Mechanism of Inspiration
Driven by impulses from the respiratory center, the primary muscles of inspiration contract. The external intercostal muscles, which feature obliquely oriented fibers, elevate the ribs. Simultaneously, the diaphragm contracts and flattens.
As a result of these muscle contractions, the geometry of the thoracic cavity changes:
- Transverse and anteroposterior diameters increase.
- The thorax expands inferiorly.
Respiratory muscle energy is expended to overcome four types of resistance: the elastic resistance of the lungs and chest wall, the viscous resistance of displaced tissues, and the aerodynamic resistance to airflow.
Pressure Dynamics and Gas Exchange
Air movement is governed by Boyle's law: pressure and volume are inversely related ($P \times V = \text{const}$). An increase in thoracic cavity volume leads to lung expansion and a drop in internal pressure.
Alveolar pressure ($P_{al}$) This is the pressure inside the alveoli relative to atmospheric pressure.
- During inspiration (0–2 s): Pressure falls below atmospheric, reaching a peak rarefaction of -1 mm Hg midway through inspiration. Air is passively drawn into the alveoli along the gradient.
- During expiration (2–4 s): Pressure becomes positive (up to +1 mm Hg), driving air outward.
- At phase transition points (0, 2, 4 s): Airflow ceases, and $P_{al}$ equals 0.
Pleural pressure ($P_{pl}$) Pressure in the pleural space is always subatmospheric (negative). It is generated as the difference between atmospheric pressure and the elastic recoil of the lungs ($P_{atm} - P_{elastic\_recoil} = P_{negative}$).
- At rest (end-expiration), it is approximately -5 mm Hg.
- During inspiration, the chest wall expands, the elastic recoil of the lungs increases, and pressure drops to -8 to -9 mm Hg.
- During expiration, the thorax recoils, and values return to baseline (approximately -6 mm Hg).
Mechanism of Expiration
Expiration can be passive or active depending on metabolic demands.
- Quiet expiration is entirely passive. The thorax descends under its own weight, aided by the elastic recoil of lung tissue, abdominal organ pressure shifting the diaphragm upward, and the elastic recoil of cartilages stretched during inspiration.
- Active (forced) expiration is recruited during physical exercise or expiration against resistance (e.g., underwater). Passive forces are supplemented by muscle work: the internal intercostal muscles and abdominal wall muscles contract.
An important factor for structural stability of the lungs during expiration is alveolar interdependence: compression of larger alveoli causes tension in tethered smaller ones, preventing the collapse of small bronchi.
Airway Aerodynamics
Airflow rate ($Q$) is calculated using the formula: $Q = \Delta P / R$, where $\Delta P$ is the difference between atmospheric and alveolar pressure, and $R$ is aerodynamic resistance.
Interestingly, the primary resistance to airflow is generated in the upper respiratory tract. Although the lower airways have smaller individual calibers, their number multiplies exponentially with each successive generation of bronchial branching. Consequently, the total cross-sectional area of the lower respiratory divisions becomes so massive that their overall resistance drops significantly.