Pulmonary ventilation, or breathing, is the process of moving air into and out of the lungs. This continuous exchange of gases supplies the body with oxygen and removes carbon dioxide. Air moves between the atmosphere and the lungs solely because of pressure differences, flowing naturally from a region of higher pressure to one of lower pressure. The respiratory system is functionally divided into two distinct zones, which represent the sequential path the air travels.
The Conducting Zone
The initial stage of air passage involves the conducting zone, which transports air deep into the lungs. This zone begins with the nasal and oral cavities and continues through the pharynx, larynx, and trachea. These structures ensure air reaches the specialized areas for gas exchange.
The primary function of the conducting zone is to condition the air before it reaches the delicate lung tissue. As air travels through the nasal passages, it is warmed, humidified, and filtered of debris and pathogens. This preparation is accomplished by the respiratory mucosa lining the airways, which uses mucus to trap particles and cilia to move the contaminated mucus upward to be swallowed.
The trachea, supported by C-shaped rings of cartilage, splits into the right and left primary bronchi, which enter the respective lungs. These primary bronchi progressively branch into smaller secondary and tertiary bronchi, eventually leading to fine tubes called terminal bronchioles. This extensive branching pattern is often referred to as the bronchial tree. Notably, no gas exchange occurs throughout the conducting zone, as its purpose is solely to deliver clean, warm, and moist air to the next zone.
The Respiratory Zone
The air passage ends in the respiratory zone, where the exchange of gases takes place. This zone begins with the respiratory bronchioles, which are extensions of the terminal bronchioles, and continues through the alveolar ducts. These ducts terminate in alveolar sacs, which are clusters of tiny air sacs called alveoli.
Each individual alveolus is a cup-shaped structure with elastic walls that stretch during inhalation. The walls are composed primarily of Type I alveolar cells, forming an extremely thin simple squamous epithelium. Interspersed among these are Type II alveolar cells, which secrete pulmonary surfactant to lower surface tension and help maintain the shape of the alveoli.
The alveoli are densely wrapped in a network of capillaries, forming the respiratory membrane. This membrane, which is the barrier between the air and the blood, is extremely thin, measuring about 0.5 micrometers thick. This minimal distance allows oxygen (O2) to rapidly diffuse from the air inside the alveoli into the blood, while carbon dioxide (CO2) diffuses from the blood into the alveoli to be exhaled.
The Mechanics of Breathing
The physical act that moves air through both the conducting and respiratory zones relies on generating pressure gradients. This process is governed by Boyle’s Law, which states that the pressure of a gas is inversely proportional to its volume, meaning an increase in lung volume decreases the internal pressure. Air movement depends on the difference between the atmospheric pressure outside the body and the intra-alveolar pressure inside the lungs.
Inspiration, or breathing in, is an active process initiated by the contraction of respiratory muscles. The diaphragm, the primary muscle of breathing, contracts and moves downward, significantly increasing the vertical dimension of the thoracic cavity. Concurrently, the external intercostal muscles contract, pulling the ribs upward and outward, which expands the chest’s anterior-posterior dimension.
The expansion of the thoracic cavity forces the lungs to stretch and expand due to the cohesive connection maintained by the pleural fluid. This increase in lung volume lowers the intra-alveolar pressure to a value slightly below the atmospheric pressure. The pressure gradient causes air to rush into the lungs until the pressure equalizes.
Normal expiration, or breathing out, is typically a passive process that does not require muscle contraction. The diaphragm and external intercostal muscles simply relax, returning to their resting positions. This muscle relaxation, combined with the natural elastic recoil of the stretched lung tissue, reduces the volume of the thoracic cavity.
The decrease in lung volume compresses the air inside, which raises the intra-alveolar pressure to a point slightly higher than the atmospheric pressure. This reversed pressure gradient immediately forces the air to move out of the lungs. Forced expiration, such as during exercise, becomes an active process, engaging accessory muscles like the abdominal muscles to push the diaphragm upward and further decrease the chest volume.