Introduction

Every cell in the body constantly breaks down fuel molecules — glucose, amino acids, fatty acids — to release the energy it needs for its everyday activities. This breakdown depends on a steady supply of oxygen (O2), and it leaves behind carbon dioxide (CO2), a harmful by-product that has to be cleared away. So the body needs a continuous two-way exchange: O2 has to keep moving in from the atmosphere to reach the cells, and the CO2 the cells generate has to keep moving back out. This ongoing exchange of O2 from the air for CO2 from the cells is what breathing means physiologically — the everyday word for what is more formally called respiration.

It is something you can feel directly: rest a hand on your chest and notice it rise and fall with every breath. That visible movement is just the outward sign of a much bigger process happening inside — how the body actually organises this exchange of gases, starting with the organs that carry it out, is taken up in the sections that follow.