How can a boundary separate things and allow exchange?

A boundary can keep two regions distinct while allowing selected things to pass. In the lungs, a very thin barrier separates air from blood. Oxygen can cross into blood while carbon dioxide crosses toward the air. The air and blood do not have to mix for their molecules to exchange.

Each gas diffuses according to its own partial-pressure difference: the pressure associated with that gas, rather than the combined pressure of all gases. Incoming blood has a lower oxygen partial pressure than air in the lung sacs, so oxygen has a net movement into blood. Carbon dioxide has a difference in the opposite direction. Breathing and circulating blood keep renewing the conditions on each side.

A cell membrane is selective in another way. Its oily interior makes passage easier for some molecules than for charged ions or many water-loving substances. Transport proteins provide particular routes. A dissolved substance has not disappeared: its particles remain available to move, but their movement depends on both their properties and the boundary they encounter.

A heat exchanger reveals why energy and matter need separate accounts. Two fluids flow on opposite sides of a conducting wall. Energy passes through that wall from the warmer side toward the cooler side while the fluids remain separated. A boundary can therefore allow gas diffusion, select molecular traffic or transfer energy without mixing fluids. Its structure determines which kind of exchange occurs.