What is The mammalian lung?

A lung is an invagination of the body surface into a moist internal chamber, which keeps the surface wet in dry air. The mammalian lung is a tree of airways ending in some three hundred million alveoli, sacs $0.2\,\mathrm{mm}$ across whose walls — one epithelial cell and one capillary endothelium, $0.5\,\text{µ}\mathrm{m}$ in all — add up to $100\,\mathrm{m}^{2}$ in a human, wrapped in a capillary net through which the whole cardiac output passes. It is ventilated tidally: the diaphragm and rib muscles enlarge the chest, air flows in; they relax, air flows out by the same path. A breath moves $500\,\mathrm{mL}$, of which $150\,\mathrm{mL}$ only fills the airways (dead space) and never reaches an alveolus; the alveolar gas, renewed by a seventh at each breath, stays at $P_{\mathrm{O_2}} = 13.3\,\mathrm{kPa}$ and $P_{\mathrm{CO_2}} = 5.3\,\mathrm{kPa}$ — the pressures the blood equilibrates with.