Cells require oxygen for glucose metabolism and release carbon dioxide as a byproduct. This process requires constant gas exchange between the human body and the environment to replenish the oxygen supply and remove carbon dioxide. This exchange is accomplished through the efforts of the respiratory system, in which powerful muscles force oxygen-rich air into the lungs and carbon dioxide-rich air out of the body.
Gas exchange takes place in the lungs. Humans have two lungs, a right and a left, with the right being slightly larger than the left due to the heart’s placement in the left side of the chest cavity. The right lung has three lobes, and the left has two. The lungs are surrounded by a thick membrane called the pleura.
In anatomy, the terms right and left are used with respect to the subject, not the observer.
Air enters the body through the mouth or nares (the opening of the nasal cavity) and passes through the trachea (sometimes called the windpipe) and into the two bronchi, each of which leads to one lung. Within the lung, the bronchi branch into smaller passageways called bronchioles and then terminate in sac-like structures called alveoli, which is where gas exchange between the air and the capillaries occurs. The large surface area of the alveoli allows for efficient exchange of gases through diffusion (movement of particles from areas of high to low concentration). Alveoli are covered in a layer of surfactant, which lubricates the sacs and prevents the lungs from collapsing.
The diaphragm contributes to the activity of ventilation—the process of inhalation and exhalation. The contraction of the diaphragm creates a vacuum, forcing air into the lungs. Relaxation of the diaphragm compresses the lungs, forcing carbon dioxide-enriched gas out in exhalation. The amount of air breathed in and out is the tidal volume, and the residual capacity is the small volume of air left in the lungs after exhalation.
Respiratory disorders can be caused by infection or by pathophysiological processes that damage respiratory tract tissues.
A bronchospasm is a sudden constriction of the bronchioles that restricts airflow, most commonly caused by asthma or COPD. They are treated with inhaled albuterol (which reopens the airway).