Zoology · Ch 6 — Respiration
Mechanism of Breathing
Mechanism of Breathing
Breathing works on a simple pressure principle: air always flows from a region of higher pressure to a region of lower pressure. Inspiration happens when the pressure inside the lungs (intrapulmonary pressure) drops below atmospheric pressure, so air rushes in; expiration happens when intrapulmonary pressure rises above atmospheric pressure, so air is pushed out.
During inspiration, the diaphragm - a dome-shaped sheet of muscle separating the thorax from the abdomen - contracts and flattens, while the external intercostal muscles between the ribs contract and pull the ribs and sternum upward and outward. Together these movements enlarge the thoracic cavity, which drops the pressure inside the lungs below atmospheric pressure and draws fresh air in.
Expiration, at rest, is largely a passive process: the diaphragm and external intercostal muscles simply relax. The diaphragm domes back upward and the internal intercostal muscles pull the ribs down, shrinking the thoracic cavity and raising intrapulmonary pressure slightly above atmospheric pressure, which pushes air back out.
| Event | Inspiration | Expiration |
|---|---|---|
| Respiratory centre | Initiates the stimulus | Terminates the stimulus |
| Diaphragm & inspiratory muscles | Contract | Relax |
| Thoracic volume | Increases as the chest wall expands | Decreases as the chest wall contracts |
| Alveolar vs atmospheric pressure | Alveolar pressure falls below atmospheric | Alveolar pressure rises above atmospheric |
| Result | Air flows in until pressures equalise; alveoli inflate | Air flows out until pressures equalise; alveoli deflate |
What this figure shows. A paired diagram comparing inspiration and expiration: on inhalation the rib cage expands and the diaphragm flattens/moves down as air is drawn into the lungs; on exhalation the rib cage becomes smaller and the diaphragm domes back upward as air is expe …