Normal arterial blood pH = 7.35-7.45 (slightly alkaline). The respiratory system helps maintain this through CO2 regulation. Acidosis (pH 7.45): Low CO2 - Respiratory alkalosis (hyperventilation, anxiety) or Metabolic alkalosis (vomiting). CO2 + H2O = H2CO3 = H+ + HCO3- - this equation links breathing to blood pH. Increasing ventilation blows off CO2 (raises pH); decreasing ventilation retains CO2 (lowers pH).

The Pulmonary Arteries are unique because they carry deoxygenated blood (unlike all other arteries which carry oxygenated blood - a common exam trap). Pulmonary trunk arises from the right ventricle. Divides into right and left pulmonary arteries which carry deoxygenated blood to each lung. In the lungs, blood is oxygenated in the alveolar capillaries. Pulmonary veins (4 total - 2 from each lung) carry oxygenated blood back to the left atrium (also unique - veins carrying oxygenated blood). This is the pulmonary circulation or lesser circulation.

Atelectasis is the partial or complete collapse of a lung or a region of lung tissue, resulting in reduced or absent gas exchange in the affected area. Causes: Obstructive (blocked airway - mucus plug, foreign body, tumour - air absorbed beyond blockage), Compressive (external pressure on lung - pleural effusion, pneumothorax, tumour), Surfactant deficiency (premature infants - NRDS), Post-operative (most common cause of post-op fever in first 24-48 hours). Symptoms: breathlessness, reduced breath sounds, cyanosis. Treatment: physiotherapy, deep breathing exercises, removal of obstruction.

Minute Ventilation (VE) = Tidal Volume x Respiratory Rate = 500 mL x 12 breaths/min = 6000 mL (6 litres/min) at rest. During intense exercise, minute ventilation can increase to 150-200 litres/min due to increased tidal volume and breathing rate. The alveolar ventilation rate (more relevant for gas exchange) = (Tidal volume - Dead space) x Respiratory rate = (500 - 150) x 12 = 4200 mL/min. Minute ventilation is an important measure in critically ill patients on mechanical ventilation.

Bronchiectasis is a chronic condition characterised by permanent, abnormal widening (dilation) and destruction of the bronchial walls, resulting in impaired mucociliary clearance and recurrent infections. Features: Chronic productive cough with large amounts of purulent (pus-like) sputum (three-layer sputum), Recurrent chest infections, Haemoptysis (coughing blood), and Clubbing of fingers. Causes: Cystic fibrosis, whooping cough, TB, immunodeficiency. Diagnosis: HRCT (High-Resolution CT) scan - shows tram-track or signet ring appearance.

Thoracocentesis (Pleural tap / Pleurocentesis) is a procedure where a needle or catheter is inserted through the chest wall into the pleural space to drain pleural effusion (excess fluid). Used for both diagnosis (fluid analysis - Light's criteria to distinguish transudate vs. exudate) and treatment (relieving breathlessness from large effusions). It is performed under ultrasound guidance to reduce complications. The needle is inserted in the triangle of safety in the axilla. Chest tube (intercostal drain) is used for ongoing drainage of large effusions, empyema, or pneumothorax.

Haemoptysis is the medical term for coughing up blood or blood-stained sputum from the respiratory tract (lungs, bronchi, or trachea). Important causes: Tuberculosis (most common cause in India), Lung cancer, Bronchiectasis, Pulmonary embolism, Pneumonia, and Mitral stenosis (cardiac cause). Important distinctions: Haematemesis = vomiting blood (from GI tract), Haematuria = blood in urine, Epistaxis = nosebleed. Haemoptysis is always a serious symptom requiring urgent investigation including chest X-ray, CT scan, bronchoscopy, and sputum culture.

Tracheostomy is a surgical procedure in which an opening (stoma) is created in the anterior neck through the trachea to establish a direct airway. A tracheostomy tube is inserted to maintain the airway. Indications: Prolonged mechanical ventilation, Airway obstruction (tumour, trauma, severe epiglottitis), Unable to protect airway (neurological conditions), and Severe obstructive sleep apnoea. Cricothyroidotomy = emergency airway access through the cricothyroid membrane (faster emergency procedure). Laryngectomy = surgical removal of the larynx (for laryngeal cancer).

Pulmonary Embolism (PE) occurs when a blood clot (thrombus), usually originating from a Deep Vein Thrombosis (DVT) in the leg, travels through the circulation and lodges in a pulmonary artery, blocking blood flow to a portion of the lung. Symptoms: sudden onset breathlessness, pleuritic chest pain, haemoptysis, tachycardia (classic triad). Risk factors (Virchow's triad): stasis, hypercoagulability, endothelial damage. Diagnosis: CT Pulmonary Angiography (CTPA) - gold standard; D-dimer blood test. Treatment: anticoagulants (heparin, warfarin), thrombolysis in massive PE.

The Thyroid cartilage is the largest cartilage of the larynx and forms the Adam's apple. The larynx has 9 cartilages: Unpaired (3) = Thyroid, Cricoid (ring-shaped, only complete ring in the airway), Epiglottis. Paired (3 pairs = 6) = Arytenoid (most important - move vocal cords), Corniculate, Cuneiform. The Cricoid cartilage lies below the thyroid cartilage and is the only complete ring of cartilage in the entire airway. The vocal cords (vocal folds) are attached anteriorly to the thyroid cartilage and posteriorly to the arytenoid cartilages.