The lungs are the central organs of the respiratory system, responsible for the exchange of gases between the atmosphere and the bloodstream. Located on either side of the heart, these spongy, air-filled organs facilitate the intake of oxygen and the removal of carbon dioxide. This process is fundamental for cellular function throughout the body.
Major Air Passageways
The journey of air into the lungs begins with the trachea, commonly known as the windpipe. This tube is reinforced with C-shaped rings of cartilage, which provide support and prevent it from collapsing. The trachea descends into the chest and divides into two main branches: the right and left primary bronchi. Each primary bronchus enters a lung, marking the beginning of the bronchial tree.
This branching pattern continues within the lungs, resembling an inverted tree. The primary bronchi split into smaller secondary bronchi, which in turn divide into even smaller tertiary bronchi. This network distributes inhaled air throughout the lungs. The walls of these larger airways are also lined with cilia and mucus to trap and remove inhaled particles.
Microscopic Structures for Gas Exchange
From the tertiary bronchi, the airways continue to narrow, becoming microscopic tubes called bronchioles. These are the final branches of the conducting zone, leading air into the sites of gas exchange. Unlike the larger bronchi, bronchioles have walls that are less than one millimeter in diameter and lack cartilage, relying on smooth muscle for their shape. This transition marks the shift from air conduction to the respiratory zone.
At the end of the bronchioles are clusters of tiny, balloon-like air sacs known as alveoli. Humans have between 300 to 500 million alveoli, providing a large surface area for gas exchange. The walls of each alveolus are exceptionally thin, consisting of a single layer of epithelial cells. This thin barrier is in direct contact with a dense network of capillaries.
Across this thin barrier, oxygen from inhaled air diffuses from the alveoli into the blood. Simultaneously, carbon dioxide diffuses from the blood into the alveoli to be exhaled. This efficient exchange is driven by the difference in gas concentrations between the air and the blood.
Lung Lobes and Protective Linings
The lungs are not symmetrical. The right lung is divided into three sections, or lobes: the superior, middle, and inferior. In contrast, the left lung has only two lobes, a superior and an inferior one, to accommodate the heart. These lobes are separated by deep grooves called fissures.
Each lung is enclosed within a two-layered membrane called the pleura. The visceral pleura is the inner layer that covers the lungs, while the parietal pleura is the outer layer that lines the chest wall. Between these two layers is the pleural cavity, which contains a thin film of pleural fluid. This fluid acts as a lubricant, allowing the lungs to expand and contract smoothly during breathing.
Breathing is powered by the diaphragm, a large muscle below the lungs. When the diaphragm contracts, it flattens and moves downward, increasing the volume of the chest cavity and drawing air into the lungs. When it relaxes, it moves upward, decreasing the thoracic volume and pushing air out.