Sequence of Events in One Complete Breath Cycle
INSPIRATION PHASE:
- Medullary rhythmicity centre fires → nerve impulses travel to inspiratory muscles
- Diaphragm receives signal → contracts, moves downward and flattens
- External intercostal muscles receive signal → contract, lift ribs upward and outward
- Thoracic cavity volume increases (in all three dimensions)
- Lung tissue stretches → intra-pulmonary pressure falls below atmospheric (~–1 to –3 mmHg)
- Pressure gradient established: atmosphere (higher) → lungs (lower)
- Air flows through: external nares → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
- Alveoli expand with fresh air (pO2 = 104 mmHg, pCO2 = 40 mmHg)
- Gas exchange begins at alveolar-capillary interface (O2 into blood, CO2 out)
EXPIRATION PHASE (quiet): 10. Pneumotaxic centre in pons sends inhibitory signals → inspiratory centre quietens 11. Diaphragm and external intercostals RELAX 12. Elastic recoil of lung tissue compresses alveoli → intra-pulmonary pressure rises above atmospheric 13. Pressure gradient reversed: lungs (higher) → atmosphere (lower) 14. Air flows OUT: alveoli → bronchioles → bronchi → trachea → larynx → pharynx → nasal cavity → external nares 15. CO2 is exhaled; O2-enriched blood leaves alveolar capillaries
GAS TRANSPORT CYCLE (simultaneous): 16. O2 loads onto haemoglobin in alveolar capillaries → oxyhaemoglobin formed 17. Oxygenated blood travels via pulmonary veins → left atrium → left ventricle → systemic circulation 18. Blood reaches tissue capillaries (pO2 ~40 mmHg) → Bohr effect → O2 released from Hb 19. O2 diffuses into tissues; CO2 produced by metabolism enters blood 20. CO2 processed in RBCs (carbonic anhydrase → HCO3–, chloride shift); some binds Hb (carbaminoHb) 21. Deoxygenated blood returns via systemic veins → right heart → pulmonary circulation → back to Step 9