
In summary, water is necessary for life and makes up most of our cells (by weight). Maintaining a balance of water and solutes inside and outside of the cell is important for cell volume and function. Drinking excessive water can cause decreases in electrolytes in the blood and extracellular environments that cause cells to swell. This is known as water toxicity and can be associated with neurological symptoms due to brain swelling. Normal consumption of water is necessary to prevent dehydration and lead a healthy lifestyle, but excessive water intake over a very short period of time can cause brain damage or even death.