Respiratory System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Respiratory System — Key Facts for NEET PG

• Respiratory Zone: Gas exchange — respiratory bronchioles, alveolar ducts, alveoli (300 million alveoli, surface area ~70 m²)
• Conducting Zone: Warm, humidify, filter air — nose to terminal bronchioles
• Alveolar Wall: Type I pneumocytes (gas exchange) + Type II (surfactant production)
• Surfactant: Dipalmitoylphosphatidylcholine (DPPC) — reduces surface tension; Deficiency → RDS in neonates
• ⚡ Exam tip: Right lung has 3 lobes, 2 fissures; Left lung has 2 lobes, 1 fissure, cardiac notch

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Respiratory System — NEET PG Study Guide

Airway Anatomy

Upper Respiratory Tract:

• Nasal cavity, paranasal sinuses
• Pharynx (nasopharynx, oropharynx, laryngopharynx)
• Larynx (voice box)

Lower Respiratory Tract:

• Trachea: 16-20 C-shaped cartilage rings; carina at bifurcation (T4-T5)
• Main bronchi: Right (wider, shorter, more vertical)
• Bronchi → Bronchioles → Terminal bronchioles → Respiratory bronchioles → Alveoli

Lung Anatomy

Right Lung: 3 lobes (upper, middle, lower), 2 fissures (horizontal, oblique) Left Lung: 2 lobes (upper, lower), 1 fissure (oblique), cardiac notch

Hilum: Bronchus, pulmonary vessels, nerves, lymphatics

Pleura:

• Visceral pleura (covers lung surface)
• Parietal pleura (lines chest wall)
• Pleural cavity with pleural fluid (lubrication)

Mechanics of Breathing

Inspiration: Active — diaphragm contracts, external intercostals elevate ribs Expiration: Passive at rest (recoil); Active during forced (internal intercostals, abdominal muscles)

NCE Exam Pattern

Common question types:

1. Airway anatomy and differences between right and left bronchi
2. Alveolar structure and gas exchange
3. Respiratory mechanics
4. Lung volumes and capacities
5. Oxygen-hemoglobin dissociation curve

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Respiratory System — Comprehensive NEET PG Notes

Detailed Theory

1. Respiratory Epithelium

Conducting Zone (no gas exchange):

• Pseudostratified ciliated columnar epithelium
• Goblet cells (mucus) + Basal cells + Brush cells
• Submucosal glands (mixed serous and mucous)
• Cilia beat toward pharynx (mucociliary escalator)
• Mucus traps particles, cilia move mucus upward

Respiratory Zone:

• Simple cuboidal (respiratory bronchioles)
• Simple squamous (alveolar ducts, alveoli)

2. Alveolus — Detailed Structure

Type I Pneumocytes:

• 95% of alveolar surface area
• Extremely thin (0.1-0.2 μm) for gas exchange
• Cannot divide → damaged Type I → replaced by Type II
• Highly permeable to gases

Type II Pneumocytes:

• 5% of alveolar surface area
• Produce surfactant (reduces surface tension)
• Can divide → replenish Type I and II
• Cuboidal shape, foamy cytoplasm (lamellar bodies)

Alveolar Macrophages (Dust Cells):

• Phagocytose debris, bacteria
• Can leave via lymphatics or be expectorated

Blood-Gas Barrier (0.6 μm total):

1. Alveolar epithelium (Type I)
2. Fused basement membranes
3. Capillary endothelium

• Extremely thin for diffusion

Alveolar Pores of Kohn:

• Connect adjacent alveoli
• Allow collateral ventilation
• Important when bronchioles obstructed

3. Surfactant

Composition:

• Dipalmitoylphosphatidylcholine (DPPC) — 40%
• Other phospholipids
• Surfactant proteins (SP-A, SP-B, SP-C, SP-D)

Function:

• Reduces surface tension (prevents alveolar collapse)
• Increases lung compliance (easier to inflate)
• Prevents atelectasis

La Place’s Law: Pressure = 2T/r (smaller radius → higher pressure)

• Without surfactant: Small alveoli collapse into large ones
• With surfactant: Stabilizes alveoli of different sizes

RDS (Hyaline Membrane Disease):

• Deficiency of surfactant in premature infants
• Due to insufficient Type II cell development
• Risk: <34 weeks gestation
• Treatment: exogenous surfactant, CPAP

Clinical Note: Corticosteroids given to mothers at risk of preterm delivery to accelerate fetal lung maturity.

4. Pleura and Pleural Space

Visceral Pleura:

• Covers lung surface, extends into fissures
• Sensitive to pain (innervated by phrenic nerve at central dome)

Parietal Pleura:

• Cervical: Apex of lung
• Costal: Lines ribs and intercostal spaces
• Diaphragmatic: Covers diaphragm
• Mediastinal: Covers mediastinal structures

Pleural Reflections:

• Horizontal: Right midaxillary, T4-T5
• Vertebral: T10-T12

Pleural Cavity:

• Potential space (~10-20 μm)
• Pleural fluid: Ultrafiltrate of plasma
• Lubricates lung movement
• Creates surface tension (lung doesn’t collapse)

5. Respiratory Mechanics

Inspiration (active):

• Diaphragm: Most important muscle (75% of tidal volume)
  • Contracts → dome descends 1-10 cm
  • Flattens, increases vertical dimension
• External intercostals: Elevate ribs (pump handle movement)
• Accessory muscles (forced breathing): Scalenes, sternocleidomastoid, pectoralis minor

