Physiology

Anatomy of the Respiratory System

The Architecture of Breathing

Amos Asamoah
October 2025
4 min read
Respiratory System

Breathing is the first thing you do when you enter this world and the last thing you do before you leave it. Yet, between those two moments lies an incredible system of tubes, sacs, and muscles working together — effortlessly and continuously. The respiratory system isn’t just about lungs; it’s about airflow, gas exchange, and homeostasis — all wrapped in anatomical precision.

🌬️ Overview

The respiratory system is divided into two main parts:

  1. Upper respiratory tract – air conditioning system
  2. Lower respiratory tract – gas exchange machinery

🏛️ Upper Respiratory Tract

Think of it as the entrance hall of the body’s airways — it filters, warms, and humidifies the air before it enters the lungs.

Components:

  1. Nose and nasal cavity
  2. Pharynx
  3. Larynx

👃 Nose and Nasal Cavity

  • The external nose opens into two nostrils (nares).
  • Inside, the nasal cavity is divided by the nasal septum into right and left halves.
  • The nasal conchae (turbinates) increase surface area and cause turbulence, ensuring air is warmed and moistened.
High-yield: The nasal cavity is lined with ciliated pseudostratified columnar epithelium with goblet cells — forming the mucociliary escalator that traps and removes particles.
Clinical tie: Damage to cilia (like from smoking) → impaired mucus clearance → chronic bronchitis.

🧠 Paranasal Sinuses

  • Air-filled cavities in the frontal, maxillary, ethmoid, and sphenoid bones.
  • They lighten the skull, produce mucus, and resonate the voice.

🗣️ Pharynx (Throat)

A muscular tube that serves both respiratory and digestive systems. It extends from the base of the skull to the esophagus.

Part Location Function
Nasopharynx Behind nasal cavity Passage for air only
Oropharynx Behind oral cavity Air + food passage
Laryngopharynx Above larynx Air + food diverge here
Clinical pearl: The epiglottis (a leaf-shaped cartilage) prevents food from entering the airway during swallowing.

🗣️ Larynx (Voice Box)

  • Connects pharynx to trachea.
  • Formed by 9 cartilages, including: Thyroid cartilage (Adam’s apple); Cricoid cartilage; Epiglottis.
  • Houses the vocal cords — folds of mucous membrane that vibrate to produce sound.
Key muscle: The posterior cricoarytenoid — the only abductor of vocal cords; paralysis = airway obstruction.
Clinical tie: Injury to recurrent laryngeal nerve (branch of vagus) → hoarseness or voice loss.

🫁 Lower Respiratory Tract

Once air passes the larynx, it enters the lower tract — the conducting and respiratory zones that bring air deep into the lungs.

🧱 A. Trachea (Windpipe)

  • ~10–12 cm long tube extending from C6 to T5, where it divides into right and left bronchi.
  • Reinforced by C-shaped hyaline cartilage rings (open posteriorly to allow esophageal expansion).
  • Lined with ciliated pseudostratified epithelium — continues the mucociliary elevator function.
Clinical tip: The carina (the last tracheal ridge) is highly sensitive — irritation triggers a cough reflex.

🌳 B. Bronchial Tree

The airways divide repeatedly — like branches of a tree.

  1. Primary bronchi – one to each lung (right is wider, shorter, more vertical → foreign body entry site).
  2. Secondary bronchi – one per lobe (3 right, 2 left).
  3. Tertiary bronchi – supply bronchopulmonary segments.
  4. Bronchioles – smaller, non-cartilaginous tubes.
  5. Terminal bronchioles – end of conducting zone.
  6. Respiratory bronchioles – start of respiratory zone, contain alveoli.
High-yield breakdown: Conducting zone: Nose → terminal bronchioles (no gas exchange, just airflow). Respiratory zone: Respiratory bronchioles → alveolar sacs (gas exchange occurs).

🫁 C. Lungs

Each lung is a spongy, cone-shaped organ filling most of the thoracic cavity.

Lung Lobes Fissures
Right lung 3 (superior, middle, inferior) Horizontal & oblique
Left lung 2 (superior, inferior) Oblique only; cardiac notch present
Clinical note: The right main bronchus is more vertical → most aspirated objects end up in the right lower lobe.

🧩 Lung Structure

  • Each lung is enclosed by pleurae: Visceral pleura – covers lung surface; Parietal pleura – lines chest wall; Pleural cavity – thin film of fluid reduces friction during breathing.
Clinical tie: Air in pleural cavity → pneumothorax → lung collapse.

🌬️ D. Alveoli — The Respiratory Units

The alveoli are the tiny air sacs (about 300 million in total) where gas exchange actually occurs. Their combined surface area ≈ 70 m² — roughly the size of a tennis court!

Cell Type Function
Type I pneumocytes Flat cells for gas diffusion
Type II pneumocytes Secrete surfactant (reduces surface tension)
Alveolar macrophages Clean up debris and microbes
Clinical high-yield: Deficiency of surfactant → Neonatal Respiratory Distress Syndrome (RDS).

🫀 Blood and Nerve Supply of the Lungs

Blood Supply

  • Pulmonary arteries: Carry deoxygenated blood to lungs.
  • Pulmonary veins: Carry oxygenated blood back to the heart.
  • Bronchial arteries: Supply nutritional blood to lung tissue itself.

Nerve Supply

  • Parasympathetic (vagus): Bronchoconstriction, mucus secretion.
  • Sympathetic: Bronchodilation, ↓ mucus.
Drug link: β₂-agonists (like salbutamol) mimic sympathetic action → relieve asthma.

🧠 Lymphatic Drainage

  • Superficial and deep plexuses → bronchopulmonary nodes → tracheobronchial nodes → thoracic duct.
  • Helps clear debris and prevents infection spread.

🧩 Functional Divisions Summary Table

Division Includes Primary Function
Conducting zone Nose → Terminal bronchioles Filter, warm, humidify air
Respiratory zone Respiratory bronchioles → Alveoli Gas exchange
Muscles of respiration Diaphragm, intercostals, accessory muscles Drive inspiration & expiration
← Previous Topic: Lymphatic Circulation Next Topic: Mechanics of Breathing →
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