AP Biology Question of the Day

Osmolarity plays an important role in how water travels within our body. Osmolarity describes the concentration of solutes within a solvent and is expressed as the amount of solutes divided by the volume of solvent:

The higher the amount of solute is within a volume of solvent and/or the lower the volume of solvent is, then the osmolarity will be greater. In regard to fluid movement, water will move from an area with a low osmolarity to an area with a high osmolarity. Human blood has an osmolarity of roughly:

At this concentration, the osmolarity inside the cell is equal to the osmolarity of the surrounding environment; therefore, it is considered to be in an isotonic solution. When the osmolarity around the cell is higher, then water will flow out of the cell and into the blood. This type of solution is called a hypertonic solution. Conversely, a hypotonic solution exits when the osmolarity of the fluid surrounding the cell is lower than that inside the cell. In this case, water will flow from the surrounding environment and into the cell.

James was stranded on an island. He was thirsty and decided to drink the water from the sea (which has a very high osmolarity). After drinking the water, his body became even more dehydrated and he began urinating more frequently. Which of the follow choices best explains why James urinated more frequently and became more dehydrated despite drinking the seawater?