NEET Biology — Chapter 17

Breathing and Exchange of Gases

Breathing and Exchange of Gases covers the structure of the human respiratory system, the mechanics of inspiration and expiration, all the respiratory lung volumes and capacities (with their numeric values), gas exchange by diffusion, oxygen and carbon dioxide transport in blood, and neural regulation of breathing. NEET asks 3–4 questions from this chapter — the respiratory volumes table, Bohr effect, and CO₂ transport as bicarbonate are the most tested.

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1. Respiratory Organs and the Human Airway

Different animals have evolved different respiratory surfaces — skin (earthworms), gills (fish), book lungs (spiders), and lungs (mammals). In humans, the lungs are the chief respiratory organs.

Human airway: Nostrils → nasal cavity (filters, warms, moistens air) → pharynx → larynx (voice box) → trachea (C-shaped cartilaginous rings keep it open) → primary bronchi → secondary bronchi → bronchioles → terminal bronchioles → alveolar ducts → alveoli.

Each lung has ~300 million alveoli, providing a total surface area of ~70 m². The alveolar wall is extremely thin (0.2 µm) and richly supplied with blood capillaries.

NEET tip: The epiglottis prevents food from entering the trachea. The trachea bifurcates into left and right primary bronchi at the carina. The right lung has 3 lobes; the left lung has 2 lobes (cardiac notch).

2. Mechanics of Breathing

Breathing depends on creating pressure differences between the atmosphere and the lungs using the diaphragm and intercostal muscles.

Inspiration (active process):

  • Diaphragm contracts → flattens downward.
  • External intercostal muscles contract → ribs move up and out.
  • Thoracic volume increases → intrapulmonary pressure falls below atmospheric pressure → air enters lungs.

Expiration (passive during quiet breathing):

  • Diaphragm and intercostal muscles relax.
  • Thoracic volume decreases → intrapulmonary pressure rises → air expelled.

During forced expiration, the internal intercostal muscles contract to push ribs further inward, and the abdominal muscles help compress the abdomen.

3. Respiratory Volumes and Capacities

Lung volumes are measured with a spirometer. Key values to memorise for NEET:

TermValueDefinition
Tidal Volume (TV)500 mLAir moved per normal breath
Inspiratory Reserve Volume (IRV)2500–3000 mLExtra air inhaled after normal inspiration
Expiratory Reserve Volume (ERV)1000–1100 mLExtra air exhaled after normal expiration
Residual Volume (RV)1100–1200 mLAir left after maximum expiration
Vital Capacity (VC)~4600 mLTV + IRV + ERV
Total Lung Capacity (TLC)~5800 mLVC + RV
Functional Residual Capacity (FRC)~2300 mLERV + RV
NEET tip: Residual Volume cannot be measured by a spirometer alone — it requires body plethysmography or helium dilution. This is a frequent MCQ trap.

4. Gas Exchange and Transport

Gas exchange occurs by simple diffusion across the thin alveolar and capillary walls, driven by partial pressure gradients.

  • Alveolar pO₂ (~104 mmHg) > venous blood pO₂ (~40 mmHg) → O₂ diffuses into blood.
  • Venous blood pCO₂ (~45 mmHg) > alveolar pCO₂ (~40 mmHg) → CO₂ diffuses out.

Oxygen transport:

  • ~97% as oxyhaemoglobin (HbO₂) — forms in lungs, dissociates in tissues.
  • ~3% dissolved in plasma.
  • The O₂–Hb dissociation curve is sigmoid; Bohr effect: high pCO₂ and high temperature shift curve rightward (promotes O₂ release in tissues).

Carbon dioxide transport:

  • ~70% as bicarbonate ions (HCO₃⁻) — carbonic anhydrase catalyses CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.
  • ~23% as carbaminohaemoglobin (CO₂ bound to Hb).
  • ~7% dissolved in plasma.

5. Regulation of Breathing and Disorders

Breathing is regulated by the respiratory centre in the medulla oblongata and pons.

  • Pneumotaxic centre (pons) — controls rate and depth of breathing.
  • Apneustic centre (pons) — prolongs inspiration.
  • Peripheral chemoreceptors (carotid and aortic bodies) detect pO₂ and pCO₂ changes.
  • CO₂ is the primary chemical stimulus for breathing — rising pCO₂ → increased breathing rate.

Respiratory disorders:

  • Asthma — inflammation of bronchioles; wheezing; triggered by allergens.
  • Emphysema — alveolar walls damaged; reduced surface area; caused mainly by smoking.
  • Occupational lung diseases — silicosis (silica), asbestosis (asbestos), black lung disease (coal dust).
NEET caution: O₂ is not the primary driver of the breathing rate — CO₂ is. Low pO₂ (hypoxia) does stimulate breathing, but through peripheral chemoreceptors, not the medullary centre directly.
NEET Bio Breathing Notes
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Chapter note placement for Breathing and Exchange of Gases.

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5 focused sessions: respiratory organs & airway, mechanics of breathing, lung volumes & capacities, gas transport (O₂ and CO₂), and regulation & disorders — 15 NEET MCQs each.

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