Expiration (normally passive):

• Relaxation of diaphragm and external intercostals
• Elastic recoil of lungs and chest wall
• No muscle contraction at rest

Forced Expiration (active):

• Internal intercostals (depress ribs)
• Abdominal muscles (compress abdominal cavity, push diaphragm up)

Lung Compliance:

• C = ΔV/ΔP (change in volume per change in pressure)
• Normal: 200 mL/cmH₂O
• Decreased: Fibrosis, atelectasis
• Increased: Emphysema (loss of elastic tissue)

Airway Resistance:

• Most resistance in bronchi (4th-5th generation)
• Bronchioles < 2mm contribute little (lots in parallel)
• Smooth muscle contraction (parasympathetic) → ↑ resistance
• Sympathetic (β2) → bronchodilation → ↓ resistance

6. Lung Volumes and Capacities

Volumes (non-divisible):

• Tidal Volume (TV): Normal breath (~500 mL)
• Inspiratory Reserve Volume (IRV): Max inspiration from TV (~3000 mL)
• Expiratory Reserve Volume (ERV): Max expiration from TV (~1200 mL)
• Residual Volume (RV): Air remaining after max expiration (~1200 mL)

Capacities (sum of volumes):

• Inspiratory Capacity = TV + IRV (~3500 mL)
• Vital Capacity = TV + IRV + ERV (~4500 mL)
• Functional Residual Capacity = ERV + RV (~2400 mL)
• Total Lung Capacity = All four volumes (~6000 mL)

Clinical Tests:

• Spirometry: Measures TV, IRV, IVC, ERV, FVC, FEV1
• FEV1/FVC ratio: Obstructive <0.7, Restrictive >0.8

7. Gas Exchange

Ventilation (V): Air reaching alveoli (~350 mL per breath) Perfusion (Q): Blood reaching alveoli (~5 L per minute)

V/Q Matching:

• Normal V/Q = 0.8
• V/Q = 0: shunt (no ventilation)
• V/Q = ∞: dead space (no perfusion)
• Anatomical dead space: Conducting airways (~150 mL)

Diffusion:

• Fick’s Law: V = (A × D × (P1-P2))/(T × √MW)
• Factors affecting: Membrane thickness, surface area, diffusion coefficient, partial pressure gradient
• CO diffusing capacity (DLCO): Measures diffusion efficiency

Oxygen Transport:

• 98.5% bound to hemoglobin (1.34 mL O₂/g Hb)
• 1.5% dissolved in plasma
• Hb-O₂ dissociation curve: Sigmoid (cooperative binding)

Carbon Dioxide Transport:

• 70% as bicarbonate (CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻)
• 23% bound to hemoglobin (carbaminohemoglobin)
• 7% dissolved

8. Control of Breathing

Respiratory Center (brainstem):

• Pons: Pneumotaxic center (regulates inspiratory duration), Apneustic center
• Medulla: Dorsal respiratory group (inspiration), Ventral respiratory group (expiration)

Chemoreceptors:

• Central (medulla): Respond to CSF pH (H⁺)
• Peripheral (carotid bodies, aortic bodies): Respond to PaO₂, PaCO₂, pH

Mechanoreceptors:

• Stretch receptors (Hering-Breuer reflex): Inhibit inspiration when lung overdistended
• J receptors ( juxtacapillary): Trigger dyspnea when pulmonary capillary pressure increased

Higher Centers: Cortex can voluntarily control breathing (speech, breath-holding)

9. Respiratory Adjustments

Altitude:

• ↓ Barometric pressure → ↓ PO₂
• Acclimatization: ↑ Ventilation, ↑ 2,3-DPG, ↑ Hb, ↑ HCO₃⁻ excretion
• Acute mountain sickness: Headache, nausea, insomnia
• High altitude pulmonary edema (HAPE), cerebral edema (HACE)

Diving:

• Boyle’s Law: Pressure ↑ → Volume ↓
• Nitrogen narcosis (rapture of the deep) at >30 m
• Decompression sickness (the bends) if ascent too fast

10. Clinical Correlations

COPD:

• Obstructive pattern
• Chronic bronchitis: Blue bloater (cyanosis, productive cough)
• Emphysema: Pink puffer (barrel chest, pursed lips)
• FEV1/FVC < 0.70

Asthma:

• Reversible airway obstruction
• Bronchospasm, inflammation, mucus hypersecretion
• Eosinophilic inflammation
• FEV1/FVC < 0.70 (reversible with bronchodilator)

ARDS:

• Acute Respiratory Distress Syndrome
• Non-cardiogenic pulmonary edema
• Bilateral infiltrates, severe hypoxemia
• Common causes: Sepsis, trauma, aspiration, pancreatitis

Pulmonary Embolism:

• Obstruction of pulmonary arterial system
• Virchow’s triad: Stasis, endothelial injury, hypercoagulability
• D-dimer: Fibrin degradation product (screening)
• CT pulmonary angiography: Gold standard

Practice Questions for NEET PG

1. Describe the structure of the alveolar wall and the blood-gas barrier.
2. Explain the mechanism of surfactant and its clinical significance.
3. Discuss lung volumes and capacities.
4. Explain the V/Q relationship in different lung regions.
5. Describe the neural control of breathing.

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.