The process of respiration has four stages: Pulmonary ventilation (breathing - moving air in and out), External (Pulmonary) Respiration (gas exchange between alveoli and pulmonary capillaries - O2 enters blood, CO2 leaves blood), Gas transport (O2 and CO2 transported by blood), Internal (Tissue) Respiration (gas exchange between systemic capillaries and body cells - O2 leaves blood, CO2 enters blood), and Cellular Respiration (O2 used by mitochondria to produce ATP/energy). External and internal respiration both occur by diffusion along partial pressure gradients.

The Cough Reflex is a vital protective mechanism that clears the airway of irritants and foreign material. Mechanism: 1) Irritant stimulus detected by receptors in larynx, trachea, bronchi (vagus nerve). 2) Deep inspiration - fills lungs. 3) Glottis closes - intrathoracic pressure rises dramatically (up to 300 mmHg). 4) Glottis suddenly opens - explosive expiration - air speed up to 960 km/h (267 m/s). 5) Irritant expelled along with mucus. A persistent dry cough is a common side effect of ACE inhibitor medications (lisinopril, ramipril) - due to bradykinin accumulation.

Carbon dioxide is transported in three ways: Bicarbonate ions (HCO3-) = ~70% (CO2 enters RBCs, combines with H2O catalysed by carbonic anhydrase, forms H2CO3, dissociates to H+ + HCO3-, HCO3- moves into plasma via chloride shift). This is the most common method. Carbaminohaemoglobin = ~23% (CO2 binds to globin portion of Hb - NOT the haem/iron part). Dissolved in plasma = ~7% (small amount dissolved directly). The chloride shift (Hamburger shift) maintains electrical neutrality when HCO3- exits RBCs.

Pulmonary Fibrosis is a condition where the lung tissue becomes scarred (fibrosed) and thickened, making it stiff and difficult to breathe. Types: Idiopathic Pulmonary Fibrosis (IPF) - unknown cause, most common, affects elderly males, poor prognosis. Secondary causes: Asbestosis, silicosis (occupational), drug-induced (methotrexate, amiodarone), radiation, autoimmune (sarcoidosis, rheumatoid arthritis). Spirometry shows a restrictive pattern: reduced FVC, normal or elevated FEV1/FVC ratio. Characteristic: Velcro crackles on auscultation; finger clubbing. Treatment: pirfenidone, nintedanib (anti-fibrotic drugs), lung transplant.

The Bohr Effect (described by Christian Bohr, 1904) states that increased CO2 and decreased pH (acidity) cause haemoglobin to release O2 more readily - i.e., the oxygen dissociation curve shifts to the RIGHT. This is physiologically important at the tissue level: actively respiring tissues produce CO2 and acid (lactic acid), Bohr effect, Hb releases O2 to the tissues that need it most. Right shift is also caused by: increased temperature, increased 2,3-DPG. Left shift (increased affinity, less O2 release): decreased CO2, alkalosis, decreased temperature, fetal Hb (HbF).

Sneezing (Sternutation) is a protective reflex triggered by irritation of the nasal mucosa (by dust, allergens, pepper, viruses, bright light - photic sneeze reflex). Mechanism: similar to cough - deep inspiration, glottis closes, pressure builds, explosive exhalation through nose AND mouth expelling the irritant. A sneeze can propel droplets at 160 km/h, up to 8 metres. It is NOT possible to keep eyes open during a sneeze (involuntary). A sneeze cannot be suppressed while closing mouth and nose - this can damage middle ear, sinuses, or blood vessels.

Carbonic Anhydrase (CA) is an enzyme found in red blood cells that catalyses the reversible reaction: CO2 + H2O = H2CO3 = H+ + HCO3-. In tissues: CO2 produced, enters RBCs, CA converts CO2 to HCO3-, transported to lungs. In lungs: Reaction reverses, HCO3- converted back to CO2, CO2 exhaled. Without carbonic anhydrase, this reaction would be too slow for efficient CO2 transport. The enzyme speeds the reaction by a factor of 10 million. Carbonic anhydrase inhibitors (e.g., acetazolamide) are used as diuretics and to treat altitude sickness.

Obstructive vs Restrictive Lung Disease: Obstructive = airflow is obstructed (narrowed airways), air cannot be exhaled properly, air trapping, FEV1 reduced, FEV1/FVC ratio <70% (reduced). Examples: Asthma, COPD, Emphysema, Chronic Bronchitis. Treatment: bronchodilators. Restrictive = lung volumes are reduced (lungs cannot fully expand), reduced TLC and FVC, FEV1/FVC ratio normal or elevated. Examples: Pulmonary fibrosis, pleural effusion, scoliosis, obesity, sarcoidosis, pneumonia. Treatment: treat underlying cause, sometimes steroids.

Altitude Sickness (Acute Mountain Sickness - AMS) occurs at high altitudes (above 2500 m) where atmospheric pressure is lower, resulting in lower partial pressure of O2 (though O2 percentage remains 21%). Effects on breathing: Hypoxia triggers hyperventilation (faster breathing). Hyperventilation blows off CO2 causing respiratory alkalosis (blood becomes too alkaline). Symptoms: headache, nausea, dizziness, fatigue, breathlessness. Serious complications: High Altitude Pulmonary Oedema (HAPE), High Altitude Cerebral Oedema (HACE). Acclimatisation: increased RBC production (polycythaemia), increased 2,3-DPG. Treatment: descent, oxygen, acetazolamide (Diamox).

Bronchoscopy is a procedure where a flexible or rigid bronchoscope (fibre-optic camera tube) is passed through the nose or mouth, down the throat, through the vocal cords, and into the bronchi to directly visualise the airways. Uses: Diagnosis (investigate haemoptysis, suspicious lesions, recurrent pneumonia, foreign body), Biopsy (take tissue samples for histology - lung cancer diagnosis), Treatment (remove foreign bodies, control bleeding, place stents, deliver medications), and BAL (Bronchoalveolar Lavage - wash and collect cells from alveoli for analysis). Laryngoscopy = visualises larynx; Thoracoscopy = visualises inside chest cavity.