The lungs are part of the respiratory system. A good way to understand the workings of the lungs is to first get an overview of their structure, or anatomy. The human respiratory system begins at the nose and includes the nasal passages, which allow air to pass by the back of the throat and to enter the windpipe, or trachea. The trachea sits below the voice box (larynx), and can be felt in the front of your neck as it descends behind the breastbone (sternum) into the upper chest. The trachea divides into two branches: the right mainstem bronchus and the left mainstem bronchus. The right mainstem bronchus leads air to and from the right lung, and the left mainstem bronchus leads air to and from the left lung.
The area where the trachea divides into the right and left mainstem bronchi is called the carina. After the split, the right and left mainstem bronchi leading to each lung subdivide further into smaller and smaller tube-like passages, via the branching tracheobronchial tree. As the bronchi continue to subdivide into successively narrower and narrower bronchi, they ultimately end in the tiniest subdivision, the bronchiole. Each bronchiole leads to the lung air sacs, the alveoli.
The alveoli are specialized lung structures. They allow for fresh, oxygen-rich (O2) inhaled air to enter the body, and for oxygen-poor, carbon dioxide-rich (CO2) air to exit (Figure IB). Oxygen is required for life; oxygen deprivation is rapidly fatal. As oxygen is provided to the body’s organs via the lungs, “used” air — comprised mostly of carbon dioxide (CO2) — is excreted by exhalation. Carbon dioxide is produced by the body’s metabolism and is considered a “waste product.”
Abnormal accumulation of carbon dioxide in the body and the bloodstream is detrimental to health and is seen in certain forms of respiratory failure. The process that is responsible for the body’s oxygen uptake and its carbon dioxide removal (or excretion) is called respiration. Respiration is the primary, crucial function of the lungs and respiratory system. Physicians occasionally refer to respiration as “gas exchange.” Gas exchange takes place in the deepest lung, at the level of the alveoli. Oxygen and carbon dioxide exchange takes place along a specialized zone where each air sac (alveolus) is in intimate contact with fine, minute blood vessels called capillaries. The capillary bed completely encircles the alveoli along the alveolar-capillary membrane. Elevations in blood carbon dioxide levels above normal lead to additional accumulation of acids in the bloodstream. As acids continue to build up in the body, respiratory failure can ensue.