The potential space between two opposing serosal surfaces is mostly empty except for the small amount of serous fluid.

Serosa is entirely different from the adventitia, a connective tissue layer which binds together structures rather than reducing friction between them.

Early in embryonic life visceral organs develop adjacent to a cavity and invaginate into the bag-like coelom.

[4] The fluid is produced by the serous membranes and stays between the two layers to reduce friction between the walls of the cavities and the internal organs when they move with respect to one another, such as when the lungs inflate or the heart beats.

[4] All serous membranes found in the human body formed ultimately from the mesoderm of the trilaminar embryo.

The trilaminar embryo consists of three relatively flat layers of ectoderm, endoderm (also known as "entoderm") and mesoderm.

The intraembryonic coelom can now be seen as a cavity within the body which is covered with serous membrane derived from the splanchnopleure.

This Wikipedia entry incorporates text from the freely licensed Connexions [1] edition of Anatomy & Physiology [2] text-book by OpenStax College