Oxygen is a vital requirement of animals. An animal may survive for days without water or for weeks without food, but life without oxygen is measured in minutes.

Delivering oxygen and removing carbon dioxide (the product of cellular respiration) are the two major functions of the respiratory system. The processes involved with these functions related to gases include ventilation (movement of air in and out of the lungs), gas exchange between air and blood in the lungs, gas transport in blood, and gas exchange between blood and cells at the level of the tissues.

Secondary functions of the respiratory system include assistance in the regulation of the pH of the body fluids, assistance in tem­perature control, and phonation (voice produc­tion). The role of the respiratory system in the regulation of the pH of blood and other body fluids is closely associated with the ability of the respiratory system to remove carbon dioxide. If carbon dioxide accumulates in the blood because the respiratory system cannot remove it, blood pH falls; this is respiratory acidosis. Blood pH rises if the respiratory system removes more carbon dioxide than is appropriate and blood levels of carbon dioxide are lower than normal; this is respiratory alka­losis. The changes in carbon dioxide and pH are closely linked because of the chemical reac­tion shown in the equation below. The bicar­bonate ions and water molecules for this reaction are readily available in body fluids.

H₂O + CO₂ θ H⁺ + HCO₃-

The respiratory system consists essentially of the lungs and the passages that conduct air into and out of the lungs. These passages include the nostrils, nasal cavity, pharynx, larynx, trachea, and bronchi.