Why are blisters filled with water?

The fluid inside a blister is primarily composed of serum, the liquid component of blood plasma. When the skin undergoes friction, pressure, heat, or other irritants, the outer layer separates from the deeper layers, creating a pocket or cavity. This cavity fills with serum, which contains water, electrolytes, proteins, and white blood cells. The serum helps protect the underlying tissue and provides a moist environment for healing.