Free Mammalian Ventilation revision notes for OCR A Level Biology – covering specification points 3.1.1(c), 3.1.1(d) and 3.1.1(e).
Ventilation (Breathing) In Mammals
Ventilation enables the effective exchange of oxygen and carbon dioxide in the alveoli by taking in oxygen and removing waste carbon dioxide; this maintains a concentration gradient to maximise diffusion.
The table below compares the events of inhalation and exhalation:
| Step | Inhalation | Exhalation |
|---|---|---|
| 1 | The diaphragm contracts and flattens | Diaphragm relaxes: returns to a dome shape |
| 2 | External intercostal muscles contract, pulling the ribs up and out | External intercostal muscles relax, allowing ribs to move down and in |
| 3 | The thoracic cavity volume increases | The thoracic cavity volume decreases |
| 4 | Pulmonary lung pressure decreases below atmospheric pressure (negative pressure) | Pulmonary lung pressure increases above atmospheric pressure (positive pressure) |
| 5 | Air flows into the lungs down the pressure gradient | Air flows out of the lungs down the pressure gradient |
| (Forced only) | Internal intercostal muscles relax (inactive during normal inhalation) | Internal intercostal muscles contract (during forced exhalation), pulling ribs down and in further, decreasing thoracic volume more rapidly |
Forced inhalation and forced exhalation are active breathing processes involving additional muscle groups beyond those used in normal, quiet breathing, such as during exercise, singing or coughing.
The table below outlines the key changes which occur during inhalation and exhalation:
| Change | Inhalation | Exhalation |
|---|---|---|
| Diaphragm | Contracts | Relaxes |
| External intercostals | Contract | Relax |
| Internal intercostals | Relax (inactive) | Contract (when forced) |
| Thoracic volume | Increases | Decreases |
| Pulmonary pressure | Decreases (below atmospheric) | Increases (above atmospheric) |
| Air movement | Into lungs | Out of lungs |
