Here's why:

* Universal Solvent: Water is an excellent solvent, meaning it can dissolve many substances. This allows it to carry nutrients, oxygen, and other essential molecules throughout the body.

* Transport Medium: Water acts as a medium for transporting these dissolved substances, as well as waste products, to and from cells.

* Temperature Regulation: Water has a high heat capacity, meaning it can absorb and retain heat well. This helps regulate body temperature.

* Lubrication and Protection: Water lubricates joints and organs, preventing friction and damage. It also protects organs and tissues.

While water is the fundamental component, body fluids also contain:

* Electrolytes: Minerals like sodium, potassium, calcium, and chloride, which are essential for nerve function, muscle contraction, and fluid balance.

* Proteins: These play various roles in blood clotting, immune function, and transporting substances.

* Other substances: These include sugars, hormones, and gases like oxygen and carbon dioxide.

The specific composition of body fluids varies depending on their location and function. For example, blood is different from cerebrospinal fluid, which is different from saliva. However, water remains the primary component and the foundation for all body fluids.