This document discusses mechanisms of gas exchange in animals. It covers three main points:
1) Breathing involves exchanging carbon dioxide produced by cells for oxygen from the air through three phases: breathing, gas transport in blood, and exchange with tissues.
2) Animals exchange gases across moist surfaces like skin or specialized structures like gills, tracheal systems, or lungs.
3) In humans, air follows a branching path from nose to bronchi and bronchioles to reach alveoli in lungs where gas exchange occurs through diffusion across thin membranes.