Ventilation is the process of moving air into and out of the lungs. In humans, this is achieved by Negative Pressure Breathing — the lungs are inflated by creating a pressure lower than atmospheric pressure inside the thoracic cavity.
Ventilation is governed by Boyle's Law:
At constant temperature, pressure and volume are inversely related. When the thoracic cavity expands, internal pressure falls; when it contracts, internal pressure rises. Air always flows from high pressure to low pressure.
Inhalation is an active process requiring muscular contraction.
| Structure | Action | Effect |
|---|---|---|
| Diaphragm | Contracts → flattens and moves downward | Increases vertical dimension of thorax |
| External intercostal muscles | Contract → pull ribs upward and outward | Increases lateral and anterior-posterior dimensions |
Result: Thoracic volume increases → intra-pulmonary pressure drops to ~758 mmHg (below atmospheric ~760 mmHg) → air flows into the lungs.
At rest, exhalation requires no muscular effort:
During exercise or forced breathing, additional muscles are recruited:
Both actions rapidly decrease thoracic volume, expelling air quickly.
The lungs are enclosed by two pleural membranes:
Between them is the pleural cavity containing pleural fluid, which:
Clinical note: If air enters the pleural cavity (pneumothorax), intrapleural pressure equalises with atmospheric pressure and the lung collapses.
Surfactant is a phospholipid mixture secreted by type II pneumocytes lining the alveoli. It:
| Phase | Muscles Active | Thoracic Volume | Intra-pulmonary Pressure | Air Movement |
|---|---|---|---|---|
| Inhalation | Diaphragm + External intercostals | Increases | Decreases (<760 mmHg) | Into lungs |
| Quiet exhalation | None (passive recoil) | Decreases | Increases (>760 mmHg) | Out of lungs |
| Forced exhalation | Internal intercostals + Abdominals | Decreases rapidly | Increases rapidly | Out of lungs |