The overarching purpose of the human respiratory system is to complete gas exchange for respiration. The organs that comprise the respiratory function work together to take in oxygen and expel carbon dioxide: oxygen is necessary for maintaining life, while carbon dioxide is a waste product of aerobic respiration. There are several parts of the respiratory system that carry out this central function.
The first part of the respiratory system is located around the face. When breathing, oxygen enters either the nose or mouth and then flows through the sinuses, which are vital for maintaining and regulating the temperature and humidity of the air destined for the lungs. The trachea filters the inhaled air to help prevent unwanted particles and debris getting into the lungs. This branches into two bronchi, the main tubes that carry inhaled air towards the lung. These also act as a filtering system: the bronchi are lined with cilia that propel mucus up and out of the lungs. This mucus is filled with unwanted particles, including bacteria and other parasites, that may be present in inhaled air.

The air then travels through to the lungs themselves, which are split into lobes. These lobes are filled with alveoli, which help to expand the surface area of the lungs and facilitate the exchange of oxygen and carbon dioxide from the cardiovascular system. The walls of the alveoli are a mixture of epithelial cells and capillaries which carry blood. These capillaries carry carbon dioxide to the lungs and oxygen away from it: this oxygenated blood is pumped through the body by the heart to provide tissues with oxygen.

The diaphragm is another part of the respiratory system that is not directly linked to the rest of the system. The purpose of the diaphragm is to assist with breathing and to separate the respiratory system from the digestive system.