Two shipwreck survivors were rescued from a life raft. One had drunk seawater while the other had not. The one who had drunk the seawater was more severely dehydrated than the one who did not. Explain.

Osmosis is a critical biological process where water molecules move through a semipermeable membrane from an area with a lower concentration of solutes (such as salts, sugars, and other compounds) to an area with a higher concentration. This movement aims to achieve an equilibrium on both sides of the membrane. In the context of dehydration from seawater, when one consumes seawater, the high concentration of salts in the water results in body fluids having a higher solute concentration than the normal levels within cells. As a result, water molecules inside the cells move outwards to dilute the external, saltier environment, reducing the amount of water within the cells and leading to dehydration.

This natural phenomenon can be observed in various everyday situations, such as when you sprinkle salt on vegetables and they begin to release water. In our bodies, osmosis can have severe consequences when the balance is disrupted by the intake of high-solute fluids like seawater.

Sodium chloride, commonly known as table salt, is the primary salt found in seawater. It comprises roughly 3.5% of seawater's composition, equating to about 35 grams of dissolved salts in one liter of seawater. Seawater's high sodium chloride content is what makes it unfit for drinking. When ingested, seawater's salty composition upsets the delicate balance of salts in the human body. This imbalance forces the body to attempt to restore equilibrium, which paradoxically results in more water being pulled from cells and tissues to dilute the excess sodium in the bloodstream, exacerbating dehydration.

A greater understanding of the properties of sodium chloride in seawater can help illustrate why drinking it is detrimental to human health. Sodium chloride dissociates into sodium and chloride ions in water, increasing the number of particles that contribute to the solution's osmotic pressure. Thus, seawater has a much higher osmotic pressure compared to our body fluids.

Solute concentration refers to the amount of a solute—that is, the dissolved substance—in a given volume of solvent, like water. It plays a crucial role in processes such as osmosis and can profoundly affect living organisms. The concept of solute concentration is integral to understanding why drinking seawater leads to dehydration. The concentration of solutes, particularly sodium and chloride ions in seawater, is significantly higher than that found in the human body's cells and blood.

Our cells are designed to operate within a narrow range of solute concentration, commonly referred to as isotonic conditions. Seawater is hypertonic to our body fluids, meaning it has a higher solute concentration. When hypertonic solutions like seawater enter the body, they disrupt the balance, causing cells to lose water as they try to equalize solute levels with their exterior environment. The body's response to this is to urinate more to expel the excess salt, which, counterintuitively, leads to a net loss of water and increased risk of dehydration.

A semipermeable membrane, sometimes also called a selectively permeable membrane, is a barrier that allows certain substances to pass through while blocking others. It is essential in biological systems, especially in the context of osmosis. Cell membranes are semipermeable, permitting water to move freely while typically preventing many solutes, such as salts and other ions, from doing so. The semipermeable nature of cell membranes enables cells to control their internal environment and maintain homeostasis.

In the scenario of dehydration from seawater, the human body's cell membranes will not allow the excess salt to pass through easily. Nevertheless, they do allow water to move out of the cells, trying to dilute the external salt concentration. It's this selective permeability that results in the osmotic flow of water out of the cells, contributing to a state of dehydration when large amounts of seawater are consumed.