The question of how much “empty space” exists in the human body requires looking at two distinct levels: the subatomic and the anatomical. At the deepest physical level, the answer seems to be that the body is almost entirely void, a concept derived from the structure of atoms. However, at the practical biological level, the body is a dense, highly occupied volume where true emptiness is rare. The difference lies in whether we define “empty space” by the quantum physics of matter or by the measurable cavities and fluids of human physiology.
The Atomic Illusion of Emptiness
The widely cited idea that all matter, including the human body, is mostly empty space stems from the composition of the atom. Every atom consists of a tiny, dense nucleus surrounded by a cloud of electrons. The nucleus accounts for virtually all of the atom’s mass but occupies an incredibly small fraction of its total volume.
If the nucleus of an atom were the size of a marble, the nearest electron would be flying around a radius of over a mile away, illustrating the enormous scale of separation. Because the subatomic particles are minute relative to the space they inhabit, the atom’s volume is estimated to be more than 99.9999999% “empty.” This figure is often simplified to 99.999% in popular science, suggesting that if all the empty space were compressed, a human body would shrink to a particle too small to see. This model is purely theoretical and relevant only when considering fundamental physics.
The Reality of Molecular Occupancy
Despite the atomic structure, the human body is functionally solid because of electron clouds and chemical bonding. Electrons exist as probability distributions, or “clouds,” that essentially occupy the entire volume of the atom. The volume of these electron clouds defines the size of an atom and the density of matter.
When atoms combine to form molecules, their electron clouds overlap, creating a powerful repulsive force that prevents atoms from passing through one another. This quantum mechanical effect gives matter its solidity, meaning the space between atomic nuclei is not a true vacuum, but a region filled with the negative charge density of shared electrons.
Anatomical Voids: The Measurable Empty Spaces
Moving away from the subatomic, the human body contains several large, measurable spaces that are truly void of solid tissue. These anatomical voids are filled with gases or fluids. The largest of these is the volume of air in the lungs and the conducting airways, known as dead space.
Anatomical dead space is the volume of the nose, trachea, and bronchi where no gas exchange occurs. For an average adult, this fixed volume of air is roughly 150 milliliters, representing air that is inspired but not used. The lungs also contain residual volume, the air that remains after a maximal exhalation, which is another significant pocket of non-solid space.
The digestive tract, from the mouth to the rectum, is essentially a long, convoluted tube that is technically external space passing through the body. This tract contains varying amounts of food, waste, and digestive gases, representing a large, non-solid volume. The major body cavities—thoracic, abdominal, and cranial—are also voids, though they are tightly packed with organs and are not empty in the classical sense.
Between the cells themselves, a significant amount of space is occupied by interstitial fluid, not solid tissue. This fluid surrounds all the body’s cells, acting as the medium for nutrient and waste exchange. In a healthy adult, this fluid alone accounts for about 12 liters of volume, or approximately 16% of total body weight. The fluid within the blood vessels (plasma) and specialized fluids like cerebrospinal fluid also contribute to the overall non-solid volume.