The main function of the respiratory system is to ensure that oxygen is brought to each cell in the body and that carbon dioxide leaves each cell and is removed from the body. Respiration is the general term that is used to describe this overall process.

Each breath contains an invisible mixture of dust, fungi, bacteria, water, nitrogen, oxygen, and carbon dioxide that enters the respiratory tract. Mucus and cilia in the upper respiratory tract trap most of the dust, bacteria, and other particles before the mixture of air reaches the bronchioles. Coughing and sneezing build strong gusts of air that force out many harmful items from the respiratory tract. 

Oxygen from the atmosphere passes over warm, moist membranes in the nose and mouth, warming the air to a temperature that will not harm the lungs.

When oxygen reaches the lungs, more than a quarter-of-a-billion alveoli facilitate gas exchange in the lungs. It is here that oxygen enters the circulatory system in exchange for carbon dioxide. The carbon dioxide then makes the reverse journey out of the body.

Read

Read "Structures of the Respiratory System" on pages 244 to 248 of your textbook. Use the Unit D printable materials to label the structures and functions of the respiratory system.

Watch

Watch the following animation that shows the voyage of a molecule of oxygen through the structures that make up the respiratory passageways and the subsequent return of carbon dioxide to the atmosphere.