Overview
This lecture details the structure and function of the respiratory tract, the mechanics and neural control of breathing, and explains relevant gas laws important for understanding respiration and clinical scenarios.
Anatomy of the Respiratory Tract
- Air enters via the nostrils (nares), passing vibrissae (nose hairs) as the first defense.
- The nasal cavity has moist, ciliated pseudostratified epithelium with goblet cells and is highly vascularized for filtering, warming, and humidifying air.
- Turbinates (conchae) in the nasal cavity create turbulence, increasing filtration and contact with olfactory neurons for smell.
- Sinuses lighten the skull and help with sound resonance; their mucosa is prone to infection.
- The pharynx (nasopharynx, oropharynx, laryngopharynx) conducts air to the larynx and esophagus.
- The epiglottis prevents food from entering the trachea during swallowing.
- Air moves past the larynx (voice box, Adam’s apple) into the trachea.
- The trachea contains 16–20 C-shaped cartilaginous rings and is lined with ciliated epithelium and goblet cells for the mucociliary escalator.
- The trachea bifurcates at the carina (T4-T5) into the right and left main bronchi; the right is wider and more vertical.
- Bronchi branch 23–25 times, becoming bronchioles (<0.5mm diameter); cartilage decreases and smooth muscle increases at smaller airways.
- Terminal bronchioles end the conducting zone; respiratory bronchioles and alveoli are sites of gas exchange.
- Alveoli have thin membranes and large surface area, containing macrophages for defense.
Gas Laws in Respiration
- Dalton’s Law: The total pressure of a mixture of gases is the sum of their partial pressures.
- Example: At sea level (760 mmHg): N₂ ≈ 597 mmHg, O₂ ≈ 159 mmHg, CO₂ ≈ 0.3 mmHg.
- Henry’s Law: Gases dissolve in liquids in proportion to their partial pressures, moving down their own pressure gradients.
- Boyle’s Law: Volume and pressure are inversely related in a closed system (increase volume, decrease pressure, and vice versa).
- Gas flows from high to low pressure.
Mechanics of Breathing
- Inspiration increases thoracic volume (diaphragm contracts/flattens, external intercostals contract), lowering lung pressure and drawing air in.
- Forced inspiration recruits accessory muscles: pectoralis major/minor, serratus anterior, sternocleidomastoid, and scalenes.
- Expiration: Relaxing inspiratory muscles causes passive exhalation; forced expiration uses abdominals, internal intercostals, obliques, transversus abdominis, and latissimus dorsi to compress the thoracic cavity.
Neural Regulation of Breathing
- The medulla (dorsal inspiratory region) generates spontaneous breathing rhythm, innervating the diaphragm via the phrenic nerve (C3-C5).
- Pons (pneumotaxic area) can modify the breathing rate by inhibiting inspiratory neurons.
- Chemoreceptors in carotid bodies and aortic arch detect increased CO₂, decreased O₂, and increased H⁺, signaling the brain to adjust respiration (via glossopharyngeal and vagus nerves).
- The main driver for increased breathing is rising CO₂ and H⁺ (acidity).
Key Terms & Definitions
- Vibrissae — nose hairs acting as the first filter for inspired air.
- Cilia — hair-like projections on epithelial cells that move mucus and trapped particles.
- Goblet cells — cells in the respiratory tract that secrete mucus.
- Turbinates (Conchae) — bony projections in the nasal cavity creating airflow turbulence.
- Mucociliary escalator — cilia move mucus and debris upwards to be swallowed.
- Carina — bifurcation point of the trachea into right and left main bronchi.
- Alveoli — tiny air sacs in lungs for gas exchange with blood.
- Partial pressure — the pressure of a single gas in a mixture.
- Phrenic nerve — nerve originating from C3-C5, innervates the diaphragm.
- Chemoreceptors — sensory receptors detecting chemical changes (e.g., CO₂, O₂, H⁺) in blood.
Action Items / Next Steps
- Review diagrams of the respiratory tract and associated muscles.
- Memorize key respiratory laws (Dalton’s, Henry’s, Boyle’s) and their clinical significance.
- Learn the muscles of inspiration and expiration and their innervation.
- Prepare questions on neural regulation for discussion in the next class.