Mechanism of inspiration
Under the action of impulses from the respiratory center, the primary inspiratory muscles contract. External intercostal muscles, which have obliquely directed fibers, elevate the ribs (the moment of force is smaller at the upper rib than at the lower one). Simultaneously, the diaphragm contracts and flattens.
As a result of these muscular contractions, the geometry of the chest changes:
- Transverse and anteroposterior diameters increase.
- The chest expands in the inferior direction.
The energy of the respiratory muscles is expended to overcome four types of resistance: the elastic resistance of the lungs and chest wall themselves, the viscous resistance of displaced tissues, and the aerodynamic resistance to airflow.
Pressure dynamics and gas exchange
Air movement is based on Boyle's law: pressure and volume are inversely related ($P \times V = \text{const}$). An increase in the volume of the thoracic cavity leads to lung expansion and a drop in pressure within them.
Alveolar pressure ($P_{al}$) This is the pressure inside the alveoli relative to atmospheric pressure.
- During inspiration (0–2 s): pressure drops below atmospheric, reaching a peak negative pressure of -1 mm Hg in the middle of inspiration. Air is passively drawn into the alveoli along the gradient.
- During expiration (2–4 s): pressure becomes positive (up to +1 mm Hg), pushing air outward.
- At phase transition moments (0, 2, 4 s): airflow is absent, and $P_{al}$ equals 0.
Pleural pressure ($P_{pl}$) The pressure in the pleural cavity is always subatmospheric (negative). It is formed as the difference between atmospheric pressure and the elastic recoil of the lungs ($P_{atm} - P_{el.recoil} = P_{neg}$).
- At rest (end of expiration), it is about -5 mm Hg.
- During inspiration, the chest expands, the elastic recoil of the lungs increases, and pressure drops to -8 to -9 mm Hg.
- During expiration, the chest collapses, and values return to baseline (about -6 mm Hg).
Mechanism of expiration
Expiration can be passive or active depending on the body's metabolic demands.
- Quiet expiration occurs entirely passively. The chest lowers under its own weight. It is aided by the elastic recoil of lung tissue, the pressure of abdominal organs shifting the diaphragm upward, and the elastic recoil of cartilages that were stretched during inspiration.
- Active (forced) expiration is engaged during physical exertion or expiration underwater. Muscle work is added to the passive forces: the internal intercostal muscles and abdominal muscles contract.
An important factor for the structural stability of the lungs during expiration is alveolar interdependence: the compression of larger alveoli causes the stretching of smaller ones connected to them, preventing the lumens of small bronchi from collapsing.
Airway aerodynamics
Airflow velocity ($Q$) is calculated by 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 formed in the upper respiratory tract. Despite the lower respiratory tract being narrower, their number increases manifold with each new level of bronchial branching. As a result, the total cross-sectional area of the lower regions becomes so large that resistance in them decreases significantly.
Briefly: answers to topical questions
What role does surfactant play in respiratory biomechanics?
Surfactant stabilizes the condition of the alveoli and participates in the mechanics of their expansion, preventing them from sticking together.
The mechanism of action depends on the phase of respiration:
- During inspiration — molecules move apart, surface tension increases, which ensures the alveoli are protected from overstretching.
- During expiration — molecules lie close together, surface tension decreases, which ensures the alveoli are protected from collapse.
In addition, this surface-active substance facilitates the diffusion of oxygen into the blood.
Pleural pressure is always negative (pulling the lungs along with the chest wall), while alveolar pressure changes sign: negative during inspiration (drawing air in), positive during expiration (pushing air out).
Class and exam questions
Why is pleural pressure negative?
This is due to the specifics of ontogenesis. During growth, the chest cavity increases in size faster than the lung tissue. Because of this discrepancy, the lungs are constantly in a stretched state (elastic recoil tends to compress them), which forms the subatmospheric pressure between the pleural layers.
What happens to alveolar pressure during pauses between inspiration and expiration?
At the moments of respiratory phase transitions (at 0, 2, and 4 seconds of the cycle), when airflow is completely absent, alveolar pressure equilibrates with atmospheric pressure and becomes zero.
Where in the respiratory system is the highest aerodynamic resistance?
The main aerodynamic resistance is localized in the upper respiratory tract. In the lower sections, due to the enormous number of branching bronchi, the total cross-sectional area increases sharply, and resistance drops.
What else to study on this topic
- Müller's maneuver and extreme pleural pressure values
- Graphs of respiratory volume dynamics and airflow
- Alveolar interdependence and maintenance of bronchial patency
- Types of respiratory resistance (elastic, viscous, and aerodynamic)
- Age-related features of intrapleural pressure formation
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