Functional Anatomy of Respiratory System

Upper & Major Airways - The Windpipe Wonders

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Lungs & Pleura - The Breathing Bags

• Lungs: Spongy organs in thoracic cavity.
  • Right Lung: 3 lobes (Superior, Middle, Inferior); Oblique & Horizontal fissures. 10 bronchopulmonary segments (BPS).
  • Left Lung: 2 lobes (Superior, Inferior); Oblique fissure. Cardiac notch, Lingula. 8-10 BPS.
  • Hilum: Structures enter/exit. 📌 RALS (Pulmonary Artery: Right Anterior, Left Superior to bronchus).
• Pleura: Double-layered serous membrane.
  • Visceral: Covers lungs.
  • Parietal: Lines thoracic cavity.
  • Pleural Cavity: Potential space; serous fluid.
  • Recesses: e.g., Costodiaphragmatic (lowest point). and Pleura (layers, recesses))

⭐ The right main bronchus is wider, shorter, and more vertical than the left, a common site for foreign body aspiration.

Mechanics of Breathing - The Pump & Push

• Pump: Thoracic cage & respiratory muscles. Push: Pressure gradients.
• Inspiration (Active):
  • Muscles: Diaphragm (main, ~75%), External Intercostals. 📌 DIE (Diaphragm, External Intercostals).
  • Accessory (forced): Scalenes, Sternocleidomastoid (SCM).
  • Thoracic volume ↑ → Intrapleural pressure (IPP) ↓ (more negative, -4 to -8 cm H₂O) → Lung volume ↑ → Alveolar pressure (Palv) ↓ (-1 cm H₂O) → Air IN.
• Expiration (Passive at Rest):
  • Elastic recoil of lungs & chest wall.
  • Thoracic volume ↓ → IPP ↑ (less negative) → Lung volume ↓ → Palv ↑ (+1 cm H₂O) → Air OUT.
• Forced Expiration (Active):
  • Muscles: Internal Intercostals, Abdominals.
• Key Pressures & Law:
  • Transpulmonary Pressure (Ptp) = Palv - IPP (keeps alveoli open).
  • Boyle's Law: $P \propto 1/V$.

⭐ Intrapleural pressure is always subatmospheric (negative) during normal quiet breathing.

Alveolar Zone - The Oxygen Exchange

• Primary site of gas exchange ($O_2$ $\leftrightarrow$ $CO_2$) in the lungs.
• Comprises: Respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
• Alveoli: Functional units; approx. 300-500 million; provide vast surface area (~70-100 m²) for diffusion.
• Key Cell Types:
  • Type I Pneumocytes: Squamous epithelial cells (95%); form the thin alveolar wall; primary sites for gas exchange.
  • Type II Pneumocytes (Septal Cells): Cuboidal cells (5%); secrete surfactant; can differentiate into Type I cells.
  • Alveolar Macrophages (Dust Cells): Phagocytose debris and pathogens.
• Respiratory Membrane (Blood-Air Barrier): Extremely thin (~0.2-0.6 µm).
  • Alveolar epithelium (Type I cell)
  • Fused basal laminae of alveolar epithelium and capillary endothelium
  • Capillary endothelium
• Surfactant: Lipoprotein complex (mainly dipalmitoylphosphatidylcholine - DPPC); reduces alveolar surface tension, prevents collapse (atelectasis), increases compliance. 📌 Mnemonic: Surfactant Stops Small alveoli from Shutting.

⭐ Surfactant synthesis begins around 24-28 weeks of gestation; deficiency leads to Neonatal Respiratory Distress Syndrome (NRDS), especially in premature infants. Type II pneumocytes are also the main progenitor cells after lung injury.

High‑Yield Points - ⚡ Biggest Takeaways

• Type II pneumocytes produce surfactant, preventing alveolar collapse and reducing surface tension.
• The right main bronchus is wider, shorter, and more vertical, a common site for foreign body aspiration.
• Diaphragm is the primary inspiration muscle, innervated by the phrenic nerve (C3-C5).
• Anatomical dead space in conducting airways is approximately 150 mL.
• Pulmonary arteries carry deoxygenated blood; pulmonary veins carry oxygenated blood.
• Cartilage supports bronchi; bronchioles lack cartilage, depending on smooth muscle tone.
• Right lung: 3 lobes, 2 fissures; Left lung: 2 lobes, 1 fissure, plus cardiac notch & lingula.