What happens to osmolality when a person is dehydrated?

When a person is dehydrated, the osmolality of their body fluids increases. Osmolality refers to the concentration of solutes in a solution, and when there is a loss of water without a corresponding loss of solutes, the concentration of solutes in the blood rises.

Dehydration reduces the overall volume of fluid in the body, resulting in the blood (plasma) becoming more concentrated with solutes, such as sodium, urea, and other electrolytes. As a result, the osmolality increases, which can lead to various physiological responses, including the sensation of thirst and the release of antidiuretic hormone (ADH) to help conserve water by reducing urine output.

In a normal hydration state, osmolality is kept within a specific range, but when dehydrated, it shifts upward due to the imbalance between fluid loss and solute retention. This response is essential for maintaining homeostasis in the body during periods of insufficient hydration.