Alveoli (singular: alveolus) are the tiny, grape-like air sacs at the end of the bronchial tree where gas exchange takes place. Each lung contains approximately 300-500 million alveoli. Total surface area is approximately 70-80 m2 (size of a tennis court). Walls are just one cell thick (type I pneumocytes) - allowing rapid diffusion. Type II pneumocytes produce surfactant - reduces surface tension and prevents alveolar collapse. Surrounded by a rich network of pulmonary capillaries.

Vital Capacity (VC) is the maximum volume of air that can be exhaled after taking the deepest possible breath. VC = Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) + Expiratory Reserve Volume (ERV) = 500 + 3000 + 1200 = ~4700 mL (~4.7 litres). Vital capacity is reduced in conditions like pneumonia, pulmonary fibrosis, pleural effusion. It is higher in trained athletes. Spirometry is the test used to measure lung volumes including vital capacity - important for diagnosing respiratory diseases like COPD and asthma.

Pleurisy (Pleuritis) is inflammation of the pleura (the double-layered membrane surrounding the lungs). The inflamed pleural layers rub against each other causing sharp, stabbing chest pain that typically worsens with breathing, coughing, or sneezing - called pleuritic chest pain. Causes: Viral infections (most common - influenza, COVID-19), Bacterial pneumonia, Tuberculosis, Pulmonary embolism, Autoimmune diseases (lupus, rheumatoid arthritis). Treatment: pain relief (NSAIDs), treating the underlying cause.

Pulmonary Surfactant is a complex mixture of phospholipids and proteins secreted by Type II alveolar cells (Type II pneumocytes). Its main function is to reduce surface tension within the alveoli, preventing them from collapsing (atelectasis) at the end of each breath. Without surfactant, the alveoli would collapse due to high surface tension of water lining. Neonatal Respiratory Distress Syndrome (NRDS) occurs in premature babies due to insufficient surfactant production, causing respiratory failure at birth.

Residual Volume (RV) is the approximately 1200 mL of air that always remains in the lungs even after the most forceful exhalation. It cannot be removed by breathing. Its importance: Prevents the lungs from completely collapsing between breaths, Allows continuous gas exchange even between breaths, and Keeps alveoli open (along with surfactant). Residual volume cannot be measured by simple spirometry - requires special techniques like helium dilution or body plethysmography. RV increases in emphysema (hyperinflation) and decreases in pulmonary fibrosis.

The respiratory tract is lined with ciliated epithelium. The cilia (tiny hair-like projections) beat rhythmically (10-20 times per second) in a coordinated wave-like motion, sweeping the mucus blanket (with trapped particles, bacteria, and debris) upward toward the pharynx where it is swallowed or expectorated. This is the Mucociliary Escalator (Mucociliary Clearance) - a key defence mechanism of the lungs. Cigarette smoke paralyses and destroys cilia - impaired clearance - increased infections and mucus accumulation (smoker's cough).

The Diaphragm is the primary muscle of respiration - a dome-shaped, flat muscle that forms the floor of the thoracic cavity and separates it from the abdominal cavity. During inhalation: the diaphragm contracts and flattens, thoracic volume increases, pressure decreases, air flows in. During exhalation: the diaphragm relaxes and domes upward, thoracic volume decreases, pressure increases, air flows out. The diaphragm is innervated by the phrenic nerve (C3, C4, C5). Mnemonic: C3, 4, 5 keeps the diaphragm alive.

COPD (Chronic Obstructive Pulmonary Disease) is a chronic, progressive lung disease characterised by irreversible airflow obstruction (unlike asthma which is reversible). It includes: Chronic Bronchitis (chronic inflammation and excess mucus production in the bronchi - blue bloater: cyanotic, oedematous) and Emphysema (destruction of alveolar walls - large air spaces - reduced surface area for gas exchange - pink puffer: breathless, barrel chest). Main cause: cigarette smoking (>80% of cases). Also: air pollution, occupational dust/chemicals. Treatment: bronchodilators, steroids, oxygen therapy.

Lung Cancer is the leading cause of cancer death worldwide. Types: Non-Small Cell Lung Cancer (NSCLC) ~85% of cases (includes Adenocarcinoma - most common, Squamous cell carcinoma, Large cell carcinoma) and Small Cell Lung Cancer (SCLC) ~15% (highly aggressive, strongly linked to smoking). Main cause: Cigarette smoking (responsible for ~85% of cases). Other causes: radon gas, asbestos, air pollution, genetic factors. Symptoms: persistent cough, haemoptysis (blood in sputum), weight loss, chest pain, hoarseness. Early detection: CT scan (low-dose), sputum cytology.

The normal respiratory rate (eupnoea) in a healthy adult at rest is 12-20 breaths per minute. Terms for abnormal breathing rates: Tachypnoea = breathing rate >20/min (too fast), Bradypnoea = breathing rate <12/min (too slow), Apnoea = complete cessation of breathing, Hyperpnoea = increased depth of breathing (e.g., during exercise), Dyspnoea = difficulty in breathing (shortness of breath). In newborns, the normal breathing rate is higher: 30-60 breaths per minute.