Water is essential to life; the human body cannot function properly without it. Dehydration is a condition where more water leaves the body than is taken in. Thirst is one sign of dehydration. There are other forms of dehydration, though, and the condition can refer to salt loss as well as simple water loss. The body works to adjust its water content to keep cells at a safe level of hydration. What happens to cells during dehydration, therefore, depends on what type of dehydration the body is experiencing.
Water accounts for approximately 50% of body weight in females, and approximately 60% in males. Water is divided between two locations: intracellular (inside the cells) and extracellular (outside the cells). The extracellular compartments contain the water in the blood as well as the water located between the cells in the tissues. For the average person, about two thirds of the body's water is intracellular. Water can be exchanged between intracellular areas and extracellular components when necessary.
Each compartment's fluid is made up of water and salts. These dissolved salts provide osmotic pressure to the compartment. Osmotic pressure represents the concentration of particular salts in each compartment relative to another compartments. The more salts in the water, the higher the osmotic pressure. Under normal circumstances, the osmotic pressure in the intracellular compartment is the same as in the extracellular compartment. When dehydration occurs, however, the concentration of salts in one or more compartments increases or decreases. This can provoke water to move from one compartment to another to even up the osmotic pressure differences between the cells and the extracellular compartment.
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Isotonic dehydration, also known as isonatremic dehydration, refers to loss of water along with the salt that is normally in the water. Examples of conditions where this happens are diarrhea and vomiting. This depletes salts and water in the extracellular compartment, and water and salts move out of the cells to replace the lost extracellular fluid. There is no change in osmotic pressure, only a change in fluid volume in both compartments.
Hypotonic dehydration means that the body's fluids have less concentrated salts dissolved in the water. Water present in the extracellular fluid then moves into the cells because the cells have more dissolved salts and thereby a higher osmotic pressure. It is possible to disrupt cell function and distort cell structure if overhydration occurs, such as when a person drinks too much water without taking in salts as well.
Hypertonic dehydration means that the body has lost more water relative to salts. The extracellular fluid therefore has a higher osmotic pressure. Cells allow water to flow outward and into the extracellular fluid to balance the osmotic pressure difference between inside the cells and outside the cells.
Overall Intracellular Changes
Overall, in conditions of dehydration, the cells of the body tend to donate water to the extracellular compartment, as the extracellular compartment is more changeable with regard to osmotic pressure than the intracellular. The cells can afford to donate water to adjust this because they contain about twice as much water as the extracellular compartment. Thus, a small change in the intracellular compartment means a more significant change to the extracellular compartment.