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Osmosis is the passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, across a selectively permeable membrane. This process continues until an equilibrium is reached and the concentration of solutes is equal on both sides of the membrane.

When Na+ ions move across a plasma membrane, they create a concentration gradient. As these ions accumulate on one side of the membrane, they create an area with a higher solute concentration relative to the other side. As a result, water molecules move passively across the membrane through osmosis to balance the concentration gradient. This is why the movement of Na+ ions is followed by the movement of water.

Oral rehydration therapy (ORT) is a treatment for dehydration, typically caused by diarrhea, cholera, or other similar conditions. ORT is based on the principles of osmosis and involves drinking an electrolyte solution composed of water, salts (sodium, potassium, and chloride), and sugar (glucose or sucrose).

The rationale behind oral rehydration therapy is that water will follow the transport of Na+ ions across the plasma membrane of the intestinal epithelial cells. By supplying the body with a solution containing a balanced concentration of electrolytes (salts) and sugar, the goal is to encourage the movement of water into the bloodstream and replace the lost fluids. The sodium-glucose co-transporter in the intestinal lining assists in this process, where glucose molecules are carried into the intestinal cells along with Na+ ions which in turn are followed by the movement of water molecules. This rehydration process helps to alleviate symptoms and restore normal bodily functions. In conclusion, oral rehydration therapy is based on the principle of osmosis, as it relies on the passive movement of water molecules following the transport of Na+ ions and other solutes across the plasma membrane to replace lost fluids and restore electrolyte balance in the body.