The human respiratory system is responsible for the exchange of carbon dioxide (CO2,) and oxygen (O2). Oxygen is essential in the production of adenosine triphosphate (ATP)—the nucleotide responsible for cell metabolism the "fuel" of the cell— by the mitochondria. Human beings cannot live without a properly functioning respiratory system unless some sort of mechanical intervention (e.g. ventilator, heart-lung bypass) is implemented.
The respiratory system is viewed in two parts: the upper respiratory tract and the lower respiratory tract. The system has built-in methods to prevent harmful substances from entering the lungs:
Upper Respiratory Tract
This part of the respiratory tract consists of the nose, oral cavity (mouth), pharynx, larynx (voice box), and trachea (wind pipe). Atmospheric air—which is mostly nitrogen— enters through the nose or oral cavity where it is filtered and humidified, passes through the structures of the throat, and then enters the lower respiratory tract. Upper respiratory tract infections (e.g. sinus infections, common cold, strep throat) are some of the most common infections in the world and are largely non-fatal.
Lower Respiratory Tract
This part of the respiratory tract consists the of the bronchi, bronchioles, and alveoli. All of these structures are contained within the lungs. The alveoli are responsible for the gas exchange mentioned above.
Breathing is an active process controlled by the hypothalamus. In humans, the diaphragm is the primary muscle of respiration. At rest, the diaphragm is a dome that projects into the chest. When it contracts, it flattens, and the volume of the chest cavity increases. This change in volume causes the pressure in the chest cavity to drop, and air moves into the lungs due to this pressure difference. Contraction and relaxation of the intercostal muscles, which stretch between the ribs, play a smaller role in respiration. The intercostal muscles can increase or decrease the volume of the chest depending on which specific muscles contract.
Gas is exchanged in the lungs by diffusion— the process of molecules moving from an area of high concentration to an area of low concentration. Upon inhalation, oxygen molecules are directed into the alveoli by the bronchioles. The oxygen then diffuses into the blood from the alveoli, and the CO2 molecules from the blood (a waste product of cellular respiration) enter the alveoli and are then exhaled. In order for this diffusion to occur, there must be a higher concentration of oxygen molecules in the alveolar air than in the blood, and a higher CO2 concentration in the blood than in the alveoli. This is achieved through constant respiration, bringing in air that is rich in oxygen and scant in CO2.