Breathing is the mechanical process of inhaling and exhaling.  The diaphragm, a dome-shaped muscle, contracts and relaxes changing the size of the chest cavity.  This draws air into the lungs and forces air out of the lungs.

Respiration occurs at the cellular level.  Mitochondria use oxygen in the production of energy  (ATP) from glucose.  Carbon dioxide is a toxic waste product of respiration. 

The body obtains oxygen by inhaling air into the lungs. The oxygen molecules diffuse across the membranes of the alveoli into capillaries.  Red blood cells carry the oxygen to body cells.  The carbon dioxide is carried by blood back to the lungs where it is released by diffusion across the alveoli.  Once inside the lungs, the carbon dioxide is exhaled. (Find out more about red blood cells.)

The nasal cavities and sinuses moisten and warm air that is inhaled through the nose.  The pharynx is the back of the throat.  The larynx contains the vocal cords.  That is why the larynx is also called the voice box.

A flap of tissue called the epiglottis closes the trachea when food is swallowed and closes the esophagus when air is inhaled.  Air passes through the trachea before entering the bronchi.  The bronchi (plural) are two tubes, one entering each lung.  In the lungs a bronchus (singular) becomes tiny branches called the bronchioles.  The bronchioles end in tiny thin walled sacs called alveoli.  Alveoli are often described as looking like a bunch of grapes.