The Human Respiratory System & Gaseous Exchange

Many students confuse breathing with respiration: • Breathing (Ventilation / External Respiration): - A physical, mechanical process of moving air into (inhalation) and out of (exhalation) the lungs. - Involves muscular movements of the ribcage, intercostal muscles, and diaphragm. - Occurs outside cells; produces no energy; uses muscular energy. • Cellular Respiration (Internal / Tissue Respiration): - A biochemical process occurring inside living cells (specifically in the mitochondria and cytoplasm) where glucose is oxidized to release energy in the form of ATP. - Catalyzed by intracellular respiratory enzymes. - Word Equation for Aerobic Respiration: Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP) - Chemical Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 2880 kJ (Energy) • Aerobic vs Anaerobic Respiration: - Aerobic Respiration: Requires oxygen; completely oxidizes glucose; yields large energy (36-38 ATP per glucose molecule). - Anaerobic Respiration: Occurs without oxygen; incomplete breakdown of glucose; yields small energy (2 ATP). In human muscles during strenuous exercise, it produces Lactic Acid (causing muscle cramps and fatigue). In yeast, it produces Ethanol and CO₂ (Alcoholic Fermentation).

Inhaled air follows a sequential anatomical pathway to reach the gas exchange surface: 1. Nose / Nostrils: - Lined with fine hairs (cilia) and sticky mucus secreted by goblet cells. - Three functions: Filters and traps dust particles and airborne microbes; moistens the incoming dry air; warms air to body temperature (37°C) via superficial blood capillaries. 2. Pharynx & Larynx: - Pharynx: Common junction for food and air. - Epiglottis: A flap of cartilage that covers the trachea during swallowing to prevent food from entering the airway ("choking"). - Larynx: The "voice box" containing vocal cords that vibrate to produce speech sounds. 3. Trachea (Windpipe): - A flexible tube connecting larynx to bronchi. - Supported by C-shaped rings of cartilage that prevent the trachea from collapsing inwards when air pressure drops during inhalation. - Inner lining has ciliated epithelial cells that sweep mucus and trapped dust upwards away from the lungs toward the throat to be swallowed. 4. Bronchi and Bronchioles: - Trachea branches into two Bronchi (singular: Bronchus)—one entering each lung. - Inside each lung, bronchi branch repeatedly into narrower tubes called Bronchioles. 5. Alveoli (Air Sacs): - Microscopic grape-like clusters of tiny air sacs at the ends of bronchioles where gaseous exchange takes place. An adult has over 600 million alveoli!

Breathing relies on changing the volume and pressure inside the thoracic (chest) cavity: • Inhalation (Breathing In / Inspiration): 1. External intercostal muscles contract, pulling the ribs UPWARDS and OUTWARDS. 2. The Diaphragm contracts and flattens DOWNWARDS. 3. The volume of the thoracic cavity INCREASES. 4. The internal pressure inside the lungs DECREASES below outside atmospheric pressure. 5. Atmospheric air rushes into the lungs through the trachea to equalize pressure. • Exhalation (Breathing Out / Expiration): 1. External intercostal muscles relax, allowing ribs to fall DOWNWARDS and INWARDS. 2. The Diaphragm relaxes and bulges UPWARDS into its natural dome shape. 3. The volume of the thoracic cavity DECREASES. 4. The internal pressure inside the lungs INCREASES above atmospheric pressure. 5. Air is forced out of the lungs into the atmosphere. • Gaseous Exchange at the Alveoli: - Deoxygenated blood from the heart (pulmonary artery) arrives at the dense capillary network surrounding each alveolus. - Oxygen dissolves in the moisture lining the alveolus, then diffuses across the thin one-cell alveolar wall into the blood capillaries, binding to hemoglobin in red blood cells to form Oxyhemoglobin. - Carbon dioxide diffuses in the opposite direction—from high concentration in blood plasma across the thin walls into the alveolus to be exhaled. • Composition of Inhaled vs Exhaled Air: - Oxygen: Inhaled ~21% → Exhaled ~16% (used in cellular respiration). - Carbon dioxide: Inhaled ~0.04% → Exhaled ~4.0% (produced as metabolic waste). - Nitrogen: Inhaled ~78% → Exhaled ~78% (unchanged).