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🫁 Respiratory System - Vocational Course (Telangana)

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1. Introduction

1. Air conduction - moving air to and from the lungs
2. Air filtration - warming, moistening, and cleaning inhaled air
3. Gas exchange (respiration) - O₂ and CO₂ exchange at the alveoli

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2. Divisions of the Respiratory System

• Upper airways = all structures above the larynx (nasal cavities, sinuses, pharynx)
• Lower airways = larynx, trachea, bronchial tree down to alveoli

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3. Organs of the Respiratory System

A. Nose and Nasal Cavity

• The nose filters, warms, and humidifies incoming air
• Divided into vestibule, respiratory region, and olfactory region
• Lined by respiratory epithelium - ciliated pseudostratified columnar cells, goblet cells, basal cells
• The olfactory region (roof of nasal cavity) contains bipolar olfactory receptor neurons with G-protein-coupled receptors for smell
• Rich vascular network warms air; mucus traps particles

B. Pharynx

• Posterior continuation of oral and nasal cavities
• Directs food into the esophagus and air into the larynx
• The two passages intersect here - hence why we can choke

C. Larynx

• Connects pharynx to trachea
• Contains vocal folds (vocal cords) - vibrate to produce sound
• Lined by respiratory mucosa except vocal cord surfaces (stratified squamous epithelium)
• The epiglottis covers the airway during swallowing

D. Trachea

• Extends from larynx into the mediastinum, then divides into right and left main (primary) bronchi
• Wall has 4 layers:
  1. Mucosa - ciliated pseudostratified epithelium on thick basement membrane
  2. Submucosa - dense irregular connective tissue
  3. Cartilaginous layer - C-shaped hyaline cartilage rings (keep airway open)
  4. Adventitia - binds trachea to adjacent structures

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4. The Bronchial Tree - Airways

Conducting Zone (Generations 0-16)

• Moves air by convection - NO gas exchange occurs here
• Forms the anatomical dead space (~150 mL)
• Airways progressively lose: cilia → mucous glands → cartilage

Respiratory Zone (Generations 17-23)

• Site of actual gas exchange
• Alveoli line the walls

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5. The Lungs

• Two lungs occupying the thoracic cavity, each enclosed in a pleural cavity lined by serous membrane (pleura)
• Right lung: 3 lobes (upper, middle, lower)
• Left lung: 2 lobes (upper, lower) - smaller due to cardiac notch
• Each lung has ~300 million alveoli, total surface area ~200 m² for gas exchange
• Pulmonary circulation: Right ventricle → pulmonary artery → lung capillaries → pulmonary vein → left atrium
• Blood flow is highest at the base and lowest at the apex when standing (gravity)

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6. The Alveoli - Site of Gas Exchange

• Alveoli are tiny air sacs (~200 µm diameter), pouchlike evaginations of respiratory bronchioles, ducts, and sacs
• Alveolar wall (septum) = the air-blood barrier, very thin for rapid diffusion

Cell Types in Alveoli

Surfactant

• Produced by Type II cells
• Reduces surface tension in alveoli
• Without surfactant - alveoli collapse (atelectasis); seen in premature babies as Respiratory Distress Syndrome

Air-Blood Barrier (from alveolar air to blood)

1. Thin layer of surfactant
2. Type I alveolar cell + basal lamina
3. Capillary endothelial cell + basal lamina (often fused with above)

• O₂ moves from alveolus (high) → blood (low)
• CO₂ moves from blood (high) → alveolus (low)

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7. Mechanics of Breathing

Inspiration (Active process)

• Diaphragm contracts and moves downward
• External intercostal muscles contract, ribs move up and out
• Thoracic volume increases → lung pressure drops below atmospheric → air flows in
• Innervated by the phrenic nerve (C3, C4, C5)

Expiration (Passive at rest)

• Respiratory muscles relax
• Elastic recoil of lungs and chest wall pushes air out
• During exercise, internal intercostal muscles and abdominal muscles actively assist

Lung Volumes (Spirometry)

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8. Control of Breathing

• Medulla oblongata - contains the Central Pattern Generator (CPG), sets the basic rhythm of inspiration/expiration
• Pons - fine-tunes the rhythm
• Respiratory-related neurons (RRNs) fire during specific phases of the respiratory cycle

Chemoreceptors (sensors that adjust breathing)

Autonomic Control of Airway Diameter

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9. Pulmonary Blood Supply

1. Pulmonary circulation - for gas exchange; right ventricle → pulmonary artery → capillaries around alveoli → pulmonary vein → left atrium
2. Bronchial circulation - for nutrition of airway walls; branches of aorta supply bronchi and bronchioles

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10. Summary: Path of Air (Inspiration)

Nostrils/Mouth
    ↓
Nasal Cavity / Oral Cavity (filters, warms, humidifies)
    ↓
Pharynx
    ↓
Larynx (past vocal cords, epiglottis protects)
    ↓
Trachea (supported by C-shaped cartilage rings)
    ↓
Right & Left Main Bronchi
    ↓
Secondary (lobar) Bronchi → Segmental Bronchi
    ↓
Bronchioles → Terminal Bronchioles (gen 16)
[End of conducting zone - dead space]
    ↓
Respiratory Bronchioles (gen 17-19)
    ↓
Alveolar Ducts (gen 20-22)
    ↓
Alveolar Sacs → Alveoli (gen 23)
[GAS EXCHANGE: O₂ in, CO₂ out]

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11. Common Disorders (Vocational Relevance)

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• Histology: A Text and Atlas with Correlated Cell and Molecular Biology (Ross & Pawlina)
• Costanzo Physiology, 7th Edition
• Medical Physiology (Boron & Boulpaep)
• Color Atlas of Human Anatomy, Vol. 2: Internal Organs