According to Estrella Mountain Community College, people need to breathe because human cells require oxygen to use food energy efficiently and to release the carbon dioxide generated by that use. The lungs are designed to maximize their surface area as they fill with air in order to allow as much of the gases to diffuse into and out of the blood as possible.
Estrella Mountain Community College states that breathing is accomplished by changing the pressure in the lungs relative to atmospheric pressure. This is done by muscular movements of the rib cage and a large sheet of muscle known as the diaphragm, which act together to increase the volume of the chest cavity for inhalation and decrease it for exhalation.
According to Estrella Mountain Community College, during inhalation air is drawn through the mouth and nose into a tube called the trachea. The trachea branches into the bronchi, which travel to each lung, where they branch further into the bronchioles. These channels carry the air to clusters of tiny sacs known as alveoli. These rounded chambers are surrounded by capillaries filled with blood which constantly absorb oxygen from the air and release carbon dioxide into it. The red blood cells carry only oxygen, while the water in the blood carries the carbon dioxide.