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.
Anatomy
The respiratory system is viewed in two parts: the upper respiratory tract and the lower respiratory tract.
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, passes through the structures of the throat, and then enters the lower respiratory tract. Upper respiratory 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.
Physiology
This section contains information on the mechanism of respiration and gas exchange. For further information on systemic oxygenation, see pulmonary circulation.[1]
Breathing is an active process controlled by the hypothalamus[2] There are muscles between the ribs known as intercostal muscles which apply pressure into the lungs for exhalation, and relax for inhalation. The diaphragm also plays an integral part in respiration.
The process of gas exchange that occurs within the lungs is 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 is then diffused into the blood from the alveoli, and the CO2 molecules from the blood (a waste product of cellular respiration) enter into the alveoli and are then exhaled. In order for the diffusion to work, there must be a higher saturation of oxygen molecules in the alveoli, and a lower CO2 saturation in the blood. This is achieved through constant respiration, bringing in air that is rich in oxygen and scant in CO2.