All animals require oxygen. Oxygen enables living things to metabolize (or burn) nutrients, which releases the energy they need to grow, reproduce, and maintain life processes. Some animals can exist for months on fats or other foods stored in their bodies, and many can live a shorter time without water. Yet few can survive for long without oxygen because little can be stored in the body. Most animals obtain oxygen from their environment. It is generally believed that life originated in the oceans, where many animals still live, obtaining their oxygen in a dissolved form from the water. In the course of evolution, various animals have become earth dwellers and have developed structures that allow them to breathe air.

The human respiratory system. Redrawn from Johnson, 1998.

Along with supplying oxygen, the respiratory system assists in the removal of carbon dioxide, preventing a dangerous and potentially lethal...